Circulatory System in Humans
Senior 3 · Biology
Circulatory System in Humans
Lesson 1 of 5
25 min
Cut open a tilapia from Lake Kivu and its heart has two chambers. A frog's heart has three. Yours, like a cow's or a bird's, has four. An insect's heart has no chambers at all: it is a tube along its back, and its blood does not even stay in blood vessels. Most animals larger than a few millimetres need a transport system, because diffusion alone is far too slow to supply cells deep inside a large body. But animals have solved the problem in different ways, and the differences explain why you can run, keep warm and think as fast as you do.
Definition
Circulatory system: a system of a pump (the heart), a fluid (blood) and, in most animals, tubes (vessels) that carries substances round the body.
Open circulation: blood is pumped out of the heart into a body cavity, where it bathes the organs directly, before returning to the heart. Insects and snails have open circulation.
Closed circulation: blood stays inside vessels all the time; substances pass in and out through the walls of tiny vessels called capillaries. Earthworms and all vertebrates have closed circulation.
Single circulation: blood passes through the heart once in each complete circuit of the body (fish). Double circulation: blood passes through the heart twice in each circuit, once for the lungs and once for the rest of the body (birds and mammals).
Figure 1: Open and closed circulation.
In open circulation the blood moves slowly and at low pressure, which is enough for an insect: its oxygen does not travel in the blood at all, but through air tubes that reach every cell. A closed system can push blood fast and at high pressure, and can send more blood to the organs that need it most, such as working muscles.
Figure 2: Single circulation in a fish and double circulation in a mammal. In the mammal, each side of the heart has an upper chamber, the atrium, and a lower one, the ventricle.
In a fish, blood loses much of its pressure squeezing through the narrow capillaries of the gills, so it flows on to the body slowly. In a mammal, blood that has picked up oxygen in the lungs returns to the heart and is pumped a second time, at high pressure, to the body. The two sides of the heart are completely separated, so oxygenated and deoxygenated blood never mix.
| Feature | Single circulation (fish) | Double circulation (mammal) |
|---|---|---|
| Times through the heart per circuit | Once | Twice |
| Chambers in the heart | Two (one atrium, one ventricle) | Four (two atria, two ventricles) |
| Pressure of blood reaching the body | Low, after the gills | High, straight from the heart |
| Supports | A cold-blooded animal supported by water | A warm-blooded, active animal with a high demand for oxygen |
The pulmonary circulation takes blood from the right side of the heart to the lungs and back to the left side. The systemic circulation takes blood from the left side to every other organ and back to the right side. Notice two exceptions to "arteries carry oxygenated blood": the pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood. An artery is defined by carrying blood away from the heart, not by its oxygen.
Figure 3: Plan of the human double circulation (not to scale). Orange: oxygenated blood; blue: deoxygenated blood.
Worked example 1: tracing a red blood cell
A red blood cell is in a capillary in the left leg. List, in order, the vessels and heart chambers it passes through until it returns to the leg, by the shortest route.
Worked example 2: two pressures
In a resting adult, the highest pressure in the pulmonary artery is about 25 mmHg, and in the aorta about 120 mmHg (1 mmHg = 0.133 kPa). (a) Convert both to kPa. (b) How many times higher is the pressure in the aorta? (c) Suggest why the lungs receive blood at a much lower pressure.
Warning
Common mistakes. (1) "Arteries carry oxygenated blood, veins carry deoxygenated blood": the pulmonary artery and pulmonary vein are the exceptions; arteries carry blood away from the heart, veins carry it towards it. (2) Saying a fish's blood goes through its heart twice: it goes once, through the gills and then the body. (3) Drawing deoxygenated blood as truly blue: it is dark red; blue is only a colour code in diagrams.
Animal Circulatory Systems: Single vs. Double Circulatory, LearnStalk. Watch the path of blood in a fish and in a mammal, and count how many times it passes through the heart in each.
Tip
Memory hook: A for Away. Arteries carry blood Away from the heart; veins enter it. Right side of the heart: deoxygenated, to the lungs. Left side: oxygenated, to the body.
Practice: transport systems
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 to 3, the table, the worked examples (typical textbook pressures) and the practice questions.
