The circulatory system is a vital component of vertebrates, consisting of two interconnected systems: the blood vascular system and the lymphatic system. While both systems play crucial roles in maintaining the body’s overall function, the circulatory system primarily refers to the blood vascular system.

In vertebrates, the blood vascular system is a closed system characterized by a contractile heart and a complex network of vessels. Arteries serve as the blood vessels that carry oxygenated blood away from the heart. These arteries then branch into thinner arterioles, which further divide into an intricate network of capillaries. Capillaries are incredibly thin, composed of a single layer of tasselated endothelial cells. The endothelial cells line all blood vessels, including the heart, forming a protective barrier between the blood and the surrounding tissues.

Capillaries are present in all body tissues, facilitating the exchange of substances between the blood and tissue cells. This exchange occurs through the tissue fluid, which fills the spaces between cells. The capillary walls allow for the passage of substances such as oxygen, nutrients, hormones, water, carbon dioxide, and nitrogenous wastes. The endothelial cells in capillaries utilize pinocytosis, a process in which small vesicles invaginate the plasma membranes, to facilitate the exchange of these substances.

As the blood releases oxygen, nutrients, and hormones to the cells, the cells simultaneously release water, carbon dioxide, and nitrogenous wastes into the blood. This exchange allows for metabolism to occur efficiently and for waste products to be transported to the excretory organs for elimination.

Once the exchange has taken place in the capillaries, the blood proceeds into thin venules, which then merge to form veins. Veins carry deoxygenated blood back towards the heart. It is important to note that not all blood passes through capillaries into venules. Some veins, such as portal veins, renal veins, and hepatic veins, possess capillaries that resemble those found in arteries.

In addition to the capillary network, there are specific anatomical structures that contribute to the regulation of blood pressure and circulation. Some organs, like the skin, possess through channels between arterioles and venules. These channels enable direct communication and exchange of materials between the arterial and venous systems. Furthermore, arteriovenous anastomoses, present in the digits, also assist in regulating blood flow and pressure. While their exact function is not entirely understood, they play a role in maintaining optimal circulation.

Certain organs exhibit unique features within their blood vessels. For instance, thin-walled spaces or sinusoids allow for material exchange between tissues and blood. In some organs, specialized networks of tiny blood vessels, known as rete mirabile, can be found. Examples of such organs include the kidneys and air bladder. These structures contribute to the organ’s specific physiological functions.

In summary, the circulatory system of vertebrates comprises the blood vascular system and the lymphatic system. The blood vascular system consists of arteries, capillaries, and veins, facilitating the exchange of substances between the blood and tissues. Capillaries form an extensive network in almost all body tissues, promoting efficient exchange. Venules and veins carry deoxygenated blood back to the heart. Additionally, specific anatomical structures and specialized vessels contribute to the regulation of blood pressure and circulation in various organs.

Parts of Circulatory System

• The circulatory system, present in chordates and annelids, is primarily of the closed type. However, molluscs and arthropods possess an open circulatory system, lacking capillaries. In these organisms, blood flows through arteries into different organs, passing through blood spaces or sinuses, and then returning to vessels (veins) that lead back to the heart.
• The main components of the circulatory system include the heart, arteries, veins, capillaries, and blood itself. The heart, a specialized muscular organ, functions as a modified blood vessel. It contracts periodically to pump blood throughout the body via specific vessels.
• Arteries and their branches form the arterial system, responsible for carrying blood away from the heart. Arteries have thick, elastic walls that allow them to withstand the high pressure generated by the pumping action of the heart. As arteries branch out, they become narrower and give rise to smaller vessels called arterioles.
• On the other hand, veins and their tributaries make up the venous system. Veins collect blood from the capillaries, which are the smallest blood vessels. Capillaries have extremely thin walls that enable the exchange of substances between the blood and surrounding tissues. Veins then transport the blood from the capillaries of the arteries or arterioles back to the heart. Veins have thinner walls compared to arteries and contain valves that prevent backflow of blood.
• In addition to the heart, arteries, veins, and capillaries, blood itself is a crucial component of the circulatory system. Blood is a specialized fluid that carries oxygen, nutrients, hormones, and waste products throughout the body. It consists of plasma, red blood cells, white blood cells, and platelets. Plasma is the liquid component of blood, while red blood cells carry oxygen using the protein hemoglobin. White blood cells are involved in immune responses, and platelets are responsible for blood clotting.
• Overall, the circulatory system is a complex network of organs and vessels that ensures the transport of oxygen, nutrients, and other essential substances to various parts of the body. The heart acts as a pump, while arteries, veins, and capillaries facilitate the flow of blood. Together, these components work harmoniously to maintain the proper functioning of the circulatory system in chordates and annelids.

Portal System

• The portal system is a unique arrangement in the circulatory system where blood does not return directly to the heart, but instead passes through an intermediate organ such as the liver or kidney. This specialized system plays a crucial role in regulating various physiological processes.
• In the portal system, a vein carrying blood begins in capillaries and ends in capillaries, with the vein serving as both an afferent (incoming) and efferent (outgoing) vessel. The afferent vessels connect with capillaries in a manner similar to arteries, and then the blood is collected and transported into systemic veins.
• Among vertebrates, all species possess a hepatic portal system, which involves blood passing through two sets of capillaries in the liver. The hepatic portal system allows for the efficient regulation of substances absorbed from the digestive system before they enter the general circulation. This arrangement ensures that nutrients, toxins, and other absorbed materials are appropriately processed and metabolized by the liver before reaching other organs and tissues.
• In addition to the hepatic portal system, some lower vertebrates and embryos of higher vertebrates possess a renal portal system. This system involves blood passing through two sets of capillaries in the kidney before returning to the heart. The renal portal system allows for the efficient filtration and processing of blood before it reaches the heart, facilitating the regulation of water balance, waste elimination, and other important renal functions.
• Furthermore, there is a specialized portal system known as the pituitary portal system, which consists of capillaries within the pituitary gland. Although relatively small, the pituitary portal system is of great significance as it allows for the direct communication between the hypothalamus and the pituitary gland. This portal system ensures the precise control and regulation of hormones released by the pituitary gland, which play vital roles in various physiological processes throughout the body.
• In summary, the portal system is a distinct arrangement in the circulatory system where blood passes through an intermediate organ before returning to the heart. The hepatic portal system is present in all vertebrates, enabling the liver to process and metabolize absorbed substances from the digestive system. Some species also possess a renal portal system, facilitating the filtration and regulation of blood within the kidneys. Additionally, the pituitary portal system is a specialized arrangement that allows for direct communication between the hypothalamus and the pituitary gland. These portal systems contribute to the precise control and regulation of various physiological functions in vertebrates.

Lymphatic System

• The lymphatic system is an important component found in chordates, excluding cyclostomes and cartilaginous fishes. It consists of lymph, lymph vessels, and lymph nodes. Lymph is a tissue fluid that exists among the body cells. It is essentially blood plasma without red blood cells and certain proteins.
• The lymphatic system begins with lymph capillaries, which form a network of thin, blind-ending vessels throughout the body. These capillaries are interspersed among the blood capillaries and serve to collect the lymph. Lymph vessels, which are thin-walled vessels, are formed by the union of lymph capillaries. These vessels gradually merge and empty into veins, eventually returning the lymph back into the bloodstream.
• Lymph nodes are an integral part of the lymphatic system, particularly in mammals. They are small, bean-shaped structures located along the lymph vessels. Lymph nodes play a crucial role in the body’s defense against diseases. Within the lymph nodes, specialized cells called lymphocytes are produced. Lymphocytes are a type of white blood cell that plays a vital role in immune responses. They help identify and neutralize foreign substances, pathogens, and abnormal cells, protecting the body from infections and diseases.
• The lymphatic system serves several important functions in the body. One of its key roles is the maintenance of fluid balance. Lymph vessels collect excess fluid, proteins, and cellular debris from the interstitial spaces between cells. This helps prevent the accumulation of fluid and maintains the appropriate balance of fluids within tissues.
• Additionally, the lymphatic system aids in the transportation of fats and fat-soluble vitamins. Specialized lymphatic vessels, known as lacteals, are present in the small intestine and assist in the absorption of dietary fats. These fats are then transported as chyle through the lymphatic system before eventually reaching the bloodstream.
• Furthermore, the lymphatic system acts as a route for immune surveillance and response. Lymph nodes play a critical role in filtering and monitoring the lymph for the presence of foreign antigens. When antigens are detected, lymphocytes within the lymph nodes are activated, leading to an immune response to eliminate the invading pathogens or foreign substances.
• In summary, the lymphatic system is a vital component of the circulatory system in chordates. It consists of lymph, lymph vessels, and lymph nodes. Lymph serves as a tissue fluid found among the body cells, collected by lymph capillaries and transported through lymph vessels back into the bloodstream. Lymph nodes play a key role in immune defense by producing lymphocytes, which are essential for fighting off infections and diseases. Additionally, the lymphatic system aids in fluid balance, fat absorption, and immune surveillance.

Evolution of Heart in Vertebrates

• The evolution of the heart in vertebrates is a fascinating process that involves the transformation of a bilateral structure into a complex, unpaired organ. During embryonic development, the heart originates from a group of endocardial cells located below the pharynx. These cells initially organize themselves into a pair of thin endothelial tubes.
• In the next stage, the two endothelial tubes merge together, forming a single endocardial tube that runs longitudinally below the pharynx. Concurrently, the splanchnic mesoderm, located beneath the endoderm, folds longitudinally around the endocardial tube. This two-layered tube serves as the foundation for the future heart, with the splanchnic mesoderm thickening to create the myocardium or muscular wall of the heart, and the outer layer forming the thin epicardium or visceral pericardium. The endocardial tube develops into the inner lining of the heart, known as the endocardium.
• As development progresses, folds of the splanchnic mesoderm meet above the heart, forming a dorsal mesocardium. This mesocardium acts as a suspensory structure, holding the heart in place within the coelom. Additionally, a transverse septum forms behind the heart, creating two chambers within the coelom: an anterior pericardial cavity that encloses the heart, and a posterior abdominal cavity.
• Initially, the heart is a straight tube, but as it grows, it undergoes changes in shape. Due to its fixed ends, the heart increases in length and eventually adopts an S-shaped configuration. Furthermore, various modifications occur, including the development of valves, constrictions, and partitions within the heart. These structural alterations, along with the differential thickenings of the heart walls, lead to the formation of three or four chambers within the heart.
• Through this intricate process of embryonic development, the heart in vertebrates evolves from a bilateral origin to an unpaired, multi-chambered organ. This transformation allows for the efficient pumping and circulation of blood throughout the body, enabling vertebrates to meet the metabolic demands of their increasingly complex physiological systems.

1. Single-Chambered Heart

• In certain primitive chordates, such as Amphioxus, a distinct, multi-chambered heart is not present. Instead, a simple structure serves the function of a heart. In Amphioxus, this heart-like structure is found as a muscular and contractile portion of the ventral aorta, located beneath the pharynx.
• The ventral aorta, which is a major blood vessel, demonstrates muscular properties and is responsible for contracting and pumping blood throughout the organism. While it is not a true heart in the conventional sense, this muscular section of the ventral aorta acts as a functional equivalent, facilitating the circulation of hemolymph, the fluid responsible for nutrient and gas exchange, within the organism.
• The absence of a multi-chambered heart in Amphioxus reflects its evolutionary position as a primitive chordate. As organisms evolve and become more complex, their circulatory systems tend to develop specialized heart structures with multiple chambers that improve efficiency and enable more precise control over blood flow. However, in the case of Amphioxus, the single-chambered muscular ventral aorta serves the essential purpose of propelling hemolymph through the body, supporting basic metabolic functions.
• The presence of a single-chambered heart in Amphioxus demonstrates the diverse range of adaptations seen within the animal kingdom, highlighting the variety of mechanisms through which organisms can achieve effective circulation. While Amphioxus lacks the intricate cardiovascular system seen in more advanced vertebrates, its simple heart-like structure exemplifies an early stage in the evolutionary development of the circulatory system.

2. Two-Chambered Heart

• The two-chambered heart is a characteristic feature found in certain groups of vertebrates, such as cyclostomes (jawless fishes) and some species of teleosts (bony fishes). In these organisms, the heart is composed of two main chambers arranged in a linear order, along with additional accessory structures.
• Cyclostomes, including lampreys and hagfish, possess a four-chambered heart with a sinus venosus, atrium (auricle), ventricle, and conus arteriosus (bulbus cordis). Among these chambers, only the atrium and ventricle can be compared to the paired atria and ventricles found in higher vertebrates. The sinus venosus serves as a thin-walled chamber that receives venous blood, while the ventricle and conus arteriosus are responsible for pumping this blood. This arrangement represents a two-chambered heart.
• In cartilaginous fishes, such as dogfish sharks, the heart is a muscular, S-shaped tube with four compartments in a linear series. The sinus venosus and conus arteriosus act as accessory chambers, while the atrium and ventricle are considered true chambers. The sinus venosus connects to the atrium through a sinu-atrial aperture guarded by valves, and the atrium opens ventrally into the ventricle through an atrio-ventricular aperture also guarded by valves. The muscular ventricle leads to a narrow conus arteriosus that contains valves in two series. The heart is enclosed within a pericardial cavity, separated from the body cavity by a transverse septum.
• In teleosts, which are a diverse group of bony fishes, the heart structure is similar to that of cartilaginous fishes. The conus arteriosus is reduced and has a single pair of valves. Additionally, the proximal part of the ventral aorta, close to the conus, is enlarged and thick-walled, forming a structure known as the bulbus arteriosus. This bulbus arteriosus is elastic and dilates during ventricular contraction. The heart in teleosts is thus considered to be two-chambered, with a single circulation of blood.
• The presence of a two-chambered heart in these organisms reflects an intermediate stage in the evolution of the circulatory system. While not as efficient as the multi-chambered hearts found in more advanced vertebrates, the two-chambered heart still allows for the essential functions of receiving and pumping blood, supporting basic physiological processes in these species.

3. Three-Chambered Heart

Dipnoi

• Dipnoi, commonly known as lungfish, exhibit interesting adaptations in their circulatory system. These unique features are associated with their utilization of the swim bladder as an organ of respiration. The circulatory system of lungfish represents a crucial step towards the development of a double-type circulatory system, wherein both oxygenated and unoxygenated blood enter the heart and are kept separate.
• One notable characteristic of the circulatory system in lungfish is the presence of a septum that divides the atrium, the receiving chamber of the heart, into right and left chambers. This division is a significant adaptation related to the lungfish’s respiratory needs. The lungfish uses its primitive lung-like gas bladder, connected to the circulatory system, for respiration.
• In the circulatory pathway, blood from the right auricle of the lungfish flows into the right ventricle, the pumping chamber of the heart. From there, it is propelled into the lung-like gas bladder through the pulmonary arteries, which branch off from the sixth pair of aortic arches. These pulmonary arteries carry oxygen-depleted blood to the gas bladder where gas exchange occurs, allowing oxygen to be absorbed from the air-filled bladder into the bloodstream.
• The oxygenated blood, now enriched with oxygen, returns to the left atrium of the heart through the pulmonary veins, similar to the circulation observed in amphibians. This oxygenated blood is then pumped out to the rest of the body, providing the necessary oxygen supply to various tissues and organs.
• The presence of a septum in the atrium and the connection between the circulatory system and the lung-like gas bladder in lungfish represent evolutionary adaptations that allow for more efficient oxygen uptake and transport. These adaptations paved the way for the development of a more advanced double-type circulatory system seen in higher vertebrates, where the separation of oxygenated and unoxygenated blood becomes more distinct.
• The circulatory system of lungfish provides an intriguing glimpse into the evolutionary transitions and adaptations that have occurred in vertebrate cardiovascular systems, highlighting the remarkable diversity and complexity of circulatory strategies across different species.

Amphibia

• Amphibians, which include frogs, toads, and salamanders, possess unique characteristics in their circulatory system that reflect their dual life in both aquatic and terrestrial environments. These adaptations enable them to efficiently transport oxygen to their tissues during different stages of their life cycle.
• One distinctive feature of the amphibian circulatory system is the anterior shift of the dorsal atrium in relation to the ventricle. Unlike in fish and lungfish, the sinus venosus, the chamber that receives venous blood, opens into the right atrium dorsally rather than posteriorly. The atrium is completely divided into right and left chambers, lacking the foramen ovale found in the inter-auricular septum of lungfish. The absence of the foramen ovale ensures that oxygenated and unoxygenated blood do not mix within the heart.
• Amphibians also exhibit unique structural modifications within the heart. Deep pockets develop in the ventricular cavity, allowing for greater mixing of oxygenated and unoxygenated blood. Additionally, the conus arteriosus, a vessel that emerges from the ventricle, divides into systemic and pulmonary vessels with the help of a spiral valve. This arrangement helps to separate oxygenated blood destined for the systemic circulation from deoxygenated blood going to the lungs or skin for gas exchange.
• In some amphibians, such as lungless salamanders, there are further variations in the circulatory system. The inter-atrial septum, which divides the atrium, may be incomplete in these species. This incomplete septum allows some mixing of oxygenated and unoxygenated blood. Furthermore, pulmonary veins, which typically carry oxygenated blood from the lungs to the heart, may be absent in lungless salamanders due to their reliance on cutaneous respiration (gas exchange through the skin) rather than lung breathing.
• The circulatory adaptations in amphibians reflect their ability to function effectively both in aquatic and terrestrial environments. During their aquatic larval stage, amphibians rely primarily on gills for respiration, and their circulatory system facilitates the efficient transport of oxygenated blood to the developing tissues. As they transition to their adult, terrestrial form, the pulmonary circulation becomes more prominent, enabling them to respire using lungs or cutaneous respiration.
• The unique characteristics of the amphibian circulatory system exemplify the remarkable adaptability of vertebrates to diverse environments and highlight the evolutionary transitions that have occurred to meet the specific respiratory demands of amphibian life cycles.

Reptilia

• Reptiles, including snakes, lizards, turtles, and crocodiles, exhibit further advancements in their circulatory system compared to amphibians. The structure and functionality of their hearts demonstrate increased separation between oxygenated and deoxygenated blood.
• In reptiles, the atrium is always completely divided into right and left chambers, providing a clear separation of oxygenated and non-oxygenated blood. In many reptile species, the sinus venosus, which receives venous blood, is incorporated into the wall of the right atrium. This integration further ensures the separation of oxygenated and deoxygenated blood within the heart.
• The ventricle of reptiles is also partially divided by a septum in most species. This septum contributes to the separation of oxygenated and deoxygenated blood, preventing significant mixing within the ventricular chamber. However, it is important to note that complete separation of the ventricle is observed only in alligators and crocodiles, making their hearts completely two-chambered.
• This separation of blood is significant because oxygenated blood returning from the lungs to the left side of the heart remains essentially separate from the deoxygenated blood returning from the body to the right side of the heart. In crocodilians, such as alligators and crocodiles, the complete separation of the two types of blood is achieved within the heart. In other reptiles, the separation is nearly complete, although some minor mixing may occur in other parts of the circulatory system.
• While the reptilian circulatory system demonstrates an increased degree of separation between oxygenated and deoxygenated blood, it is important to note that some mixing of blood may still occur in certain regions. For instance, some communication between the systemic and pulmonary circulations may exist, allowing for slight mixing of oxygenated and deoxygenated blood in specific vessels or structures.
• The evolutionary advancements seen in the reptilian circulatory system illustrate the ongoing refinement of the cardiovascular system in vertebrates. The increased separation of oxygenated and deoxygenated blood allows reptiles to have more efficient oxygen delivery to their tissues, contributing to their successful adaptation to diverse environments and lifestyles.

The embryonic conus arteriosus splits into three instead of two vessels:

During the development of the reptilian heart, the embryonic conus arteriosus undergoes a split into three vessels instead of the typical two. These vessels serve different purposes in the circulation of blood:

(i) The pulmonary arch carries blood from the right side of the ventricle to the lungs. This vessel ensures that deoxygenated blood is directed to the lungs for oxygenation before returning to the heart.

(ii) The right systemic aorta carries blood from the left side of the ventricle to the rest of the body. It delivers oxygenated blood to the systemic circulation through the right fourth aortic arch.

(iii) The left systemic artery originates from the right ventricle and connects to the left fourth aortic arch. It carries blood to specific areas of the body, contributing to the systemic circulation.

Although the reptilian heart demonstrates an increased separation between oxygenated and deoxygenated blood, there is still a point of contact between the systemic aorta from the left ventricle and the right systemic aorta. This connection, known as the foramen of Panizzae, allows for some mixing of the two types of blood. It is important to note that this mixing is relatively minor compared to amphibians, but it still represents a transitional characteristic in the reptilian heart.

Overall, the reptilian heart exhibits two complete auricles and two incomplete ventricles. The presence of the foramen of Panizzae allows for a slight mixing of blood between the right and left systemic circulations. This transitional heart structure represents an evolutionary step between the more primitive amphibian heart and the further developed hearts found in higher vertebrates.

4. Four-Chambered Heart

• In birds (Aves) and mammals (Mammalia), the circulatory system has evolved to have a more advanced structure compared to previous vertebrate groups. Both birds and mammals possess a four-chambered heart consisting of two auricles and two ventricles, resulting in complete separation of venous and arterial blood.
• In birds, the ventricle is completely divided into two chambers, allowing for a more efficient circulation. The systemic aorta, which carries oxygenated blood, originates from the left ventricle and distributes it to the head and body. On the other hand, the pulmonary artery arises from the right ventricle and carries deoxygenated blood to the lungs for oxygenation.
• This division of the ventricle and the separation of the systemic and pulmonary circulations result in a double circulation system. Importantly, there is no mixing of blood between these two circulations at any point. This ensures that oxygenated and deoxygenated blood remain separate, allowing for more efficient oxygen delivery to the body tissues.
• In birds, the sinus venosus, which receives blood from the veins, is completely incorporated into the right auricle. The right auricle receives two precaval veins and a postcaval vein, further ensuring the separation of venous and arterial blood. The left auricle, on the other hand, receives oxygenated blood through the pulmonary veins.
• Unlike in other vertebrates, birds lack a conus arteriosus, a specialized structure found in some earlier groups. Instead, the pulmonary aorta arises directly from the right ventricle, delivering deoxygenated blood to the lungs. The systemic aorta, carrying oxygenated blood, originates from the left ventricle. Both the pulmonary aorta and the systemic aorta have valves at their bases to prevent backflow of blood.
• Mammals, including humans, share a similar four-chambered heart structure with birds. The left ventricle pumps oxygenated blood to the body through the systemic aorta, while the right ventricle pumps deoxygenated blood to the lungs through the pulmonary artery.
• In summary, both birds and mammals possess a four-chambered heart with complete separation of venous and arterial blood. This advanced cardiovascular system allows for efficient oxygen delivery to the body tissues and is essential for the high metabolic demands of these animals.

Modifications of Aortic Arches in Vertebrates

Embryonic Arteries

• During the embryonic stage of vertebrates, the development of the circulatory system involves the formation of various arteries. One key blood vessel that emerges is the ventral aorta, which appears mid-ventrally below the pharynx and becomes connected to the conus arteriosus, a part of the developing heart.
• The ventral aorta, originating from the heart, extends forward beneath the pharynx and divides anteriorly into a pair of external carotid arteries that supply blood to the head. As the ventral aorta progresses, it gives rise to six pairs of lateral aortic arches at equal distances. These aortic arches traverse through the visceral arches. Each aortic arch consists of a ventral afferent branchial artery that carries venous blood to the gills and a dorsal efferent branchial artery that transports oxygenated blood away from the gills.
• The efferent branchial arteries from each side join dorsally with the lateral dorsal aorta, also known as the radix aorta, which enters the head as the internal carotid artery. Among the aortic arches, the first one is called the mandibular aortic arch, the second is the hyoid aortic arch, and the remaining arches are referred to as the third, fourth, fifth, and sixth aortic arches.
• Behind the pharynx, the lateral dorsal aorta and the fused aortic arches form the dorsal aorta, which continues dorsally into the tail as the caudal artery. From the dorsal aorta, both paired and unpaired arteries arise to supply various organs of the developing body.
• In embryos with a yolk sac, a pair of vitelline arteries emerges from the dorsal aorta and supplies the yolk sac. In embryos of amniotes, such as reptiles, birds, and mammals, a pair of umbilical or allantoic arteries arise from the dorsal aorta, providing blood supply to the allantois, a specialized embryonic structure.
• As the embryo matures into an adult, certain changes occur in the arterial system. The vitelline arteries fuse together to form the main mesenteric artery, which supplies the intestines and other abdominal organs. The allantoic arteries, however, undergo regression, and their remnants form the hypogastric or internal iliac arteries in the adult.
• In summary, the development of embryonic arteries involves the sequential formation of blood vessels, starting with the ventral aorta and progressing through the aortic arches. These arteries ensure the delivery of oxygenated blood to various regions of the developing organism, with specific arteries supplying the gills, head, yolk sac, and allantois. As development progresses, some arteries fuse or regress, resulting in the establishment of major arterial pathways in the adult body.

Aortic Arches in Vertebrates

• The arterial system in vertebrates exhibits variations among different adult species, but they all share a fundamental plan that stems from a common origin. These variations primarily arise from the evolutionary changes in the respiratory system, transitioning from gills to lungs. As vertebrates evolved, there has been a progressive reduction in the number of aortic arches throughout the vertebrate series.
• In early vertebrates, such as fish, a series of paired aortic arches are present. These aortic arches arise from the ventral aorta and connect to the gills, facilitating the exchange of oxygen and carbon dioxide. The specific number of aortic arches can vary among different fish species, with some having more arches than others.
• As we move up the evolutionary ladder, we encounter amphibians. In amphibians, such as frogs and salamanders, there is a reduction in the number of aortic arches compared to fish. This reduction reflects the transition from an aquatic lifestyle to a semi-aquatic or terrestrial one. However, amphibians still retain some of the aortic arches, albeit in a modified form, to supply the gills during the larval stage and the lungs after metamorphosis.
• In reptiles, birds, and mammals, further changes occur in the arterial system. These groups have a more developed pulmonary circulation, which involves the circulation of blood to and from the lungs for oxygenation. This change in respiratory mode leads to a reduction in the number of functional aortic arches.
• In reptiles, for instance, there is a complete separation of oxygenated and deoxygenated blood due to a fully divided ventricle. The arterial system in reptiles is more advanced compared to amphibians, with complete separation of venous and arterial blood. Although some mixing of blood may occur at certain points, such as the foramen of Panizzae in crocodilians, the overall circulation is relatively more efficient.
• Birds and mammals exhibit a four-chambered heart, enabling a complete separation of the systemic and pulmonary circulation. The pulmonary artery arises from the right ventricle and carries deoxygenated blood to the lungs, while the systemic aorta originates from the left ventricle and distributes oxygenated blood to the body. This separation ensures that oxygenated and deoxygenated blood do not mix, leading to more efficient oxygen delivery to the tissues.
• In summary, the evolution of the arterial system in vertebrates demonstrates a progressive reduction in the number of aortic arches, reflecting the changing respiratory demands and increasing complexity of the heart. From the multiple aortic arches in fish to the refined circulation in birds and mammals, these adaptations have allowed for more efficient oxygenation and distribution of blood throughout the body.

Cyclostomata

• Cyclostomata, commonly known as jawless fishes, is a group of primitive vertebrates that includes lampreys and hagfishes. These fascinating organisms exhibit unique characteristics in their cardiovascular system, particularly regarding the number of aortic arches.
• In the genus Petromyzon, which comprises the lampreys, there are typically seven pairs of aortic arches. These arches represent the arterial connections that arise from the ventral aorta and supply oxygenated blood to various regions of the body. Each pair of aortic arches branches off from the ventral aorta and distributes blood to specific regions or structures.
• On the other hand, other cyclostomes, such as the hagfishes, show variations in the number of aortic arches. The hagfish genus Myxine, for example, typically possesses six pairs of aortic arches. This represents a reduction in the number of arches compared to lampreys. Hagfishes have a more simplified cardiovascular system, reflecting their primitive nature.
• In contrast, the genus Eptatretus, another group of hagfishes, exhibits a relatively higher number of aortic arches. It is reported that Eptatretus species can have up to fifteen pairs of aortic arches. This increase in the number of arches suggests a further specialization within this particular group of cyclostomes.
• The varying numbers of aortic arches in different cyclostome species highlight the diversity and evolutionary adaptations within this group. While lampreys generally possess seven pairs, hagfishes exhibit both reductions (as seen in Myxine) and increases (as seen in Eptatretus) in the number of arches. These differences likely reflect specific ecological and physiological adaptations in their respective habitats and lifestyles.
• Overall, the study of aortic arches in cyclostomata provides valuable insights into the evolutionary history and cardiovascular adaptations of these ancient jawless fishes. By examining the variations in the number of arches among different species, scientists can gain a better understanding of the intricate cardiovascular systems found in these unique vertebrates.

Pisces

• Pisces, commonly known as fishes, encompass a diverse group of aquatic vertebrates with various adaptations in their cardiovascular system. The number of aortic arches, which are major arterial connections originating from the ventral aorta, can vary among different fish species.
• In general, fishes are believed to have six pairs of aortic arches, which serve as conduits for blood distribution. However, even among fishes, there are variations in the number of aortic arches. For instance, in sharks and rays, the number of arches is reduced to five, with the loss of the first pair known as the mandibular aortic arch. In some cases, the mandibular arch may be represented by an efferent pseudobranchial artery.
• In most bony fishes, including common teleosts, both the mandibular and hyoid aortic arches either disappear or are greatly reduced in size. These modifications in the arches are associated with the evolution of different respiratory mechanisms and the reduced reliance on gills for respiration.
• In certain fish groups, such as Polypterus and Dipnoi (lungfishes), the gills are not well-developed, and they possess additional respiratory structures like lung-like air bladders. In these species, the pulmonary artery arises from the efferent portion of the sixth aortic arch on either side. This pulmonary artery supplies blood to the air bladder or lung, facilitating respiration.
• Elasmobranchs (sharks and rays) and Dipnoi (lungfishes) exhibit a more complex arrangement of aortic arches. Each arch in these species possesses one afferent branchial artery and two efferent branchial arteries, formed by splitting, to supply the gills. In contrast, bony fishes typically have one afferent and one efferent artery per gill.
• When we look at tetrapods (four-limbed vertebrates, including amphibians, reptiles, birds, and mammals), the pattern of aortic arches changes. In tetrapods, the aortic arches do not divide into afferent and efferent parts because true internal gills are absent. Additionally, the first and second arches disappear during development.
• The variations in the number and structure of aortic arches among fishes reflect the diverse evolutionary paths and adaptations within this group. From the reduction of arches in sharks and rays to the modified arches in lungfishes and bony fishes, these adaptations are closely linked to changes in respiratory mechanisms and the specific ecological niches occupied by different fish species.

Amphibia

• Amphibia, which includes frogs, toads, salamanders, and newts, exhibit specific modifications in their aortic arches as a result of the transition from gill respiration to lung respiration during their life cycle.
• In urodeles, a group of amphibians that includes salamanders and newts, external gills serve as additional respiratory organs alongside the lungs. The aortic arches in urodeles include the third, fourth, fifth, and sixth pairs. However, the fifth pair is significantly reduced in certain species like Siren, Amphiuma, and Necturus. Unlike in fishes, the aortic arches in amphibians with external gills are not divided into afferent and efferent portions. Instead, branches arising from the fourth, fifth, and sixth aortic arches form capillaries in the external gills.
• Between the third and fourth aortic arches, the lateral dorsal aortae persist as a vascular connection known as the ductus caroticus. The sixth aortic arch gives rise to the pulmo-cutaneous arch or artery on each side, which transports blood to both the lungs and the skin. It also retains a connection with the lateral dorsal aorta called the ductus arteriosus, also known as the duct of Botalli.
• In the larval stage of anurans, which include frogs and toads, the arrangement of aortic arches resembles that of adult urodeles due to the presence of gills. However, during metamorphosis, as the gills are lost, the first, second, and fifth aortic arches disappear completely. Only the third, fourth, and sixth aortic arches remain in the adult stage. The ductus caroticus, located between the third and fourth aortic arches, also disappears. As a result, the third aortic arch, along with a portion of the ventral aorta, becomes the carotid arch, responsible for carrying oxygenated blood to the head region.
• The fourth aortic arch, along with its lateral dorsal aorta, forms the systemic arch, which supplies oxygenated blood to the systemic circulation. The sixth aortic arch transforms into the pulmocutaneous arch, delivering venous blood to both the lungs and the skin. During metamorphosis, the ductus arteriosus disappears. Therefore, adult anurans retain only the third, fourth, and sixth aortic arches, a pattern also observed in amniotes, which include reptiles, birds, and mammals.
• The modifications in the aortic arches of amphibians reflect the evolutionary changes associated with the transition from aquatic gill respiration in larvae to terrestrial lung respiration in adults. These adaptations ensure efficient oxygenation of the blood and proper distribution throughout the organism during different stages of their life cycle.

Reptilia

• Reptiles, which include snakes, lizards, turtles, and crocodiles, have fully replaced gills with lungs for respiration. In terms of aortic arches, reptiles possess only the third, fourth, and sixth pairs. The ventricle in reptiles is partially separated into two parts, leading to the division of the distal portion of the conus arteriosus and the entire ventral aorta into three vessels: two aortic or systemic vessels and one pulmonary vessel.
• The right systemic arch (fourth arch) arises from the left ventricle and carries oxygenated blood to the carotid arch (third arch). The left systemic arch (fourth arch) and the pulmonary aorta (sixth arch) originate from the right ventricle. The left systemic arch transports deoxygenated or mixed blood to the body through the dorsal aorta, while the pulmonary artery carries deoxygenated blood to the lungs.
• In reptiles, the ductus caroticus, which previously existed as a vascular connection between the third and fourth aortic arches, disappears. However, in some reptiles like snakes and certain lizards (such as Uromastix), the ductus caroticus persists. The ductus arteriosus, on the other hand, disappears in most reptiles, although it may persist in a reduced form in species like Sphenodon (a tuatara species) and some turtles.
• It is worth noting that due to the mixing of blood in reptiles, they are classified as cold-blooded animals, similar to fishes and amphibians. This mixing occurs because oxygenated and deoxygenated blood partially mix in the systemic circulation, resulting in a less efficient separation of oxygenated and deoxygenated blood compared to warm-blooded animals like mammals and birds.

Aves

• In birds, the aortic arches follow a similar pattern to reptiles, with the presence of the third, fourth, and sixth arches. However, there are some notable differences. With the complete division of the ventricle into two parts, the conus arteriosus and ventral aorta also split to form two distinct vessels: the systemic aorta, which arises from the left ventricle, and the pulmonary aorta, which arises from the right ventricle.
• The third aortic arch, along with remnants of the lateral and ventral aortae, forms the carotid arteries. These carotid arteries arise from the systemic aorta. The fourth aortic arch gives rise to the systemic aorta on the right side only. It joins with the lateral aorta of its own side, forming the dorsal aorta. A portion of the fourth aortic arch on the left side forms the left subclavian artery, while the remaining part, along with its lateral dorsal aorta, disappears.
• The sixth aortic arch forms the pulmonary aorta, which carries deoxygenated blood to the lungs. Both the ductus caroticus and the ductus arteriosus, which were present in earlier stages, disappear in birds as they develop. These ducts were vascular connections that allowed blood to bypass certain areas during embryonic development but are no longer necessary after hatching.
• Overall, the aortic arches in birds demonstrate a specialized adaptation for efficient oxygenation and circulation in the avian respiratory and cardiovascular systems.

Mammalia

In mammals, similar to birds, the aortic arches III, IV, and VI persist. The ventricle is fully divided into two separate chambers.

The conus arteriosus and ventral aorta split into two distinct vessels:

(i) The systemic aorta arises from the left ventricle, carrying oxygenated blood to the body, and

(ii) The pulmonary aorta originates from the right ventricle, transporting deoxygenated blood to the lungs for oxygenation.

The third aortic arch, along with remnants of the lateral and ventral aortae, forms the carotid arch, supplying blood to the head region.

The fourth aortic arch gives rise to the systemic aorta on the left side only. On the right side, its proximal portion forms an innominate artery and right subclavian artery, while the remaining portion, along with its lateral dorsal aorta, regresses and disappears.

The sixth aortic arch forms the pulmonary aorta, responsible for carrying deoxygenated blood from the right ventricle to the lungs.

The ductus arteriosus, which served as a vascular connection between the pulmonary artery and the systemic circulation during fetal development, undergoes degeneration in mammals. However, it persists in some mammals until hatching or birth as a thin ligament known as the ligamentum arteriosum on the left side.

These adaptations in the aortic arches of mammals contribute to the efficient oxygenation and circulation necessary for their high metabolic demands and warm-blooded nature.

Venous System

Embryonic Veins

• Embryonic veins in vertebrates exhibit a similar pattern across different species. The venous system is primarily characterized by paired and symmetrically arranged veins. However, there are some variations among different embryos.
• In embryos without a yolk sac, a sub-intestinal vein develops in the splanchnic mesoderm beneath the gut. This vein forms a loop around the anus and extends posteriorly as the caudal vein, which continues into the tail.
• In embryos with a yolk sac, a pair of vitelline veins emerges from the yolk sac and connects to the posterior part of the developing heart, known as the sinus venosus. The fusion of these vitelline veins plays a crucial role in heart formation in bony fishes, reptiles, and birds. Each vitelline vein joins the sub-intestinal vein at its posterior end, following a similar pattern observed in embryos without a yolk sac.
• Between the kidneys, a pair of subcardinal veins arises and joins the caudal vein. Additionally, paired anterior and posterior cardinal veins develop, responsible for transporting blood from the head and posterior regions of the body, respectively. The anterior and posterior cardinal veins on each side unite to form a ductus Cuvieri or common cardinal vein, which passes through the transverse septum and enters the sinus venosus.
• In fishes and salamanders (urodeles), an inferior jugular vein emerges from the ventral side of the head and joins the common cardinal vein. This vein is unique to these species and does not have a homologue in other vertebrates. In amniotes, a pair of lateral or ventral abdominal veins originates from the body wall and enters the common cardinal veins.
• These embryonic veins serve as the foundation for the development of the circulatory system in vertebrates, ensuring proper blood flow and circulation during the early stages of development.

Pisces

• In Pisces, the class of fishes, the venous system displays certain characteristics. The common cardinal vein, also known as the duct of Cuvier, enters the sinus venosus from each side and is formed by the fusion of the anterior and posterior cardinal veins. The anterior cardinal veins collect blood from the head, while the posterior cardinal veins collect blood from the kidneys and gonads.
• The ducts of Cuvier also receive the paired lateral abdominal veins, which gather blood from the body wall and appendages. The renal portal system consists of the caudal vein and two renal portal veins located laterally to the kidneys. These veins form a network of capillaries within the kidneys.
• The hepatic portal system carries blood from the stomach and intestine, returning it to the liver. The blood passes through a series of sinusoids within the liver before entering the sinus venosus via paired hepatic veins.
• In teleosts, the lateral abdominal veins are absent. As a result, blood from the subclavians, which drain the pectoral appendages, enters the sinus venosus directly. Similarly, blood from the iliac veins, which drain the pelvic appendages, flows into the postcardinals.
• In dipnoans, a single ventral abdominal vein is present, likely formed through the fusion of the lateral abdominal veins. This vein receives blood from the iliacs via paired pelvic veins and enters the right duct of Cuvier. Additionally, a new vein called the postclaval vein emerges from the right postcardinal system. It plays a significant role in higher vertebrates. The postclaval vein connects with the caudal vein and passes through the liver to reach the sinus.
• In amniote embryos, the lateral abdominal veins are referred to as umbilical or allantoic veins because they drain the allantois. However, these veins are lost at birth, and only remnants of their structures persist in the adult anatomy.

Modifications of Veins

The venous system in different vertebrates follows a basic plan but shows modifications that are specific to each group. These modifications can be observed across the vertebrate series, with the development of the venous system in higher vertebrates resembling the embryonic stages of lower forms.

1. During liver development, the proximal part of the vitelline veins or subintestinal veins gives rise to hepatic veins located between the liver and the heart. The distal part of the left vitelline vein becomes the hepatic portal vein, which forms sinusoids within the liver, creating a hepatic portal system found in all vertebrates.
2. The anterior cardinal veins persist as internal jugular veins.
3. With the exception of fishes, the common cardinal veins transform into precaval veins that enter the sinus venosus in amphibians, reptiles, or the right auricle in birds and mammals. A subclavian vein is formed in each forelimb, which joins the precaval vein.
4. The caudal vein loses its connections with the subintestinal and subcardinal veins. In most vertebrates, its anterior part splits into two branches that join the posterior cardinal veins, except in mammals.
5. The posterior cardinal veins remain as they are in fishes, but in other vertebrates, each one breaks up into two portions. The anterior portion disappears in amphibians, reptiles, and birds. In mammals, the right anterior portion forms an azygos vein, and the left one may form a hemiazygos vein that connects to the azygos vein through a transverse anastomosis. The posterior portions of the posterior cardinal veins, which have joined the caudal vein, become the renal portal veins.
6. The renal portal veins form capillaries within the kidneys, creating a renal portal system. This system is well-developed in fishes, amphibians, and reptiles but is reduced in birds and absent in mammals due to the disappearance of the posterior portions of the posterior cardinal veins.
7. In fishes and tetrapods, the vitelline veins join the subcardinal vein to form a postcaval vein. In amphibians and reptiles, the postcaval vein extends to the posterior ends of the kidneys. In birds, it joins the renal portal veins, leading to a reduction in the renal portal system. In mammals, the postcaval vein connects with the veins from the legs and tail, completely eliminating the renal portal system.
8. The two lateral abdominal veins fuse in air-breathing vertebrates to form an anterior abdominal vein. This vein joins the hepatic portal vein near the liver, establishing a connection between the renal portal and hepatic portal systems. An iliac vein forms in each hindlimb, joining the anterior abdominal vein. The importance of the anterior abdominal vein diminishes in reptiles, and in birds, it transforms into the epigastric and coccygeo-mesenteric veins. In mammals, it disappears, except in Tachyglossus.
9. Pulmonary veins emerge from the left auricle and connect to the lungs in air-breathing species.
10. The umbilical (allantoic) veins found in the embryo disappear when the lungs become the functional respiratory organs.

Tetrapoda

The venous system in tetrapods, including amphibians, reptiles, birds, and mammals, exhibits specific modifications and variations unique to each group.

1. In amphibians, the venous system closely resembles that of lungfish, with the abdominal vein entering the hepatic portal system instead of the sinus venosus. The anterior cardinal veins persist as internal jugular veins in all tetrapods, and inferior jugular veins are absent. The common cardinal veins in amphibians and reptiles become the anterior vena cavae or precavals, which join the sinus venosus.
2. In birds and mammals, the precaval veins directly enter the right auricle of the heart since the sinus venosus is no longer present. In some species, such as humans and cats, the left precaval vein is lost, resulting in blood from the left side entering the right precaval vein through a brachiocephalic branch. Lungless salamanders lack pulmonary veins due to an incomplete interatrial septum.
3. In larval frogs and Necturus, the postcardinal veins join the caudal vein posteriorly and the precavals anteriorly, forming the common cardinal veins.
4. Reptiles exhibit a greater development of pulmonary veins and the postcaval vein, with a reduction in the significance of the renal portal system responsible for carrying blood from the posterior part of the body to the kidneys.
5. In birds, there are two functional precaval veins formed by the union of the jugular and subclavian veins on each side, along with a complete postcaval vein. The blood from the limbs is received through the renal portals, which pass through the kidneys without breaking into capillaries, differing from the renal portals of lower vertebrates.
6. In mammals, there may be one or two precaval veins. The postcaval vein, which has complex embryological development, is a single vein. The caval veins directly enter the right auricle, as the sinus venosus is absorbed into the wall of this heart chamber during embryonic life. Mammals lack a renal portal system, although their hepatic portal system closely resembles that of other vertebrates.

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