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The circulatory system of a shark compises a heart, capillary beds and veins, the same way like humans have. But in distinction form a human heart a shart has a two-chambered heart, there respiratory gases are exchanged (oxygen and carbon dioxide). In this process also gill take part while humans have lungs.

All the data is based on the course of «Sharks! Global Biodiversity, Biology, and Conservation» by Cornell University and the University of Queensland.

Let us take a 500-pound bull shark heart, its weight is about 220 kg! But if to compare the proportions of an animal and a human, its heart is only half the size of a human’s organ. How that is possible? It is evident that the heart pumps blood through a closed vessel system with two aims: 1) to distribute molecules from one part to another one (like hormones secreted by glands or nutrients); 2) to exchange respiratory gases. The first function is the same for both humans and sharks, but the second one possesses differences. The thing is that gills of sharks can function just in water whereas human’s lungs function only in the air.
The loading on the shark's heart is less than on a human's one because of its simplier structure. Sharks have only one atrium and one ventricle, But human's heart has two atria and two ventricles.

BREATHING IN THE WATER

The first important peculiarity is that water takes much energy to pump and, of course, there are some challenges to make this pumping ready for respiration. Actually there are two ways of pumping water:

1. Swimming directly into the water so that the flow goes inside.
2. Using pumping mechanism with muscles in head.
   The truth is that mostly sharks rely on the second method.

Screenshot from the course

For successful gas exchange gills have special structure: there are two layes of them. The first one is red and is easily visible, but the second one cannot be recognized without a microscope. The blood and water must be located very close to each other to perform an exchange, at the same time they must be separated to minimize the diffusion distance. This separation is done by a thin membrane.

As there is no active meachanism of oxygen transport from the water, gas molecules diffuse from places of high concentration to areas of lower one. The point is that water and blood in the gills capillaries will run in opposite directions, because this is the most efficient way to get maximum oxygen. In general, it looks like this: the air is sucked into the gills, gas exchange occurs and the air is pushed out.

BALANCE OF SALTS AND WATER

Sure, it is evident that sharks must have some mechanism to regulate salt and water in their bodies. For humans these are kidneys that accomplish this function.

But what about sharks?
Sharks have internal surrounding a bit saltier than the outer one. Moreover, there is a special salt-concentrating gland (rectal gland). It transports sodium chloride across the membranes for going out.

Screenshot from the course

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