The basic vertebrate cardiovascular system includes a heart that contracts to propel blood out to the body through arteries, and a series of blood vessels. The blood enters the heart through the upper chamber(s), the atrium (or atria). Passing through a valve, blood enters the lower chamber(s), the ventricle(s). Contraction of the ventricle forces blood from the heart through the arteries.
Fish have a two-chambered heart in which a single-loop circulatory system takes blood from the heart to the gills and then to the body. Amphibians have a three-chambered heart with two atria and one ventricle. A loop from the heart goes to the pulmonary capillary beds, where gas exchange occurs. Blood then is returned to the heart. Blood exiting the ventricle is diverted, some to the pulmonary circuit, some to systemic circuit. The disadvantage of the three-chambered heart is the mixing of oxygenated and deoxygenated blood. Some reptiles have partial separation of the ventricle. Crocodilian reptiles, birds and mammals (including humans), have a four-chambered heart, with complete separation of both systemic and pulmonary circuits.
Below, we describe the main characteristics of different groups of vertebrates:
1. Mammalian (and human) heart
2. Avian heart
3. Crocodilian heart
4. Reptilian heart
5. Amphibian heart
6. Fish heart
1. Mammalian (and human) heart:
Mammals have a double circulation; the pulmonary circulation carries blood to and from the lungs and the systemic circulation carries blood to and from the rest of the body. The heart thus acts as a double pump in which the two sides are completely separated by a wall (septum). The right side pumps deoxygenated blood to the lungs for gas exchange with the alveolar air, while the left side pumps oxygenated blood to the body for gas exchange with the tissues. The heart has four chambers, the right and left atria and the right and left ventricles. The atria pass blood to the ventricles. The right atrium receives deoxygenated blood from the body via the anterior and posterior venae cavae. This blood is pumped by the right ventricle to the lungs via the pulmonary arch and pulmonary arteries. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. This blood is pumped by the left ventricle to the body via the aortic arch and aorta. Human cardiac anatomy is more extensively discussed here.
2. Avian heart:
Birds, like mammals, have a four-chambered heart. Two atria and two ventricles allow for complete separation of oxygenated and de-oxygenated blood. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body. Because the left ventricle must generate greater pressure to pump blood throughout the body (in contrast to the right ventricle that pumps blood to the lungs), the walls of the left ventricle are much thicker and more muscular. Birds tend to have larger hearts than mammals (relative to body size and mass). The relatively large hearts of birds is probably required to meet the high metabolic demands of flight. Among birds, smaller birds have relatively larger hearts than larger birds. Hummingbirds have the largest hearts (relative to body mass) of all birds, probably because hovering takes so much energy. Cardiac output for birds is typically greater than that for mammals of the same body mass. Cardiac output is influenced by both heart rate and stroke volume (blood pumped with each beat). 'Active' birds increase cardiac output primarily by increasing heart rate.
3. Crocodilian heart:
Crocodilians are the only reptiles which possess four chambered hearts comparable to mammals. Even so, crocodilian cardiac anatomy is quite different from what is seen in birds and mammals. Crocodilians possess two aortas; the right arising from the left ventricle and the left from the right ventricle. Both aortas route blood to the systemic circulation. The right and left aortas are connected near the base of the heart by the foramen of Panizza. The foramen allows blood from the right ventricle to bypass the pulmonary circulation when necessary. A valve exists at the opening of the pulmonary artery which has interdigitating muscular projections, hence the commonly used name "cog-wheel valve". When the animal holds its breath, the cog-wheel valve closes and blood that would have normally entered the pulmonary circulation is diverted into the left aorta. It should be noted that most veterinary texts incorrectly report that the location of the foramen of Panizza is in the ventricular septum or atrial septum.
4. Reptilian heart:
The cardiac structure of non-crocodilian reptiles is significantly different from that of mammals. Most reptiles have three chambered hearts with two atria and one common ventricle. The right atrium receives blood returning from the systemic circulation via the sinus venosus, which is formed by the confluence of the right and left precaval veins and the single postcaval vein. The walls of the sinus venosus contain cardiac muscle and the pacemaker of the heart. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. The atrioventricular valves are bicuspid, membranous structures. Under normal conditions the three chambered heart functions much like a four chambered structure, therefore relatively little mixing of oxygenated and de-oxygenated blood occurs. Three cavities exist within the ventricle and can be functionally separate; the cavum venosum, cavum arteriosum and the cavum pulmonale. These cavities are partially separated by two muscular ridges found within the ventricle. These ridges vary in prominence in different species, but are generally well-developed in chelonians (turtles). The muscular ridge divides the cavum pulmonale and the cavum venosum. The vertical ridge divides the cavum venosum and cavum arteriosum. The cavum pulmonale receives blood from the right atrium through the cavum venosum and directs flow into the pulmonary circulation. The cavum arteriosum receives blood from the pulmonary veins and then directs oxygenated blood to the cavum venosum. The paired aortic arches arise from the cavum venosum and lead to the systemic circulation. The right and left aortic arches come together to form a single aorta at variable distances caudal to the heart. Differential blood flow and separation of oxygenated and de-oxygenated blood is maintained by pressure differences of the outflow tracts and the muscular ridges that partially divide the ventricle. In most non-crocodilian reptiles the ventricle function as a single pump, meaning that the same pressures are generated by both the cavum pulmonale and cavum venosum. Due the unique anatomy, both right to left and left to right shunts are possible in the reptilian heart for example during apnea.
The purpose of right to left shunting in reptiles is still under debate and it is proposed to be useful for the conservation of cardiac energy, facilitation of warming, reduction of plasma filtration into the lungs, reduction of carbon dioxide flux into the lungs and the metering of oxygen stores from the lungs during apnea. Theories to explain the purpose of left to right shunting include facilitation of carbon dioxide elimination from the lungs, minimization of ventilation/perfusion mismatches and improvement of systemic oxygen transport. In times of oxygen deprivation (diving in some reptiles, consumption of large prey in snakes), reptiles can shunt blood away from the lungs.
5. Amphibian heart:
Amphibians have a three-chambered heart consisting on two atria and one ventricle. The two atria receive blood from the two different circuits (the lungs and the systemic circulation). Mixing of the blood in the heart's ventricle, reduces the efficiency of oxygenation. This is partially mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit where gas exchange occurs.
Blood leaves the heart from the ventricle through a single truncus arteriosus which is short and soon branches into two aortic arches which loop left and right and dorsal to the heart to rejoin as a single aorta in the mid dorsal region of the body cavity. Each aortic arch has a branch leading to the lungs and skin where oxygenation occurs. Carotid arteries also branch off the aortic arches and supply the head region. Veins bring blood to the left and right atria. Both atria then empty into the single ventricle.
The pacemaker is the sinus venosus, an enlarged region between the vena cava and the right atrium. This the cells of the pacemaker are termed “leaky”, meaning that calcium ions leak into the cells. Leaking of positive ions causes a slow depolarization to threshold, thus initiating an action potential that quickly spreads throughout the muscle. The atria are very conductive, and the action potential spreads readily across these two chambers. The major route for the transmission of action potentials from the SA node to the ventricle(s) is by way of a set of modified conductive muscle cells that compose the bundle of His embedded in the septum separating the two atria.
6. Fish heart:
The circulatory systems of all vertebrates, are closed. Fish have the simplest circulatory system, consisting of only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation.
Fish have a two-chambered heart consisting of one atrium to receive blood and one ventricle to eject it. Entry and exit compartments are often referred as accessory chambers. The four compartments are arranged sequentially. The sinus venosus (first accessory chamber), collects deoxygenated blood through the incoming hepatic and cardinal veins. The atrium, a thicker-walled, muscular chamber sends blood to the ventricle. The ventricle, a thick-walled, muscular chamber that pumps the blood to the fourth part, the outflow tract (second accessory chamber). The outflow tract connects to the ventral aorta, and consists of the tubular conus arteriosus, bulbus arteriosus, or both. The conus arteriosus (in more primitive species), contracts to assist blood flow to the aorta. The bulbus anteriosus does not contract. The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the dorsal aorta, into the rest of the body.
Ostial valves, consisting of flap-like connective tissues, prevent blood from flowing backward through the compartments. The ostial valve between the sinus venosus and atrium is called the sino-atrial valve, which closes during ventricular contraction. Between the atrium and ventricle is an ostial valve called the atrio-ventricular valve, and between the bulbus arteriosus and ventricle is an ostial valve called the bulbo-ventricular valve. The conus arteriosus has a variable number of semilunar valves. In the adult fish, the four compartments are not arranged in a straight row but, instead form an S-shape with the latter two compartments lying above the former two. This relatively simpler pattern is found in cartilaginous fish and in the ray-finned fish. In teleosts, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper.
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