The Cardiovascular System: Differences in Anatomy and Physiology Between Humans and Frogs

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In evolutionary history, the amphibian heart represents and intermediate step between that of a fish, which is two-chambered (one atrium and one ventricle) and the hearts of reptiles, birds, and mammals.

The upper chambers of the heart are called atria (singular atrium), while the lower chambers are called ventricles. Between arteries and capillaries and between capillaries and veins are intermediate categories of blood vessels, known as arterioles and venules, respectively. Each group of capillaries, known as a capillary bed is fed by an arteriole, which can open or close, depending on the needs of the tissue. Only approximately 22% of carbon dioxide (CO2) is carried by hemoglobin and other blood proteins. 68% of CO2 is carried as bicarbonate (HCO3-) ion, while another 10% is dissolved physically in plasma.

Dissolved CO2 is in equilibrium with HCO3- by way of the enzyme carbonic anhydrase, which is produced in red blood cells. Production of HCO3- from CO2 also releases protons. Consequently, a large increase in CO2 concentration causes an overall reduction on the pH of blood and body tissues, known as acidosis, while an unusually high increase in removal results in the opposite condition, alkalosis. Molecular oxygen (O2) is much less soluble in blood compared to CO2, with only a tiny percentage of being dissolved physically rather than being carried by red blood cells.

However, under high pressure (hyperbaric) conditions, much more O2 can be made to dissolve. This is part of the basis for hyperbaric O2 therapy for certain conditions. Indeed, there have been experiments in which animals lacking red blood cells have been kept alive for short periods of time using only dissolved O2.

You may recall dissecting a frog during biology class, back in high school or college. Many interesting differences are notable between the cardiovascular systems of humans and frogs. While in humans all gas exchange takes place in the lungs, the frog exchanges O2 and CO2 through the lungs as well as the skin. If skin is moist, the amount of oxygen that can enter into the frog’s blood this way is substantial. Thus, a frog can receive O2 from, and release CO2 into, the water in which it is swimming.


A frog’s heart has only three chambers, consisting of two atria and one ventricle. The single ventricle receives both deoxygenated blood from the body tissues via the right atrium and oxygenated blood from the lungs and skin via the left atrium. While there is some mixing of blood in the ventricle, the mixing is not extensive. Thus, blood pumped to body tissues is mostly oxygenated, while blood pumped to lungs and skin is mostly deoxygenated.

A frog is an amphibian. In evolutionary history, the amphibian heart represents and intermediate step between that of a fish, which is two-chambered (one atrium and one ventricle) and the hearts of reptiles, birds, and mammals. Reptile hearts are three-chambered, but unlike those of amphibians the ventricle partly divided. In birds and mammals, the division of the ventricle is complete, so that there are four-chambers.

During development, the changes in embryonic and fetal circulation recapitulate evolutionary history, so that there is mixing between blood on the left and right side of the heart, including through a gap between the left and right ventricles of the fetal heart. When a gap persists between the ventricles, this is known as a ventricular septal defect, while the hole between the atria is called the foramen ovale, which normally it closes at birth. In some cases, however, the openings between the left and right chambers do not close completely, leading to a condition known as a left-to-right shunt, which can require surgical repair.


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