Veins Need One-Way Valves Because Blood Must Climb Back Up From Your Feet
Compare arteries, veins and capillaries and see how each wall suits its job, name the blood vessels that enter and leave the liver, kidneys and lungs, follow plasma as it leaks out to form tissue fluid and drains away as lymph, and learn what the spleen and tonsils do.
How does blood travel out to every cell and back to the heart?
Stand still for a few minutes with one hand hanging by your side and look at its back. The veins stand out as raised bluish lines. Raise the hand above your head and within seconds they flatten and almost disappear. Blood in those veins is flowing back towards the heart at low pressure, and gravity makes its return easier or harder.
Part 2 described the pump. This part describes the pipes and the drainage.
- Arteries carry blood away from the heart at high pressure
- Capillaries are microscopic vessels where materials are exchanged with the cells
- Veins return blood to the heart at low pressure, helped by one-way valves
- The lymphatic system collects the fluid that leaks out of capillaries and returns it to the blood, while filtering out germs
This part covers:
- The structure and function of arteries, veins and capillaries, compared point by point
- The blood vessels of three important organs — the liver, the kidneys and the lungs
- Tissue fluid and lymph — how they form, what they contain, and what the lymphatic system does
- The spleen and tonsils, two lymphatic organs that help defend the body
Why the vessels matter as much as the heart. A strong pump is useless if the pipes cannot carry the pressure or the exchange cannot happen. Almost every living cell in the body lies very close to a capillary, because oxygen and food can diffuse only a very short distance. The design of each kind of vessel matches its job.
An everyday connection. When blood is drawn for a test, the needle usually goes into a vein on the inside of the elbow, not an artery. Veins there lie near the surface, their walls are thin and the pressure inside them is low, so the puncture is easy to make and bleeding stops quickly with gentle pressure — reasons that follow directly from the comparison in this lesson.
The link to Part 1. The white blood cells described in Part 1 do much of their work in the lymphatic system, in lymph nodes, the spleen and the tonsils — which is where this chapter ends.
This page covers the third part of the ICSE Class 10 Biology chapter on the circulatory system: arteries, veins and capillaries, the blood vessels of major organs, tissue fluid and lymph, and the spleen and tonsils.
Part 2 described the pump. This part describes the pipes and the drainage.
- Arteries carry blood away from the heart at high pressure
- Capillaries are microscopic vessels where materials are exchanged with the cells
- Veins return blood to the heart at low pressure, helped by one-way valves
- The lymphatic system collects the fluid that leaks out of capillaries and returns it to the blood, while filtering out germs
This part covers:
- The structure and function of arteries, veins and capillaries, compared point by point
- The blood vessels of three important organs — the liver, the kidneys and the lungs
- Tissue fluid and lymph — how they form, what they contain, and what the lymphatic system does
- The spleen and tonsils, two lymphatic organs that help defend the body
Why the vessels matter as much as the heart. A strong pump is useless if the pipes cannot carry the pressure or the exchange cannot happen. Almost every living cell in the body lies very close to a capillary, because oxygen and food can diffuse only a very short distance. The design of each kind of vessel matches its job.
An everyday connection. When blood is drawn for a test, the needle usually goes into a vein on the inside of the elbow, not an artery. Veins there lie near the surface, their walls are thin and the pressure inside them is low, so the puncture is easy to make and bleeding stops quickly with gentle pressure — reasons that follow directly from the comparison in this lesson.
The link to Part 1. The white blood cells described in Part 1 do much of their work in the lymphatic system, in lymph nodes, the spleen and the tonsils — which is where this chapter ends.
This page covers the third part of the ICSE Class 10 Biology chapter on the circulatory system: arteries, veins and capillaries, the blood vessels of major organs, tissue fluid and lymph, and the spleen and tonsils.
How do arteries, veins and capillaries differ in structure and function?
Arteries have thick, muscular, elastic walls and carry blood away from the heart under high pressure; veins have thinner walls, wider cavities and valves, and carry blood back to the heart under low pressure; capillaries have walls only one cell thick, so materials can be exchanged between blood and cells.
The walls of arteries and veins have three layers:
- An inner lining of thin, flat cells, the endothelium, smooth so that blood flows easily
- A middle layer of smooth muscle and elastic fibres
- An outer layer of tough fibrous connective tissue
Capillaries have only the inner lining.
Comparison point by point:
- Direction of flow: arteries — away from the heart; veins — towards the heart; capillaries — from arterioles to venules, linking the two
- Wall: arteries — thick, muscular and elastic; veins — thinner, with less muscle; capillaries — one cell thick
- Lumen, the central cavity: arteries — narrow; veins — wide; capillaries — very narrow, often just wide enough for red blood cells to pass in single file
- Valves: arteries — none along their length; veins — present, to prevent backflow; capillaries — none
- Blood pressure: arteries — high, rising and falling with each beat; veins — low and steady; capillaries — falling along their length
- Pulse: arteries — yes; veins — no; capillaries — no
- Position: arteries — mostly deep in the body; veins — many near the surface; capillaries — within the tissues, close to cells
- Blood carried: arteries — oxygenated, except the pulmonary arteries; veins — deoxygenated, except the pulmonary veins
- Function: arteries — deliver blood to organs; veins — return blood to the heart; capillaries — exchange of oxygen, food, carbon dioxide and wastes
How each structure suits its function.
- Arteries receive spurts of blood at high pressure from the ventricles, so thick elastic walls stretch and recoil without bursting, smoothing the flow
- Veins carry blood at low pressure, often uphill from the legs, so valves stop it flowing backwards and a wide lumen lets it flow with little resistance
- Capillaries must let substances through, so a wall one cell thick gives the shortest possible distance for diffusion, and their huge number gives a very large surface area
Worked example — why blood slows down in the capillaries. Suppose blood flows along an artery with a cross-section of at . The artery branches into capillaries whose cross-sections add up to . The same volume of blood passes each second, so:
**The blood moves times more slowly — giving time for oxygen and food to diffuse out and for wastes to diffuse in.
An everyday example. A graze on the knee oozes blood slowly from many tiny capillaries. Bleeding from a cut vein flows out steadily, while a cut artery makes blood spurt in time with the heartbeat. First-aid courses teach these differences because they show how serious a wound is.
The boundary case — veins are not blue. Blood in veins is dark red, not blue. Veins look bluish only because of the way light passes into the skin and is reflected back** — blood drawn from a vein into a syringe is plainly red.
The walls of arteries and veins have three layers:
- An inner lining of thin, flat cells, the endothelium, smooth so that blood flows easily
- A middle layer of smooth muscle and elastic fibres
- An outer layer of tough fibrous connective tissue
Capillaries have only the inner lining.
Comparison point by point:
- Direction of flow: arteries — away from the heart; veins — towards the heart; capillaries — from arterioles to venules, linking the two
- Wall: arteries — thick, muscular and elastic; veins — thinner, with less muscle; capillaries — one cell thick
- Lumen, the central cavity: arteries — narrow; veins — wide; capillaries — very narrow, often just wide enough for red blood cells to pass in single file
- Valves: arteries — none along their length; veins — present, to prevent backflow; capillaries — none
- Blood pressure: arteries — high, rising and falling with each beat; veins — low and steady; capillaries — falling along their length
- Pulse: arteries — yes; veins — no; capillaries — no
- Position: arteries — mostly deep in the body; veins — many near the surface; capillaries — within the tissues, close to cells
- Blood carried: arteries — oxygenated, except the pulmonary arteries; veins — deoxygenated, except the pulmonary veins
- Function: arteries — deliver blood to organs; veins — return blood to the heart; capillaries — exchange of oxygen, food, carbon dioxide and wastes
How each structure suits its function.
- Arteries receive spurts of blood at high pressure from the ventricles, so thick elastic walls stretch and recoil without bursting, smoothing the flow
- Veins carry blood at low pressure, often uphill from the legs, so valves stop it flowing backwards and a wide lumen lets it flow with little resistance
- Capillaries must let substances through, so a wall one cell thick gives the shortest possible distance for diffusion, and their huge number gives a very large surface area
Worked example — why blood slows down in the capillaries. Suppose blood flows along an artery with a cross-section of at . The artery branches into capillaries whose cross-sections add up to . The same volume of blood passes each second, so:
**The blood moves times more slowly — giving time for oxygen and food to diffuse out and for wastes to diffuse in.
An everyday example. A graze on the knee oozes blood slowly from many tiny capillaries. Bleeding from a cut vein flows out steadily, while a cut artery makes blood spurt in time with the heartbeat. First-aid courses teach these differences because they show how serious a wound is.
The boundary case — veins are not blue. Blood in veins is dark red, not blue. Veins look bluish only because of the way light passes into the skin and is reflected back** — blood drawn from a vein into a syringe is plainly red.
Which main blood vessels enter and leave the liver, the kidneys and the lungs?
The liver receives blood through the hepatic artery and the hepatic portal vein and returns it through the hepatic vein; each kidney receives blood through a renal artery and returns it through a renal vein; the lungs receive deoxygenated blood through the pulmonary arteries and return oxygenated blood through the pulmonary veins.
1. The liver.
- Enters: the hepatic artery, arising from the aorta, bringing oxygenated blood
- Enters: the hepatic portal vein, bringing blood from the stomach, intestines, spleen and pancreas, rich in absorbed food after a meal
- Leaves: the hepatic vein, which joins the inferior vena cava
2. The kidneys.
- Enters: the renal artery, a branch of the aorta, bringing oxygenated blood containing urea and other wastes
- Leaves: the renal vein, which joins the inferior vena cava, carrying blood from which much of the urea has been removed
3. The lungs.
- Enter: the pulmonary arteries, one to each lung, bringing deoxygenated blood from the right ventricle
- Leave: the pulmonary veins, bringing oxygenated blood to the left atrium
- The tissue of the lungs itself is nourished by small separate vessels, the bronchial arteries
Worked example — which vessel?
- Least urea: the renal vein, because the kidneys have just filtered the blood
- Most absorbed glucose soon after a meal: the hepatic portal vein, coming straight from the intestine
- Most oxygen: the pulmonary veins, coming straight from the lungs
Worked example — tracing a glucose molecule. A glucose molecule absorbed in the small intestine reaches a muscle in the arm. Name the route.
It passes through the liver once and through the heart twice — once through each side — before it reaches the muscle.
An everyday example. In a kidney transplant, surgeons join the artery and vein of the donated kidney to large blood vessels in the patient's lower abdomen, and connect its ureter to the bladder. Once blood flows in through the new kidney's artery and out through its vein, the kidney can begin filtering.
The boundary case — two supplies for one organ. Most organs receive their blood only through arteries. The liver receives blood through both an artery and a vein — the hepatic artery for oxygen and the hepatic portal vein for absorbed food — which is why the portal vein is treated as a special case.
1. The liver.
- Enters: the hepatic artery, arising from the aorta, bringing oxygenated blood
- Enters: the hepatic portal vein, bringing blood from the stomach, intestines, spleen and pancreas, rich in absorbed food after a meal
- Leaves: the hepatic vein, which joins the inferior vena cava
2. The kidneys.
- Enters: the renal artery, a branch of the aorta, bringing oxygenated blood containing urea and other wastes
- Leaves: the renal vein, which joins the inferior vena cava, carrying blood from which much of the urea has been removed
3. The lungs.
- Enter: the pulmonary arteries, one to each lung, bringing deoxygenated blood from the right ventricle
- Leave: the pulmonary veins, bringing oxygenated blood to the left atrium
- The tissue of the lungs itself is nourished by small separate vessels, the bronchial arteries
Worked example — which vessel?
- Least urea: the renal vein, because the kidneys have just filtered the blood
- Most absorbed glucose soon after a meal: the hepatic portal vein, coming straight from the intestine
- Most oxygen: the pulmonary veins, coming straight from the lungs
Worked example — tracing a glucose molecule. A glucose molecule absorbed in the small intestine reaches a muscle in the arm. Name the route.
It passes through the liver once and through the heart twice — once through each side — before it reaches the muscle.
An everyday example. In a kidney transplant, surgeons join the artery and vein of the donated kidney to large blood vessels in the patient's lower abdomen, and connect its ureter to the bladder. Once blood flows in through the new kidney's artery and out through its vein, the kidney can begin filtering.
The boundary case — two supplies for one organ. Most organs receive their blood only through arteries. The liver receives blood through both an artery and a vein — the hepatic artery for oxygen and the hepatic portal vein for absorbed food — which is why the portal vein is treated as a special case.
How are tissue fluid and lymph formed, and what are the functions of the lymphatic system?
At the arterial end of a capillary, blood pressure pushes water and small dissolved substances out through the thin wall to form tissue fluid, which bathes the cells; most of it re-enters the capillaries at the venous end, and the rest drains into lymph vessels as lymph, which the lymphatic system returns to the blood while filtering out germs and carrying absorbed fats.
1. Formation of tissue fluid.
- At the arterial end of a capillary, the blood pressure is high
- It forces water and small dissolved substances out through the capillary wall — glucose, amino acids, salts and oxygen
- Red blood cells, platelets and most plasma proteins are too large and stay in the blood; some white blood cells squeeze out between the wall cells
- This fluid, called tissue fluid or intercellular fluid, fills the spaces between cells and bathes them
2. Exchange with the cells.
- Cells take in oxygen and food from the tissue fluid
- Cells release carbon dioxide and wastes into it
3. Return of the fluid.
- At the venous end the blood pressure has fallen, and the proteins left behind in the blood draw water back in by osmosis
- Most of the tissue fluid re-enters the capillaries this way, carrying carbon dioxide and wastes
- The rest drains into lymph capillaries — tiny, thin-walled tubes closed at one end — and is then called lymph
Worked example — how much becomes lymph? Suppose that in one day litres of fluid leaves the blood capillaries and of it re-enters them at the venous ends.
**Without the lymphatic system, those litres would build up in the tissues every day, and the body would swell.
Blood, tissue fluid and lymph compared:
- Blood: plasma with red cells, white cells, platelets and plenty of proteins
- Tissue fluid: like plasma but with far fewer proteins, and no red cells or platelets
- Lymph: similar to tissue fluid, with no red cells or platelets, but rich in lymphocytes; lymph from the intestine after a fatty meal also carries fat droplets, which make it milky
4. The lymphatic system. Lymph capillaries join to form larger lymph vessels, which have valves like veins. Lymph is moved along by the squeezing of nearby muscles during body movements, since it has no pump of its own. On the way it passes through lymph nodes, and finally drains into large veins near the base of the neck.
Functions of the lymphatic system:
- Returns tissue fluid to the blood, keeping the volume of blood steady and preventing swelling of the tissues
- Absorbs fats from the small intestine through lacteals, the lymph vessels in the villi, and carries them to the blood
- Defends the body — lymph nodes filter out germs and debris, and lymphocytes produced in lymphatic tissue make antibodies
- Carries lymphocytes into the blood
An everyday example. After a long train or bus journey spent sitting still, many people find their ankles and feet slightly swollen. With the leg muscles hardly moving, less lymph and venous blood is pushed upwards, and tissue fluid collects in the lower legs until walking about moves it on.
The boundary case — lymph does not circulate. Blood travels round a closed loop driven by the heart, but lymph flows in one direction only, from the tissues towards the veins of the neck. The lymphatic system is a drainage system, not a second circulation.**
1. Formation of tissue fluid.
- At the arterial end of a capillary, the blood pressure is high
- It forces water and small dissolved substances out through the capillary wall — glucose, amino acids, salts and oxygen
- Red blood cells, platelets and most plasma proteins are too large and stay in the blood; some white blood cells squeeze out between the wall cells
- This fluid, called tissue fluid or intercellular fluid, fills the spaces between cells and bathes them
2. Exchange with the cells.
- Cells take in oxygen and food from the tissue fluid
- Cells release carbon dioxide and wastes into it
3. Return of the fluid.
- At the venous end the blood pressure has fallen, and the proteins left behind in the blood draw water back in by osmosis
- Most of the tissue fluid re-enters the capillaries this way, carrying carbon dioxide and wastes
- The rest drains into lymph capillaries — tiny, thin-walled tubes closed at one end — and is then called lymph
Worked example — how much becomes lymph? Suppose that in one day litres of fluid leaves the blood capillaries and of it re-enters them at the venous ends.
**Without the lymphatic system, those litres would build up in the tissues every day, and the body would swell.
Blood, tissue fluid and lymph compared:
- Blood: plasma with red cells, white cells, platelets and plenty of proteins
- Tissue fluid: like plasma but with far fewer proteins, and no red cells or platelets
- Lymph: similar to tissue fluid, with no red cells or platelets, but rich in lymphocytes; lymph from the intestine after a fatty meal also carries fat droplets, which make it milky
4. The lymphatic system. Lymph capillaries join to form larger lymph vessels, which have valves like veins. Lymph is moved along by the squeezing of nearby muscles during body movements, since it has no pump of its own. On the way it passes through lymph nodes, and finally drains into large veins near the base of the neck.
Functions of the lymphatic system:
- Returns tissue fluid to the blood, keeping the volume of blood steady and preventing swelling of the tissues
- Absorbs fats from the small intestine through lacteals, the lymph vessels in the villi, and carries them to the blood
- Defends the body — lymph nodes filter out germs and debris, and lymphocytes produced in lymphatic tissue make antibodies
- Carries lymphocytes into the blood
An everyday example. After a long train or bus journey spent sitting still, many people find their ankles and feet slightly swollen. With the leg muscles hardly moving, less lymph and venous blood is pushed upwards, and tissue fluid collects in the lower legs until walking about moves it on.
The boundary case — lymph does not circulate. Blood travels round a closed loop driven by the heart, but lymph flows in one direction only, from the tissues towards the veins of the neck. The lymphatic system is a drainage system, not a second circulation.**
What are the spleen and tonsils, and what are their functions?
The spleen is the largest lymphatic organ, lying in the upper left part of the abdomen; it destroys worn-out red blood cells, filters germs from the blood, produces lymphocytes and holds a reserve of blood. The tonsils are masses of lymphatic tissue at the back of the throat that trap and destroy germs entering through the mouth and nose.
1. The spleen.
- Position: in the upper left part of the abdomen, behind the stomach and just below the diaphragm
- Appearance: a soft, dark red, bean-shaped organ
- Nature: the largest lymphatic organ in the body
Functions of the spleen:
- Destroys old and worn-out red blood cells, which is why it is sometimes called the graveyard of red blood cells; iron from their haemoglobin is saved and reused to make new cells
- Filters the blood, trapping and destroying germs with the help of phagocytic white cells
- Produces lymphocytes, which make antibodies
- Acts as a reservoir of blood, which can be released into circulation when needed
- Helps form red blood cells in the foetus, before the bone marrow takes over this job
Worked example — how many red cells are removed each day? The body has about red blood cells, each lasting about days. If cells are replaced at a steady rate, how many are removed each day, and each second?
**About million red blood cells every second — removed mainly by the spleen and liver, and replaced by the bone marrow.
Can a person live without a spleen? Yes. After a severe injury, the spleen is sometimes removed by surgery. The liver and other tissues take over its work of removing old red cells, but the person becomes more likely to suffer certain infections, and doctors often advise extra vaccinations.
2. The tonsils.
- Position: two oval masses of lymphatic tissue at the back of the throat, one on each side
- Functions: trap germs entering with food, water and air through the mouth and nose, and produce lymphocytes that help destroy them
- Tonsillitis — inflammation of the tonsils due to infection, causing a sore throat, pain on swallowing and fever
- If tonsillitis keeps coming back, a doctor may advise removing the tonsils
An everyday example. When a child has a sore throat, a doctor often shines a torch into the mouth to look at the tonsils and gently feels under the jaw. Red, swollen tonsils and tender lumps under the jaw — swollen lymph nodes — show the lymphatic tissue actively fighting an infection.
The boundary case — swollen is not always bad. Swollen tonsils and lymph nodes during an infection are usually a sign that the defence is working, as lymphocytes multiply to fight the germs. They normally shrink back once the infection clears**; swelling that lasts a long time without an obvious cause needs medical attention.
1. The spleen.
- Position: in the upper left part of the abdomen, behind the stomach and just below the diaphragm
- Appearance: a soft, dark red, bean-shaped organ
- Nature: the largest lymphatic organ in the body
Functions of the spleen:
- Destroys old and worn-out red blood cells, which is why it is sometimes called the graveyard of red blood cells; iron from their haemoglobin is saved and reused to make new cells
- Filters the blood, trapping and destroying germs with the help of phagocytic white cells
- Produces lymphocytes, which make antibodies
- Acts as a reservoir of blood, which can be released into circulation when needed
- Helps form red blood cells in the foetus, before the bone marrow takes over this job
Worked example — how many red cells are removed each day? The body has about red blood cells, each lasting about days. If cells are replaced at a steady rate, how many are removed each day, and each second?
**About million red blood cells every second — removed mainly by the spleen and liver, and replaced by the bone marrow.
Can a person live without a spleen? Yes. After a severe injury, the spleen is sometimes removed by surgery. The liver and other tissues take over its work of removing old red cells, but the person becomes more likely to suffer certain infections, and doctors often advise extra vaccinations.
2. The tonsils.
- Position: two oval masses of lymphatic tissue at the back of the throat, one on each side
- Functions: trap germs entering with food, water and air through the mouth and nose, and produce lymphocytes that help destroy them
- Tonsillitis — inflammation of the tonsils due to infection, causing a sore throat, pain on swallowing and fever
- If tonsillitis keeps coming back, a doctor may advise removing the tonsils
An everyday example. When a child has a sore throat, a doctor often shines a torch into the mouth to look at the tonsils and gently feels under the jaw. Red, swollen tonsils and tender lumps under the jaw — swollen lymph nodes — show the lymphatic tissue actively fighting an infection.
The boundary case — swollen is not always bad. Swollen tonsils and lymph nodes during an infection are usually a sign that the defence is working, as lymphocytes multiply to fight the germs. They normally shrink back once the infection clears**; swelling that lasts a long time without an obvious cause needs medical attention.
Exam tip
What earns full marks on blood vessels, lymph and lymphatic organs?
Compare the vessels on matching points, name the vessels in and out of each organ with the kind of blood they carry, explain tissue fluid formation as a pressure-driven sequence, and list the functions of the spleen and tonsils separately.
- Draw labelled cross-sections of an artery, a vein and a capillary, showing wall thickness, lumen size and a valve in the vein
- Compare on the same points — direction, wall, lumen, valves, pressure, pulse, position and function
- Link each structure to its function, such as elastic walls with high pressure and valves with low pressure
- For the liver, name the hepatic artery, hepatic portal vein and hepatic vein
- For the kidneys, name the renal artery and renal vein, and state that the renal vein has the least urea
- For the lungs, name the pulmonary arteries and pulmonary veins with the kind of blood each carries
- Explain tissue fluid formation — pressure at the arterial end, what stays behind, return at the venous end
- Define lymph and compare its composition with blood
- List lymphatic functions — return of fluid, fat absorption through lacteals, defence
- For the spleen, give its position and at least four functions
- For the tonsils, give position, function and tonsillitis
The misconception to name. Lymph is not simply colourless blood. It has no red blood cells or platelets and far fewer proteins, but it is rich in lymphocytes — and it flows one way only, with no pump.
A second trap. Writing that the hepatic portal vein carries blood away from the liver. It carries blood to the liver from the digestive organs; the hepatic vein carries blood away from the liver.
- Draw labelled cross-sections of an artery, a vein and a capillary, showing wall thickness, lumen size and a valve in the vein
- Compare on the same points — direction, wall, lumen, valves, pressure, pulse, position and function
- Link each structure to its function, such as elastic walls with high pressure and valves with low pressure
- For the liver, name the hepatic artery, hepatic portal vein and hepatic vein
- For the kidneys, name the renal artery and renal vein, and state that the renal vein has the least urea
- For the lungs, name the pulmonary arteries and pulmonary veins with the kind of blood each carries
- Explain tissue fluid formation — pressure at the arterial end, what stays behind, return at the venous end
- Define lymph and compare its composition with blood
- List lymphatic functions — return of fluid, fat absorption through lacteals, defence
- For the spleen, give its position and at least four functions
- For the tonsils, give position, function and tonsillitis
The misconception to name. Lymph is not simply colourless blood. It has no red blood cells or platelets and far fewer proteins, but it is rich in lymphocytes — and it flows one way only, with no pump.
A second trap. Writing that the hepatic portal vein carries blood away from the liver. It carries blood to the liver from the digestive organs; the hepatic vein carries blood away from the liver.
Did you know
Why can standing still for a long time make a person faint?
Students standing in the sun through a long school assembly, or soldiers standing motionless on parade, sometimes feel dizzy and even faint. They have not run or lifted anything. The cause lies in the veins of their legs — and in the muscles that normally push blood home.
Blood returning from the feet has to travel upwards, against gravity, at the low pressure found in veins. Most of the push from the heart has been used up by the time blood has passed through the capillaries.
Two features solve this problem.
- The deep leg veins run between muscles. When the calf and thigh muscles contract during walking, they squeeze these veins
- One-way valves inside the veins let blood move only upwards, towards the heart, so each squeeze pushes blood a little higher and the valves stop it sinking back
Together, muscles and valves form a muscle pump that does much of the work of returning blood from the legs.
Standing perfectly still switches that pump off.
- Blood collects in the leg veins, which stretch to hold it
- Less blood returns to the heart, so less is pumped out with each beat
- Less blood reaches the brain, which is very sensitive to a shortage of oxygen
- The person feels light-headed and may faint
Fainting itself helps. Once the person is lying flat, the brain is no longer above the heart, blood reaches it easily again, and recovery is usually quick.
This is why people who must stand for a long time are advised to gently tighten and relax their calf muscles, or shift their weight from foot to foot — keeping the muscle pump working without visibly moving.
The same pump explains two other everyday observations. Ankles swell after a long journey sitting still, as described earlier in this lesson. And doctors encourage patients to get up and walk soon after many operations, partly so that blood keeps moving through the leg veins instead of collecting there and forming clots.
Blood returning from the feet has to travel upwards, against gravity, at the low pressure found in veins. Most of the push from the heart has been used up by the time blood has passed through the capillaries.
Two features solve this problem.
- The deep leg veins run between muscles. When the calf and thigh muscles contract during walking, they squeeze these veins
- One-way valves inside the veins let blood move only upwards, towards the heart, so each squeeze pushes blood a little higher and the valves stop it sinking back
Together, muscles and valves form a muscle pump that does much of the work of returning blood from the legs.
Standing perfectly still switches that pump off.
- Blood collects in the leg veins, which stretch to hold it
- Less blood returns to the heart, so less is pumped out with each beat
- Less blood reaches the brain, which is very sensitive to a shortage of oxygen
- The person feels light-headed and may faint
Fainting itself helps. Once the person is lying flat, the brain is no longer above the heart, blood reaches it easily again, and recovery is usually quick.
This is why people who must stand for a long time are advised to gently tighten and relax their calf muscles, or shift their weight from foot to foot — keeping the muscle pump working without visibly moving.
The same pump explains two other everyday observations. Ankles swell after a long journey sitting still, as described earlier in this lesson. And doctors encourage patients to get up and walk soon after many operations, partly so that blood keeps moving through the leg veins instead of collecting there and forming clots.
Exam relevance
How do blood vessels, lymph and lymphoid organs appear in NEET Biology?
This is foundation work for Class 11 Body Fluids and Circulation and Class 12 Human Health and Disease, both part of NEET Biology, with links to Class 11 Excretory Products and their Elimination.
Where blood vessels lead. Class 11 Body Fluids and Circulation describes the walls of arteries and veins as three layers — an inner tunica intima of squamous endothelium, a middle tunica media of smooth muscle and elastic fibres, and an outer tunica externa of fibrous connective tissue — and notes that the tunica media is comparatively thin in veins. Matching each layer to its tissue is a common question type.
Where tissue fluid and lymph lead. The same chapter explains how fluid with small dissolved substances passes out of capillaries into the spaces between cells, and that lymph carries lymphocytes, transports absorbed fats from the lacteals and drains back into the major veins. Questions compare plasma, tissue fluid and lymph.
Where organ blood supply leads. Body Fluids and Circulation covers the hepatic portal system and coronary circulation, and Excretory Products and their Elimination follows the renal artery into the kidney through the afferent and efferent arterioles of each nephron and the capillary networks around it. Tracing blood through these vessels builds directly on the organ-by-organ list in this lesson.
Where the spleen and tonsils lead. Class 12 Human Health and Disease divides lymphoid organs into primary lymphoid organs — the bone marrow and thymus, where lymphocytes form and mature — and secondary lymphoid organs — including the spleen, lymph nodes, tonsils and patches of lymphoid tissue in the small intestine — where lymphocytes meet germs. The spleen is described there as a filter of the blood and a reservoir of red blood cells.
Question types to expect. At this level: vessel comparisons and diagrams, blood vessels of organs, tissue fluid and lymph, and functions of the spleen and tonsils. In NEET: wall layers, composition of lymph, portal and renal circulation, and primary versus secondary lymphoid organs, often as match-the-column or statement questions.
The single trap that costs marks. Calling the spleen a primary lymphoid organ. The spleen and tonsils are secondary lymphoid organs; the bone marrow and thymus are the primary ones.
A second trap. Thinking lymph contains red blood cells. Lymph has no red blood cells or platelets, though it is rich in lymphocytes.
Board versus competitive emphasis. The ICSE paper marks comparison lists, labelled diagrams and organ functions; NEET marks tissue layers, the classification of lymphoid organs and precise composition. The transferable habit is linking every structure to the job it performs.
Where blood vessels lead. Class 11 Body Fluids and Circulation describes the walls of arteries and veins as three layers — an inner tunica intima of squamous endothelium, a middle tunica media of smooth muscle and elastic fibres, and an outer tunica externa of fibrous connective tissue — and notes that the tunica media is comparatively thin in veins. Matching each layer to its tissue is a common question type.
Where tissue fluid and lymph lead. The same chapter explains how fluid with small dissolved substances passes out of capillaries into the spaces between cells, and that lymph carries lymphocytes, transports absorbed fats from the lacteals and drains back into the major veins. Questions compare plasma, tissue fluid and lymph.
Where organ blood supply leads. Body Fluids and Circulation covers the hepatic portal system and coronary circulation, and Excretory Products and their Elimination follows the renal artery into the kidney through the afferent and efferent arterioles of each nephron and the capillary networks around it. Tracing blood through these vessels builds directly on the organ-by-organ list in this lesson.
Where the spleen and tonsils lead. Class 12 Human Health and Disease divides lymphoid organs into primary lymphoid organs — the bone marrow and thymus, where lymphocytes form and mature — and secondary lymphoid organs — including the spleen, lymph nodes, tonsils and patches of lymphoid tissue in the small intestine — where lymphocytes meet germs. The spleen is described there as a filter of the blood and a reservoir of red blood cells.
Question types to expect. At this level: vessel comparisons and diagrams, blood vessels of organs, tissue fluid and lymph, and functions of the spleen and tonsils. In NEET: wall layers, composition of lymph, portal and renal circulation, and primary versus secondary lymphoid organs, often as match-the-column or statement questions.
The single trap that costs marks. Calling the spleen a primary lymphoid organ. The spleen and tonsils are secondary lymphoid organs; the bone marrow and thymus are the primary ones.
A second trap. Thinking lymph contains red blood cells. Lymph has no red blood cells or platelets, though it is rich in lymphocytes.
Board versus competitive emphasis. The ICSE paper marks comparison lists, labelled diagrams and organ functions; NEET marks tissue layers, the classification of lymphoid organs and precise composition. The transferable habit is linking every structure to the job it performs.
Key takeaways
What must you be able to do from this part?
Three kinds of vessel, three organs, one drainage system and two lymphatic organs.
- Arteries: away from the heart, thick muscular elastic walls, narrow lumen, no valves, high pressure, pulse, mostly deep
- Veins: towards the heart, thinner walls, wide lumen, valves, low pressure, no pulse, many near the surface
- Capillaries: one cell thick, link arterioles to venules, exchange materials with cells
- Blood slows in capillaries: at spreading into gives , times slower
- Veins look bluish but carry dark red blood
- Liver: in by the hepatic artery and hepatic portal vein; out by the hepatic vein
- Kidneys: in by the renal artery; out by the renal vein, which has the least urea
- Lungs: in by the pulmonary arteries with deoxygenated blood; out by the pulmonary veins with oxygenated blood
- Tissue fluid: forced out at the arterial end of capillaries by blood pressure; bathes cells; mostly returns at the venous end
- Lymph: fluid drained into lymph capillaries — no red cells or platelets, rich in lymphocytes; of every litres in the worked example
- Lymphatic system: returns fluid to the blood, absorbs fats through lacteals, filters germs in lymph nodes, flows one way to veins in the neck
- Spleen: largest lymphatic organ, upper left abdomen — destroys worn-out red cells, filters blood, makes lymphocytes, stores blood
- **About million red cells are removed and replaced every second
- Tonsils: lymphatic tissue at the back of the throat — trap germs; inflamed in tonsillitis
- Standing still** stops the leg muscle pump, so blood pools in the veins and a person may faint
The sharpest self-test is a drop of plasma on a round trip. Follow it out of an arterial capillary into the tissue fluid, into a lymph capillary, through a lymph node and back into a vein in the neck — naming what happens at each stop and which parts of the blood are left behind.
- Arteries: away from the heart, thick muscular elastic walls, narrow lumen, no valves, high pressure, pulse, mostly deep
- Veins: towards the heart, thinner walls, wide lumen, valves, low pressure, no pulse, many near the surface
- Capillaries: one cell thick, link arterioles to venules, exchange materials with cells
- Blood slows in capillaries: at spreading into gives , times slower
- Veins look bluish but carry dark red blood
- Liver: in by the hepatic artery and hepatic portal vein; out by the hepatic vein
- Kidneys: in by the renal artery; out by the renal vein, which has the least urea
- Lungs: in by the pulmonary arteries with deoxygenated blood; out by the pulmonary veins with oxygenated blood
- Tissue fluid: forced out at the arterial end of capillaries by blood pressure; bathes cells; mostly returns at the venous end
- Lymph: fluid drained into lymph capillaries — no red cells or platelets, rich in lymphocytes; of every litres in the worked example
- Lymphatic system: returns fluid to the blood, absorbs fats through lacteals, filters germs in lymph nodes, flows one way to veins in the neck
- Spleen: largest lymphatic organ, upper left abdomen — destroys worn-out red cells, filters blood, makes lymphocytes, stores blood
- **About million red cells are removed and replaced every second
- Tonsils: lymphatic tissue at the back of the throat — trap germs; inflamed in tonsillitis
- Standing still** stops the leg muscle pump, so blood pools in the veins and a person may faint
The sharpest self-test is a drop of plasma on a round trip. Follow it out of an arterial capillary into the tissue fluid, into a lymph capillary, through a lymph node and back into a vein in the neck — naming what happens at each stop and which parts of the blood are left behind.