Double circulation

Where it happens

Double circulation takes place in the heart and blood vessels, where the heart is divided into a right side and a left side that pump blood through two separate circuits: the pulmonary circulation and the systemic circulation.

The heart is the meeting point of the two circuits. The right atrium and right ventricle form the pulmonary pump, receiving blood from the vena cava and sending it out through the pulmonary artery.

The left atrium and left ventricle form the systemic pump, receiving blood from the pulmonary veins and sending it out through the aorta. A muscular wall, the septum, runs down the middle so oxygenated and deoxygenated blood never mix.

The left ventricle wall is much thicker than the right because it must push blood around the whole body, whereas the right ventricle only pushes blood the short distance to the lungs. Double circulation is a feature of mammals and birds; fish have a single circulation in which blood passes through the two-chambered heart only once per circuit.

Inputs and outputs

• Input to the pulmonary circulation: deoxygenated blood, rich in carbon dioxide, arriving in the right atrium from the superior and inferior vena cava.
• Input to the systemic circulation: oxygenated blood arriving in the left atrium from the four pulmonary veins.
• Input: rhythmic contraction of cardiac muscle, triggered by the pacemaker in the wall of the right atrium, which supplies the pressure for both circuits.
• Output of the pulmonary circulation: blood that has gained oxygen and lost carbon dioxide at the alveoli, at low pressure.
• Output of the systemic circulation: blood that has delivered oxygen and glucose to respiring tissues and collected carbon dioxide and other wastes.
• Output: a continuous supply of oxygen to every organ at a pressure high enough to drive tissue fluid out of capillaries and support a high metabolic rate.

The steps

1. Deoxygenated blood from the body enters the right atrium through the superior and inferior vena cava.
2. The right atrium contracts and pushes blood through the tricuspid valve into the right ventricle.
3. The right ventricle contracts and pumps blood through the semilunar valve into the pulmonary artery, which carries it to the lungs, the start of the pulmonary circulation.
4. At the alveoli, carbon dioxide diffuses out of the blood and oxygen diffuses in; the blood is now oxygenated but its pressure has dropped after passing through the narrow lung capillaries.
5. Oxygenated blood returns through the pulmonary veins to the left atrium, which contracts and pushes it through the bicuspid valve into the left ventricle.
6. The thick-walled left ventricle contracts and pumps blood at high pressure through the semilunar valve into the aorta, the start of the systemic circulation.
7. Arteries branch into arterioles and capillaries in every organ, where oxygen and nutrients diffuse into cells and carbon dioxide diffuses into the blood.
8. Venules and veins collect the deoxygenated blood and return it through the vena cava to the right atrium, completing one full circuit in which blood has passed through the heart twice.

Why it matters and how it is controlled

Passing through the heart twice lets blood be re-pressurised after it loses pressure crossing the narrow capillaries of the lungs. This means oxygenated blood reaches body tissues at high pressure, so it flows faster and supplies oxygen more efficiently than a single circulation would.

Keeping the two circuits separate has a second advantage: the pulmonary circuit can run at low pressure, which protects the delicate, thin-walled alveolar capillaries from bursting and gives blood enough time in the lungs to load up with oxygen, while the systemic circuit runs at high pressure to reach distant organs. Because oxygenated and deoxygenated blood never mix, the blood leaving the left side of the heart carries the maximum possible oxygen, which supports the high metabolic rate needed to maintain a constant body temperature in mammals and birds.

The rate of both circuits is controlled together, because the same heart drives them. The pacemaker in the right atrium sets the resting heart rate.

During exercise, the medulla oblongata sends nerve impulses along the sympathetic nerve and the adrenal glands release adrenaline, so the heart beats faster and more forcefully; the volume pumped by each side of the heart per beat stays matched, otherwise blood would pool in one circuit. Valves prevent backflow so that each contraction moves blood forward, and the elastic walls of the aorta smooth out the pressure surges from the left ventricle.

Pulmonary and systemic circulation compared

How it is examined

You may be asked to trace the path of blood through the pulmonary and systemic circulations, to label a diagram of double circulation, or to explain the advantage of double circulation compared with a single circulation where blood passes through the heart only once.

Structured questions typically show an outline diagram of the heart, lungs and body with unlabelled vessels, and ask you to name the vessels, shade the oxygenated side, or draw arrows showing the direction of flow. A frequent explanation question compares the left and right ventricle walls, and the mark scheme expects the link between wall thickness, pressure generated and the distance blood must travel.

Essay questions may ask you to compare double circulation with the single circulation of a fish, and answers gain credit for naming the number of times blood passes through the heart, the pressure reached, and the consequence for oxygen delivery and metabolic rate.

Common misconceptions

Worked exam-style question

Question. Figure 1 is a simplified diagram of double circulation in a human. Vessel P leaves the right ventricle, vessel Q enters the left atrium, and vessel R leaves the left ventricle.

(a) Name vessels P, Q and R. (b) State which of these vessels carries deoxygenated blood.

(c) Explain why the wall of the left ventricle is thicker than the wall of the right ventricle. (d) A fish has a single circulation.

Explain one advantage that double circulation gives a mammal compared with a fish.

Model answer. (a) P: pulmonary artery; Q: pulmonary vein; R: aorta. (b) P, the pulmonary artery, it carries blood from the right ventricle to the lungs before gas exchange.

(c) The left ventricle pumps blood to all parts of the body through the systemic circulation, so it must generate a higher pressure to push blood over a longer distance; a thicker muscular wall contracts more forcefully. The right ventricle pumps blood only the short distance to the lungs at low pressure, which also protects the thin alveolar capillaries.

(d) In a mammal, blood is pumped a second time after leaving the lungs, so oxygenated blood reaches the tissues at high pressure and flows faster; in a fish, blood loses pressure at the gill capillaries and then travels slowly to the body. Faster delivery of oxygen supports a higher metabolic rate.

Keeping oxygenated and deoxygenated blood completely separate so that the tissues receive fully oxygenated blood is also accepted.

Source:SRC-DSKP-EN

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