Comparative Physiology: Cardiovascular Systems

Understand the primary functions of the vertebrate circulatory system

The circulatory system helps deliver all kinds of things all over the body. These include nutrients, wastes, immune system cells, heat, other regulatory molecules and so on. The circulatory system is much more efficient than using simple diffusion alone- if we relied on simple diffusion, it would take roughly 9.26 minutes for something like oxygen just to get through our skin! Imagine how long it would take then for something to diffuse to the middle of our bodies!

Compare the anatomy & efficiency of open & closed circulatory systems

Many invertebrates have an open circulatory system, in which there is no separation between blood and the interstitial fluid. They have a heart to pump things around, but the blood vessels quickly end in massive sinuses. Due to the lack of separation between blood and interstitial fluid, the circulating fluid in these invertebrates is known as "haemolymph." Invertebrates with an open circulatory system still need a certain amount of pressure in order to keep the fluid moving (despite also having a heart), and thus many of these creatures have a tough exoskeleton.

All vertebrates, as well as some invertebrates, have a closed circulatory system in which blood is combined to the vessels. However, solutes can diffuse into the interstitial fluid. This system requires more energy, as the vessels offer up quite a bit of resistance. On the other hand, it is much faster than an open circulatory system and has other advantages, such as conveying blood directly to the organs and its ability to "shunt" blood to different organs via constriction of vessels etc.

Understand the evolutionary changes in the vertebrate heart from primitive fish to amphibians, reptiles, birds & mammals in terms of structure & overall circulatory efficiency

Flatworm

The flatworm is probably the simplest in terms of getting its nutrients: it can be over a metre long, but is very flat (hence its name). Its "flatness" means that it can get everything it needs via simple diffusion alone.

Hagfish

The hagfish is a primitive eel-like fish. Its circulatory system is considered to be partially open: while it has closed blood vessels, it also has several sinuses. It has five hearts: a main heart that pumps to the gills and four accessory hearts. This is probably necessary because the gills offer up a lot of resistance which slows down the blood considerably. The four accessory hearts (but not the main heart) are under direct neural control and can increase their output under sympathetic stimulation. When their output increases, venous return to the main heart increases, increasing the cardiac output of the main heart.

Elasmobranchs and teleosts

Elasmobranchs and teleosts are kinds of fish. They have a single heart with four chambers, but the four chambers are in a row (kind of like the foetal human heart before it folds and divides), so it is considered to be 2-chambered. These four chambers are called the sinus venosus, atrium, ventricle and conus arteriosus (a.k.a. "bulbus cordis").

Lungfish

Lungfish are, well, fish with lungs. As opposed to hagfish, lungfish only have one heart. The atrium and ventricle of this heart are partly divided, but not entirely (i.e. the lungfish heart still only has two chambers). This heart pumps into five "branchial arteries," three of which pass through gills and two which do not. From here, blood can either go to the lungs or to the rest of the body. Two veins return blood to the heart: one from the lung and one from the body. There is some mixing of the oxygenated blood from the lungs and deoxygenated blood from the body, but this is relatively limited as there are spiral folds in the bulbus cordis that separate the blood.

Amphibians

Amphibians have a 3-chambered heart (two atria and one ventricle) as opposed to the two-chambered hearts of fish. Even though the ventricle is not fully divided, it has a functional division called the "dense trabeculation of the spongy myocardium" and a spiral fold in the conus arteriosus (just like the spiral folds in the bulbus cordis of lungfish).

The artery that leaves the amphibian heart splits into two: a pulmocutaneous artery that goes to the lungs and skin (which is also an accessory breathing organ in amphibians) and a systemic artery that goes to the rest of the body. The left atrium receives oxygenated blood from the lungs, whereas the right atrium receives a mixture of oxygenated blood from the skin and deoxygenated blood from the tissues (so there is some mixing of blood).

Reptiles (except for crocodiles because they're special)

Unlike amphibians, reptiles only use their lungs for gas exchange, so their circulatory system must be simpler too, right? Wrong.

The atria of reptiles are divided completely, so that part's easy. The ventricle, however? Not so easy. The ventricle of reptile hearts is divided incompletely into three chambers: the cavum venosum (CV), cavum arteriosum (CA) and cavum pulmonale (CP). These connect into two systemic arteries (the right and left systemic arch) and the pulmonary artery.

During ventricular systole, the pressure in the pulnonary artery is relatively low, allowing blood to flow in from the CP and CV. (The intercaval canal- the passage between the CA and CV- is still closed at this point.) As the pressure increases, the intercaval canal opens, allowing blood from the CA to enter the CV, while a muscular ridge forms between the CV and CP (preventing blood from flowing between these two sections). Blood from the CV then enters the systemic arches.

Well, that was confusing. Why are reptiles so confusing? Well, this confusing system allows reptiles to bypass the lungs, which is useful when diving. During diving, PA resistance increases, causing a right-to-left cardiac shunt (i.e. deoxygenated blood gets to flow to the rest of the body too). When they're back on land, they have a left-to-right shunt.

Crocodiles

Crocodiles are different to other reptiles in that they have a proper four-chambered heart. Just like in humans, the right atria receives deoxygenated blood from the body, whereas the left atria receives oxygenated blood from the lungs. Output from the ventricles is a bit more complicated: the right ventricle is connected to a pulmonary artery and to the left systemic arch, whereas the left ventricle is only connected to the right systemic arch. The two systemic arteries are connected by the foramen of Panizza. Usually, there is a higher pressure in the left systemic arch, which prevents the valve from the right ventricle to the left systemic arch from opening. However, when crocodiles are diving, pulmonary vasculature resistance increases, closing off the valve from the right ventricle to the pulmonary artery, which shunts blood from the right ventricle into the left systemic arch instead. This results in a right-to-left shunt. (Note that crocodiles can't have a left-to-right shunt as the valve from the left ventricle to the right systemic arch is pretty much always open.)

Birds

Birds also have a four-chambered heart. Oxygenated and deoxygenated blood are completely separated (unlike in crocodiles when they can mix during diving).

Mammals

I've already spoken a lot about humans, so not much to say here. Fun fact though: the sinus venosus, which I mentioned back when I wrote about elasmobranchs and teleosts, still remains in the mammalian heart: it's the sinoatrial node.