Blood Passes Through the Heart Twice on Every Complete Round
Learn the four chambers of the heart and where each valve sits, trace the whole path of blood through both circuits, understand heartbeat, pulse and the heart's own pacemaker, and see what tissue fluid and lymph do.
Why does blood go through the heart twice instead of once?
So that oxygenated and deoxygenated blood never mix, and so that the blood sent round the body leaves at full pressure.
Blood returning from the body is deoxygenated. It enters the right side of the heart, is pumped to the lungs to pick up oxygen, and comes straight back to the heart — this time to the left side. Only then is it pumped out to the rest of the body.
So one complete circuit means two passes through the heart, and that is what double circulation means. The alternative — one pass — would mix the two kinds of blood and deliver a half-oxygenated supply at falling pressure. This page covers the second part of the ICSE Class 8 Biology chapter on the circulatory system.
Blood returning from the body is deoxygenated. It enters the right side of the heart, is pumped to the lungs to pick up oxygen, and comes straight back to the heart — this time to the left side. Only then is it pumped out to the rest of the body.
So one complete circuit means two passes through the heart, and that is what double circulation means. The alternative — one pass — would mix the two kinds of blood and deliver a half-oxygenated supply at falling pressure. This page covers the second part of the ICSE Class 8 Biology chapter on the circulatory system.
What is the structure of the human heart?
A muscular pump lying in the chest cavity between the lungs, tilted slightly to the left, enclosed in a protective double membrane called the pericardium. The fluid between its layers lets the beating heart move without friction.
Four chambers, in two pairs:
- Right atrium and left atrium — the two upper, thinner-walled chambers that receive blood
- Right ventricle and left ventricle — the two lower, thicker-walled chambers that pump blood out
A wall called the septum separates the right side from the left, so that the two kinds of blood cannot mix.
Why the walls differ in thickness. Atria only push blood down into the ventricles just below, so their walls are thin. Ventricles must push blood out of the heart, so their walls are thick. And the left ventricle has the thickest wall of all, because it pumps blood to the whole body, while the right ventricle pumps only as far as the lungs.
Four valves, all with the single job of preventing backflow so that blood moves one way only:
- Tricuspid valve — between the right atrium and right ventricle
- Bicuspid valve, also called the mitral valve — between the left atrium and left ventricle
- Semilunar valves — one at the base of the pulmonary artery and one at the base of the aorta
Remembering which is which: tri for three flaps on the right, bi for two flaps on the left.
Four great vessels attached to it:
- Superior and inferior vena cava bring deoxygenated blood from the body into the right atrium
- The pulmonary artery leaves the right ventricle for the lungs
- The pulmonary veins bring oxygenated blood from the lungs into the left atrium
- The aorta leaves the left ventricle for the whole body
The pattern that makes this easy to recall. Veins enter atria; arteries leave ventricles. Every one of the four fits that rule, which also explains why the pulmonary artery and pulmonary vein carry the opposite blood to what their names suggest — an artery leaves the heart whatever it carries.
Four chambers, in two pairs:
- Right atrium and left atrium — the two upper, thinner-walled chambers that receive blood
- Right ventricle and left ventricle — the two lower, thicker-walled chambers that pump blood out
A wall called the septum separates the right side from the left, so that the two kinds of blood cannot mix.
Why the walls differ in thickness. Atria only push blood down into the ventricles just below, so their walls are thin. Ventricles must push blood out of the heart, so their walls are thick. And the left ventricle has the thickest wall of all, because it pumps blood to the whole body, while the right ventricle pumps only as far as the lungs.
Four valves, all with the single job of preventing backflow so that blood moves one way only:
- Tricuspid valve — between the right atrium and right ventricle
- Bicuspid valve, also called the mitral valve — between the left atrium and left ventricle
- Semilunar valves — one at the base of the pulmonary artery and one at the base of the aorta
Remembering which is which: tri for three flaps on the right, bi for two flaps on the left.
Four great vessels attached to it:
- Superior and inferior vena cava bring deoxygenated blood from the body into the right atrium
- The pulmonary artery leaves the right ventricle for the lungs
- The pulmonary veins bring oxygenated blood from the lungs into the left atrium
- The aorta leaves the left ventricle for the whole body
The pattern that makes this easy to recall. Veins enter atria; arteries leave ventricles. Every one of the four fits that rule, which also explains why the pulmonary artery and pulmonary vein carry the opposite blood to what their names suggest — an artery leaves the heart whatever it carries.
What path does blood take through the heart?
Follow one drop of blood returning from the toes all the way round.
The pulmonary circuit — the short one, picking up oxygen:
- Deoxygenated blood from the body enters the right atrium through the vena cava
- The right atrium contracts, pushing it through the tricuspid valve
- Into the right ventricle
- The right ventricle contracts, pushing it through a semilunar valve
- Into the pulmonary artery, which carries it to the lungs
- In the lungs it releases carbon dioxide and takes up oxygen
The systemic circuit — the long one, delivering oxygen:
- Oxygenated blood returns through the pulmonary veins
- Into the left atrium
- The left atrium contracts, pushing it through the bicuspid valve
- Into the left ventricle
- The left ventricle contracts powerfully, pushing it through a semilunar valve
- Into the aorta, which branches to carry it to every part of the body
- In the tissues it releases oxygen and picks up carbon dioxide, becoming deoxygenated again
- It returns through the veins to the vena cava, and the circuit begins again
The two circuits named:
- Pulmonary circulation — heart to lungs to heart. Short, and at lower pressure, which suits the delicate lung capillaries.
- Systemic circulation — heart to the body to heart. Long, and at high pressure, which is why the left ventricle is so muscular.
The advantage of keeping them separate, which is the answer to almost every question on this topic:
- Oxygenated and deoxygenated blood never mix, so tissues receive fully oxygenated blood
- Blood leaves for the body at full pressure, having been re-pumped after the lungs
- Oxygen is therefore delivered rapidly and efficiently, which is what a warm-blooded animal with high energy needs requires
Notice that both sides of the heart work together, not alternately. The two atria contract at the same moment, and then the two ventricles contract at the same moment. So the two circuits run simultaneously — the heart is not one pump used twice but two pumps side by side, sharing a wall and a rhythm.
The pulmonary circuit — the short one, picking up oxygen:
- Deoxygenated blood from the body enters the right atrium through the vena cava
- The right atrium contracts, pushing it through the tricuspid valve
- Into the right ventricle
- The right ventricle contracts, pushing it through a semilunar valve
- Into the pulmonary artery, which carries it to the lungs
- In the lungs it releases carbon dioxide and takes up oxygen
The systemic circuit — the long one, delivering oxygen:
- Oxygenated blood returns through the pulmonary veins
- Into the left atrium
- The left atrium contracts, pushing it through the bicuspid valve
- Into the left ventricle
- The left ventricle contracts powerfully, pushing it through a semilunar valve
- Into the aorta, which branches to carry it to every part of the body
- In the tissues it releases oxygen and picks up carbon dioxide, becoming deoxygenated again
- It returns through the veins to the vena cava, and the circuit begins again
The two circuits named:
- Pulmonary circulation — heart to lungs to heart. Short, and at lower pressure, which suits the delicate lung capillaries.
- Systemic circulation — heart to the body to heart. Long, and at high pressure, which is why the left ventricle is so muscular.
The advantage of keeping them separate, which is the answer to almost every question on this topic:
- Oxygenated and deoxygenated blood never mix, so tissues receive fully oxygenated blood
- Blood leaves for the body at full pressure, having been re-pumped after the lungs
- Oxygen is therefore delivered rapidly and efficiently, which is what a warm-blooded animal with high energy needs requires
Notice that both sides of the heart work together, not alternately. The two atria contract at the same moment, and then the two ventricles contract at the same moment. So the two circuits run simultaneously — the heart is not one pump used twice but two pumps side by side, sharing a wall and a rhythm.
What are heartbeat, pulse and the pacemaker?
A heartbeat is one complete contraction followed by relaxation of the heart. Contraction is called systole and relaxation diastole.
In a resting adult the heart beats about 72 times a minute, and the rate rises with exercise, excitement or fever — the effect of adrenaline from the previous chapter.
The two sounds. A heartbeat heard through a stethoscope makes a lub-dub. Both sounds are valves closing:
- Lub — the tricuspid and bicuspid valves closing as the ventricles begin to contract
- Dub — the semilunar valves closing as the ventricles relax
So listening to a heart is really listening to its valves, which is why a doctor can detect a faulty valve by the sound it makes.
A pulse is the rhythmic expansion of an artery each time the left ventricle forces blood into it. The surge travels along the artery wall as a wave, and where an artery runs close to the skin over a bone it can be felt — at the wrist and at the neck.
Since one pulse corresponds to one contraction of the left ventricle, the pulse rate equals the heart rate. This is why counting the pulse at the wrist for a minute measures the heartbeat without any instrument.
A pulse is felt in arteries only, never in veins — the surge exists because arteries receive blood in spurts at high pressure, while flow in veins is slow and steady.
The pacemaker is a small patch of specialised muscle tissue in the wall of the right atrium, also called the sino-atrial node. It generates the electrical impulse that starts each heartbeat and sets the rate, and the impulse then spreads through the heart muscle so the chambers contract in the correct order.
Why this is remarkable. The heart therefore beats on its own, without needing any instruction from the brain — which is how a heart continues beating in a person who is unconscious. Nerves and hormones can speed it up or slow it down, but they do not start it.
When it fails. If the natural pacemaker works irregularly, an artificial pacemaker — a small battery-operated device — is fitted surgically to supply the electrical impulses instead. That it can be replaced by an electrical device is the clearest evidence of what the natural one actually does.
In a resting adult the heart beats about 72 times a minute, and the rate rises with exercise, excitement or fever — the effect of adrenaline from the previous chapter.
The two sounds. A heartbeat heard through a stethoscope makes a lub-dub. Both sounds are valves closing:
- Lub — the tricuspid and bicuspid valves closing as the ventricles begin to contract
- Dub — the semilunar valves closing as the ventricles relax
So listening to a heart is really listening to its valves, which is why a doctor can detect a faulty valve by the sound it makes.
A pulse is the rhythmic expansion of an artery each time the left ventricle forces blood into it. The surge travels along the artery wall as a wave, and where an artery runs close to the skin over a bone it can be felt — at the wrist and at the neck.
Since one pulse corresponds to one contraction of the left ventricle, the pulse rate equals the heart rate. This is why counting the pulse at the wrist for a minute measures the heartbeat without any instrument.
A pulse is felt in arteries only, never in veins — the surge exists because arteries receive blood in spurts at high pressure, while flow in veins is slow and steady.
The pacemaker is a small patch of specialised muscle tissue in the wall of the right atrium, also called the sino-atrial node. It generates the electrical impulse that starts each heartbeat and sets the rate, and the impulse then spreads through the heart muscle so the chambers contract in the correct order.
Why this is remarkable. The heart therefore beats on its own, without needing any instruction from the brain — which is how a heart continues beating in a person who is unconscious. Nerves and hormones can speed it up or slow it down, but they do not start it.
When it fails. If the natural pacemaker works irregularly, an artificial pacemaker — a small battery-operated device — is fitted surgically to supply the electrical impulses instead. That it can be replaced by an electrical device is the clearest evidence of what the natural one actually does.
What are tissue fluid and lymph, and how do you keep a heart healthy?
Tissue fluid is the plasma that leaks out of the capillaries into the spaces between cells. Lymph is the tissue fluid that drains away into a separate set of vessels.
How tissue fluid forms. Capillary walls are one cell thick, and the blood inside is under pressure. So some plasma, carrying dissolved oxygen, glucose, amino acids and minerals, is forced out through the wall into the spaces around the cells. Red blood cells are too large to leave and stay behind.
The cells then take oxygen and nutrients from this fluid and pass carbon dioxide and wastes into it. So tissue fluid is the actual go-between — blood never touches most body cells directly, and everything reaching a cell passes through tissue fluid first.
Most of it seeps back into the capillaries further along. The remainder is collected by lymph capillaries, and from that point it is called lymph.
What lymph is like. A colourless fluid, similar to plasma but with less protein, containing lymphocytes — a kind of white blood cell — and no red blood cells. It flows slowly through lymph vessels which, like veins, contain valves to keep it moving one way. It passes through lymph nodes and is finally returned to the blood through large veins near the neck.
The functions of lymph:
- Returns excess tissue fluid and leaked protein to the blood, preventing swelling in the tissues
- Absorbs and transports digested fats from the small intestine, through vessels called lacteals
- Filters out germs in the lymph nodes, which trap and destroy them
- Produces lymphocytes, which make antibodies — so lymph is part of the body's defence
The swollen glands felt in the neck during a throat infection are lymph nodes at work, enlarged because they are filtering and fighting the infection.
Habits that keep the heart healthy:
- Regular exercise — the heart is muscle, and like any muscle it strengthens with use
- A balanced diet low in saturated fat and salt, with plenty of fruit, vegetables and whole grains
- Maintaining a healthy weight, since excess weight makes the heart work harder
- Avoiding tobacco in every form — smoking damages the blood vessels and raises blood pressure
- Limiting alcohol
- Managing stress, and getting enough sleep
- Having blood pressure checked from time to time, since high blood pressure usually gives no symptoms at all
Why the last point deserves its place. Most of what damages a heart does so slowly and silently over years. Nothing on that list produces a noticeable effect this week, which is exactly why the habits have to be formed before there is any reason to worry.
How tissue fluid forms. Capillary walls are one cell thick, and the blood inside is under pressure. So some plasma, carrying dissolved oxygen, glucose, amino acids and minerals, is forced out through the wall into the spaces around the cells. Red blood cells are too large to leave and stay behind.
The cells then take oxygen and nutrients from this fluid and pass carbon dioxide and wastes into it. So tissue fluid is the actual go-between — blood never touches most body cells directly, and everything reaching a cell passes through tissue fluid first.
Most of it seeps back into the capillaries further along. The remainder is collected by lymph capillaries, and from that point it is called lymph.
What lymph is like. A colourless fluid, similar to plasma but with less protein, containing lymphocytes — a kind of white blood cell — and no red blood cells. It flows slowly through lymph vessels which, like veins, contain valves to keep it moving one way. It passes through lymph nodes and is finally returned to the blood through large veins near the neck.
The functions of lymph:
- Returns excess tissue fluid and leaked protein to the blood, preventing swelling in the tissues
- Absorbs and transports digested fats from the small intestine, through vessels called lacteals
- Filters out germs in the lymph nodes, which trap and destroy them
- Produces lymphocytes, which make antibodies — so lymph is part of the body's defence
The swollen glands felt in the neck during a throat infection are lymph nodes at work, enlarged because they are filtering and fighting the infection.
Habits that keep the heart healthy:
- Regular exercise — the heart is muscle, and like any muscle it strengthens with use
- A balanced diet low in saturated fat and salt, with plenty of fruit, vegetables and whole grains
- Maintaining a healthy weight, since excess weight makes the heart work harder
- Avoiding tobacco in every form — smoking damages the blood vessels and raises blood pressure
- Limiting alcohol
- Managing stress, and getting enough sleep
- Having blood pressure checked from time to time, since high blood pressure usually gives no symptoms at all
Why the last point deserves its place. Most of what damages a heart does so slowly and silently over years. Nothing on that list produces a noticeable effect this week, which is exactly why the habits have to be formed before there is any reason to worry.
Exam tip
Exam tip: tricuspid on the right, bicuspid on the left
Learn the valve positions exactly: tricuspid between the right atrium and right ventricle, bicuspid between the left atrium and left ventricle, and semilunar valves at the bases of the pulmonary artery and aorta. Swapping tricuspid and bicuspid is the single most frequent error here.
Give every valve the same function — preventing backflow, so blood flows one way.
Say the left ventricle has the thickest wall and give the reason: it pumps to the whole body, while the right pumps only to the lungs.
Use the rule veins enter atria, arteries leave ventricles to get the four great vessels right.
When tracing the path of blood, name the chambers and the valves in order — a route that skips the valves is incomplete.
For double circulation, name both circuits — pulmonary and systemic — and give the advantage: the two kinds of blood never mix, so tissues get fully oxygenated blood at full pressure.
Say both heart sounds are valves closing, and that a pulse is felt in arteries only, with pulse rate equal to heart rate.
Locate the pacemaker in the wall of the right atrium and say it initiates and sets the rate of the beat, so the heart beats without instruction from the brain.
And distinguish tissue fluid from lymph — tissue fluid is the plasma in the spaces between cells; lymph is that fluid once it has drained into lymph vessels.
Give every valve the same function — preventing backflow, so blood flows one way.
Say the left ventricle has the thickest wall and give the reason: it pumps to the whole body, while the right pumps only to the lungs.
Use the rule veins enter atria, arteries leave ventricles to get the four great vessels right.
When tracing the path of blood, name the chambers and the valves in order — a route that skips the valves is incomplete.
For double circulation, name both circuits — pulmonary and systemic — and give the advantage: the two kinds of blood never mix, so tissues get fully oxygenated blood at full pressure.
Say both heart sounds are valves closing, and that a pulse is felt in arteries only, with pulse rate equal to heart rate.
Locate the pacemaker in the wall of the right atrium and say it initiates and sets the rate of the beat, so the heart beats without instruction from the brain.
And distinguish tissue fluid from lymph — tissue fluid is the plasma in the spaces between cells; lymph is that fluid once it has drained into lymph vessels.
Did you know
Why does the heart have its own blood supply?
Blood flows through the heart in enormous quantities every minute, and none of it feeds the heart.
The heart wall is thick muscle, and blood passing through the chambers is simply passing through — it is separated from the muscle by the lining, and it cannot supply the wall any more than water in a pipe can feed the pipe. Diffusion alone could never reach the inner layers of something that thick.
So the heart is served by its own arteries, which branch off the aorta just as it leaves the left ventricle and spread over the heart's outer surface. The heart is, in effect, the first customer of every beat it makes.
That arrangement explains why a blockage in one of those small arteries is so serious. The muscle beyond it is cut off from oxygen even though the chambers beside it are full of oxygenated blood — plenty of blood, in the wrong place. It is also why the heart is the organ least able to take a break: a muscle that pumps its own supply cannot stop to rest.
The heart wall is thick muscle, and blood passing through the chambers is simply passing through — it is separated from the muscle by the lining, and it cannot supply the wall any more than water in a pipe can feed the pipe. Diffusion alone could never reach the inner layers of something that thick.
So the heart is served by its own arteries, which branch off the aorta just as it leaves the left ventricle and spread over the heart's outer surface. The heart is, in effect, the first customer of every beat it makes.
That arrangement explains why a blockage in one of those small arteries is so serious. The muscle beyond it is cut off from oxygen even though the chambers beside it are full of oxygenated blood — plenty of blood, in the wrong place. It is also why the heart is the organ least able to take a break: a muscle that pumps its own supply cannot stop to rest.
Key takeaways
The heart, circulation and lymph: quick revision
- The heart lies in the chest between the lungs, tilted left, in a double membrane called the pericardium.
- Four chambers: two thin-walled atria that receive, two thick-walled ventricles that pump, separated left from right by the septum. The left ventricle has the thickest wall because it pumps to the whole body.
- Four valves, all preventing backflow: tricuspid (right atrium to right ventricle), bicuspid (left atrium to left ventricle), and semilunar valves at the pulmonary artery and aorta.
- Veins enter atria; arteries leave ventricles — vena cava to the right atrium, pulmonary artery from the right ventricle, pulmonary veins to the left atrium, aorta from the left ventricle.
- Path of blood: vena cava right atrium tricuspid right ventricle pulmonary artery lungs pulmonary veins left atrium bicuspid left ventricle aorta body vena cava.
- Double circulation — pulmonary (heart to lungs to heart, short, lower pressure) and systemic (heart to body to heart, long, high pressure). Blood passes through the heart twice per circuit.
- Its advantage: the two kinds of blood never mix, tissues get fully oxygenated blood, and it leaves at full pressure. Both atria contract together, then both ventricles — so the circuits run simultaneously.
- A heartbeat is one systole and diastole, about 72 per minute at rest. Lub is the tricuspid and bicuspid closing; dub is the semilunar valves closing.
- A pulse is an artery expanding with each left-ventricle contraction, felt at the wrist and neck, in arteries only, and pulse rate equals heart rate.
- The pacemaker (sino-atrial node) in the wall of the right atrium generates the impulse and sets the rate, so the heart beats without the brain. An artificial pacemaker replaces it when it fails.
- Tissue fluid is plasma forced out of the one-cell-thick capillaries into the spaces between cells; it is the go-between for every exchange with a cell. Most seeps back; the rest becomes lymph.
- Lymph is colourless, with less protein, containing lymphocytes and no red cells, flowing in valved vessels through lymph nodes back into veins near the neck.
- Lymph returns fluid and protein, transports digested fats via lacteals, filters germs in the nodes, and produces lymphocytes for defence.
- Heart-healthy habits: regular exercise, a diet low in saturated fat and salt, healthy weight, no tobacco, limited alcohol, managed stress, enough sleep, and occasional blood pressure checks.
Try tracing the whole path of blood naming every chamber and valve in order, then listing the four functions of lymph — those two answers carry most of the marks in this chapter.
- Four chambers: two thin-walled atria that receive, two thick-walled ventricles that pump, separated left from right by the septum. The left ventricle has the thickest wall because it pumps to the whole body.
- Four valves, all preventing backflow: tricuspid (right atrium to right ventricle), bicuspid (left atrium to left ventricle), and semilunar valves at the pulmonary artery and aorta.
- Veins enter atria; arteries leave ventricles — vena cava to the right atrium, pulmonary artery from the right ventricle, pulmonary veins to the left atrium, aorta from the left ventricle.
- Path of blood: vena cava right atrium tricuspid right ventricle pulmonary artery lungs pulmonary veins left atrium bicuspid left ventricle aorta body vena cava.
- Double circulation — pulmonary (heart to lungs to heart, short, lower pressure) and systemic (heart to body to heart, long, high pressure). Blood passes through the heart twice per circuit.
- Its advantage: the two kinds of blood never mix, tissues get fully oxygenated blood, and it leaves at full pressure. Both atria contract together, then both ventricles — so the circuits run simultaneously.
- A heartbeat is one systole and diastole, about 72 per minute at rest. Lub is the tricuspid and bicuspid closing; dub is the semilunar valves closing.
- A pulse is an artery expanding with each left-ventricle contraction, felt at the wrist and neck, in arteries only, and pulse rate equals heart rate.
- The pacemaker (sino-atrial node) in the wall of the right atrium generates the impulse and sets the rate, so the heart beats without the brain. An artificial pacemaker replaces it when it fails.
- Tissue fluid is plasma forced out of the one-cell-thick capillaries into the spaces between cells; it is the go-between for every exchange with a cell. Most seeps back; the rest becomes lymph.
- Lymph is colourless, with less protein, containing lymphocytes and no red cells, flowing in valved vessels through lymph nodes back into veins near the neck.
- Lymph returns fluid and protein, transports digested fats via lacteals, filters germs in the nodes, and produces lymphocytes for defence.
- Heart-healthy habits: regular exercise, a diet low in saturated fat and salt, healthy weight, no tobacco, limited alcohol, managed stress, enough sleep, and occasional blood pressure checks.
Try tracing the whole path of blood naming every chamber and valve in order, then listing the four functions of lymph — those two answers carry most of the marks in this chapter.