The Circulatory System | ICSE Class 8 Biology Notes

This note covers blood and its components, blood vessels, the structure and working of the human heart, blood flow through the lungs and body, blood groups, pulse, blood pressure, the transport of tissue fluid and conditions affecting the heart.

Why does the human body need a circulatory system?

The circulatory system transports materials around the body. It consists of the heart, blood and blood vessels. The heart is a muscular pump, blood is the fluid it pumps, and blood vessels are the tubes through which blood travels.

What must reach the cells?

Cells, the living units of the body, need nutrients and oxygen. Nutrients are substances obtained from food that support the body's activities. Blood carries digested food from the small intestine and oxygen from the lungs to other parts of the body.

A tissue is a group of cells working together for a particular function. These materials enter the body at particular places, but cells throughout the body need them. Transport connects the places where materials are absorbed with the places where they are used. The heart's pumping action keeps blood moving through the vessels.

Cells also produce waste substances. Blood carries carbon dioxide towards the lungs for removal and carries other wastes towards organs that remove them. Delivery and waste collection are therefore linked functions of circulation.

How do the three main parts work together?

Human blood circulates through a closed network of vessels reaching the tissues. This arrangement is called a closed circulatory system. Exchange occurs across the walls of the smallest vessels.

Definition: Circulation is the movement of blood through the heart and blood vessels, supplying useful materials to tissues and carrying waste materials away.

The transport system links organs with different jobs. The lungs exchange gases, the small intestine supplies absorbed food, and the heart drives the movement of blood between these organs and the rest of the body.

What are the components of blood and their functions?

Plasma is the liquid part of blood in which its cells and cell fragments are suspended. It is straw coloured and contains water, proteins and dissolved substances. It transports food, carbon dioxide and nitrogenous wastes, which are wastes containing nitrogen.

How do the different components contribute?

Red blood cells, abbreviated as RBCs, contain haemoglobin, a red, iron-containing protein that binds oxygen. Haemoglobin helps carry oxygen from the lungs to the body's cells. Its presence accounts for the red colour of these cells and of blood.

White blood cells, abbreviated as WBCs, help defend the body against germs. They lack haemoglobin and are colourless. Different kinds contribute to defence in different ways, including destroying foreign organisms and taking part in the body's protective responses.

Platelets are small cell fragments that help blood clot. A clot is a mass formed at an injury that helps prevent excessive blood loss. Platelets release substances involved in clotting; they do not perform the oxygen-carrying role of red blood cells.

Formed elements is the collective name for red blood cells, white blood cells and platelets.

How should a blood diagram be read?

The drawing shows a red blood cell, a group of small platelets and several separately labelled white blood cells. The red cell is shown in red; the white-cell drawings have different internal patterns.

See Fig. 15.1 in your NCERT textbook

Plasma is the surrounding liquid, so it belongs to a different category. Blood's transport, defence and clotting functions depend on these different components working together.

How do arteries, veins and capillaries differ?

An artery carries blood away from the heart. A vein carries blood towards the heart. These definitions depend on the direction of flow. They do not depend on whether the blood contains a relatively large or small amount of oxygen.

Capillaries are the smallest blood vessels, with walls one cell thick. Their thin walls allow materials to pass between blood and the surrounding cells. Arteries branch into smaller vessels that lead to capillaries, and capillaries join into vessels leading to veins.

Oxygenated blood means oxygen-rich blood. Deoxygenated blood means blood relatively poor in oxygen after supplying tissues. The word pulmonary means related to the lungs. The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary veins bring oxygenated blood back.

How does structure suit the direction of flow?

Blood pressure is the force blood exerts against a vessel wall. Blood leaving the heart is under high pressure, so arteries have thick, elastic walls. Veins carry blood at lower pressure and have thinner walls. Valves, structures that prevent backward flow, help blood in veins move towards the heart.

Why are the lung vessels important exceptions?

Note: Identify an artery or vein by its direction relative to the heart. The pulmonary artery is an artery because blood travels away from the heart, even though this blood is deoxygenated.

Capillaries have a different emphasis: exchange. Thick walls would make exchange more difficult, whereas their one-cell-thick walls place blood close to the surrounding cells. Thus the vessels that carry blood over distances and the vessels that permit exchange have different structures.

How is the human heart organised internally?

The heart lies in the chest between the lungs, slightly tilted to the left. It is roughly the size of a clenched fist. A pericardium, a double-walled membranous covering enclosing fluid, protects it. Its muscular walls provide the force needed for pumping.

What are the four chambers?

The two upper chambers are the atria; one is an atrium. They receive blood entering the heart. The two lower chambers are the ventricles, which pump blood out. Each side therefore has an upper receiving chamber and a lower pumping chamber.

A septum is a partition separating chambers. The partition between the atria is called the interatrial septum, and the partition between the ventricles is the interventricular septum. These partitions separate the right and left sides and help prevent mixing of their blood.

Why are ventricular walls thicker?

The ventricles pump blood out towards the lungs or body organs. Their walls are much thicker than the atrial walls. The atria transfer blood to the ventricles on the same side, while the ventricles provide the pumping force for the outgoing routes.

The aorta is the main artery leaving the left ventricle. The vena cava is a major vein returning blood from the body to the right atrium. These vessel names identify connections, so they should be learnt with the chamber each serves.

The cutaway drawing labels the right and left atria, right and left ventricles, and the interventricular septum. Large vessels enter or leave above the chambers. The aorta is red, while the vena cava and pulmonary artery are blue.

See Fig. 15.2 in your NCERT textbook

How do valves and the pacemaker control a heartbeat?

The tricuspid valve guards the opening between the right atrium and right ventricle. The bicuspid valve, also called the mitral valve, guards the opening between the left atrium and left ventricle. They allow blood to pass from atria into ventricles and prevent backward flow.

Semilunar valves guard the openings from the ventricles into the pulmonary artery and aorta. They prevent blood returning from these arteries into the ventricles. The valves therefore maintain direction while changes in pressure move blood through the heart. A heartbeat is the rhythmic contraction and relaxation of heart muscle.

Where does the rhythm begin?

The sinoatrial node is a patch of specialised heart muscle in the upper part of the right atrium. It generates electrical signals that initiate the heart's rhythmic contractions. It is called the natural pacemaker, meaning the structure that sets the pace of the heartbeat.

Systole means contraction of a heart chamber, and diastole means relaxation. The cardiac cycle is the repeating sequence of contraction and relaxation in the atria and ventricles. Here, cardiac means relating to the heart.

What is the sequence of pumping?

1. When the chambers are relaxed, blood enters the atria and passes through the open tricuspid and bicuspid valves into the ventricles.
2. The pacemaker initiates a signal, and the atria contract, moving more blood into the ventricles.
3. The ventricles contract while the atria relax. The tricuspid and bicuspid valves close, preventing backward movement into the atria.
4. As ventricular pressure rises further, the semilunar valves open and blood enters the pulmonary artery and aorta.
5. The ventricles relax, the semilunar valves close, and falling ventricular pressure eventually allows filling to begin again.

The familiar lub and dub are heart sounds associated with valve closure. The first accompanies closure of the tricuspid and bicuspid valves; the second accompanies closure of the semilunar valves. A stethoscope is an instrument used to listen to sounds such as these.

What happens during pulmonary circulation?

Pulmonary circulation is the movement of blood from the right ventricle to the lungs and back to the left atrium. It takes deoxygenated blood to the lungs, where carbon dioxide is removed and oxygen enters the blood.

The pulmonary trunk is the main pulmonary artery leaving the right ventricle before dividing towards the lungs. The pulmonary veins provide the return route. Both the outward and return journeys belong to pulmonary circulation, even though the oxygen content changes in the lungs.

How can the lung route be followed?

1. The right ventricle contracts and sends deoxygenated blood into the pulmonary artery.
2. The pulmonary artery carries this blood away from the heart towards the lungs.
3. In the lung capillaries, blood gives up carbon dioxide and receives oxygen.
4. The pulmonary veins carry the oxygenated blood from the lungs to the left atrium.

The right atrium receives blood returning from the body before it enters the right ventricle. The left atrium receives blood returning from the lungs before it passes to the left ventricle. Keeping these receiving and pumping roles distinct makes the complete route easier to follow.

Why must blood return to the heart?

After receiving oxygen in the lungs, blood returns to the heart for pumping to the rest of the body. The left ventricle supplies the outgoing body route. The lungs change the blood's gas content; the heart supplies the pumping action.

Note: The pulmonary artery and pulmonary veins have opposite oxygen relationships to the familiar body arteries and veins. Their names still follow the same rule: arteries leave the heart, while veins return to it.

In a flow description, each vessel should connect the correct chamber to the correct destination. Starting with the right ventricle and ending with the left atrium gives the lung circuit without confusing it with the separate route through the body's other tissues.

Why is human blood flow called double circulation?

Systemic circulation is the route from the left ventricle through the body tissues and back to the right atrium. It distributes oxygen and nutrients to tissues and collects carbon dioxide and other wastes. It links the heart with the body's organs outside the lung circuit.

What is the body route?

1. The left ventricle contracts and pumps oxygenated blood into the aorta.
2. The aorta supplies branching arteries. Smaller arterial branches, called arterioles, lead towards capillaries in the tissues.
3. Exchange takes place across capillary walls: tissues receive useful substances and blood takes away waste materials.
4. Small collecting vessels, called venules, lead into veins that carry deoxygenated blood back towards the heart.
5. Blood returns through the vena cava to the right atrium, from which it passes into the right ventricle.

Double circulation means that a complete journey through the lung and body circuits involves two passages through the heart. One circuit connects the heart and lungs; the other connects the heart and body tissues. The circuits operate as parts of one continuous system.

The right and left sides remain separate, keeping oxygenated and deoxygenated blood from mixing. This permits an efficient oxygen supply. The separation is useful in humans and other mammals, whose bodies have high energy needs.

How does the heart muscle receive blood?

The coronary circulation is the blood supply to and from the heart muscle itself. Coronary arteries deliver blood to this muscle, and coronary veins carry blood away from it. The heart therefore has its own blood supply as well as pumping blood for other organs.

The schematic places the lungs above the heart and body parts below it. Arrows trace the pulmonary artery, pulmonary vein, aorta and vena cava. Separate vein and artery cross-sections flank the lower part of the body circuit, with a capillary drawing underneath.

See Fig. 15.4 in your NCERT textbook

When tracing the complete pathway, join the two circuits through the chambers: body to right atrium, right ventricle to lungs, lungs to left atrium, and left ventricle to body. The valves preserve forward movement within this continuous route.

What do pulse and blood pressure tell us about circulation?

The pulse is the throbbing felt in an artery as blood flows through it. Pulse rate is the number of pulse beats in one minute. It provides a way to observe the rhythm associated with the heart's pumping action.

How can the pulse be observed?

Place the index and middle fingers of the right hand on the inner side of the left wrist. Feel for the throbbing and count the beats for one minute. Record the actual count rather than replacing it with a memorised value.

A resting person usually has a pulse rate between 72 and 80 beats per minute. The word usually matters: this is not a statement that every individual must give the same reading. Observations should preserve any differences actually found.

How is pressure different from a count?

Blood pressure is greater in arteries than in veins. Systolic pressure is arterial pressure during ventricular contraction; diastolic pressure is arterial pressure during ventricular relaxation.

An instrument called a sphygmomanometer measures blood pressure. A pulse count records beats in a time interval, whereas a pressure measurement describes the force against vessel walls. They describe related aspects of circulation but are different measurements.

The heart's activity can also be represented by an electrocardiogram, a record of its electrical activity. This is abbreviated as ECG. Listening to sounds, counting a pulse and recording electrical activity observe different features of the same working heart.

How do blood groups affect donor compatibility?

The ABO blood group system classifies blood into groups A, B, AB and O. These letters are group labels. The classification depends on the presence or absence of A and B antigens, surface substances on red blood cells that can produce a protective response.

Antibodies are proteins produced in response to antigens. The labels anti-A and anti-B mean antibodies directed against A and B antigens respectively. Understanding what is on the cells and what is in the plasma helps explain compatibility.

How should the compatibility table be read?

A donor gives blood and a recipient receives it. A blood transfusion is the transfer of donated blood to a recipient. In the table, the final column lists donor groups for the group named in the first column. Nil means none of the A or B antigens or antibodies referred to in that column. The entry “anti-A, B” means antibodies against both A and B.

What do universal donor and universal recipient mean here?

In this ABO comparison, group O is called the universal donor because it appears among the donor groups for all four groups. Group AB is the universal recipient, also called universal acceptor, because its row lists all four donor groups.

Note: Blood must be carefully matched before transfusion. The Rh factor, another red-cell antigen used in blood grouping, must also be considered. The ABO labels “universal donor” and “universal recipient” do not remove the need for matching.

Incompatible blood can cause red blood cells to clump. This is why the direction of the table matters: the donor groups listed for one recipient are not automatically the groups that can receive that person's blood.

How does lymph support circulation, and what heart conditions should be distinguished?

What happens to fluid around tissue cells?

Tissue fluid is the fluid in the spaces between tissue cells. It forms when some water and small dissolved substances leave blood capillaries. Larger proteins and most of the formed elements remain in the blood vessels.

The lymphatic system is a network of vessels that collects this fluid and returns it to major veins. Fluid inside this system is called lymph. It is colourless and contains less protein than blood plasma.

Lymph contains lymphocytes, white blood cells involved in protective responses. It also transports absorbed fat from the intestine. The lymphatic system therefore provides a parallel transport route that supports blood circulation, particularly by returning excess tissue fluid.

1. Some water and small dissolved substances move out of blood capillaries into tissue spaces.
2. The tissue fluid provides a medium through which blood and tissue cells exchange materials.
3. Lymphatic capillaries collect fluid from the spaces between the cells.
4. Larger lymph vessels carry the collected fluid back towards major veins, returning it to the bloodstream.

Which terms describe different heart-related conditions?

Palpitations are a noticeable sensation of the heart beating, such as pounding, racing or fluttering. Hypertension means blood pressure higher than normal. These terms refer to different observations: awareness of the heartbeat and raised blood pressure respectively.

Cardiac arrest means that the heart stops beating. Heart failure means that the heart does not pump blood effectively enough to meet the body's needs. These definitions should not be exchanged simply because both conditions involve the heart.

A heart attack involves sudden damage to heart muscle caused by an inadequate blood supply. The coronary circulation is therefore important to the heart's own muscle, while effective pumping is important to every tissue served by the circulation.

The key distinction is between transport fluid, transport routes and pumping function. Lymph returns fluid to blood; coronary vessels serve heart muscle; valves direct blood flow; and the heart's muscular activity drives circulation through the lungs and body.

Glossary

• Circulatory system — The heart, blood and blood vessels working together to transport materials throughout the body.
• Plasma — The liquid part of blood carrying dissolved substances and suspending blood cells and fragments.
• Haemoglobin — The red, iron-containing protein in red blood cells that binds and transports oxygen.
• Platelets — Small cell fragments in blood that release substances helping the clotting process.
• Artery — A blood vessel carrying blood away from the heart towards other parts of the circulation.
• Vein — A blood vessel returning blood towards the heart from other parts of the circulation.
• Capillary — A tiny vessel with a one-cell-thick wall across which blood and surrounding cells exchange materials.
• Atrium — One of the two upper heart chambers that receive blood returning to the heart.
• Ventricle — One of the two lower heart chambers that pump blood towards the lungs or body.
• Septum — A partition separating heart chambers and helping keep blood in the right and left sides separate.
• Pacemaker — The natural signal-generating structure that initiates and maintains the rhythm of the heartbeat.
• Double circulation — The arrangement in which blood passes through the heart twice during a complete lung-and-body journey.
• Pulse rate — The number of throbbing arterial pulse beats counted during one minute.
• Blood pressure — The force exerted by circulating blood against the wall of a blood vessel.
• Lymph — Colourless fluid in lymphatic vessels that supports transport and returns tissue fluid towards the blood.

Common errors and misconceptions

• Misconception: Every artery carries oxygenated blood. Correct: Arteries carry blood away from the heart; the pulmonary artery carries deoxygenated blood towards the lungs.
• Misconception: Veins are defined by having deoxygenated blood. Correct: Veins return blood towards the heart; pulmonary veins return oxygenated blood.
• Misconception: The atria pump blood directly to the lungs and body. Correct: The atria pass blood to the ventricles, which pump it out of the heart.
• Misconception: Valves produce the pumping force. Correct: Heart muscle produces the pumping action, while valves prevent backward flow.
• Misconception: Double circulation means blood passes through the heart only once. Correct: A complete journey through the pulmonary and systemic circuits includes two passages through the heart.
• Misconception: A universal-donor label removes the need for blood matching. Correct: ABO compatibility is one consideration; blood must be carefully matched, including the Rh group.
• Misconception: Cardiac arrest and heart failure mean the same thing. Correct: Arrest means the heart stops beating; failure means its pumping is inadequate for the body's needs.

Exam-style questions with model answers

Q1. Red blood cells contain oxygen-binding haemoglobin, while platelets release substances involved in clotting. State one function of each component using this information. [2 marks]

1. Red blood cells transport oxygen because their haemoglobin binds it.
2. Platelets help blood clot, limiting excessive blood loss at an injury.

Q2. An artery carries blood away from the heart; a vein carries it towards the heart. A pulmonary artery takes deoxygenated blood to the lungs, and pulmonary veins return oxygenated blood. Explain why these vessel names are correct. [2 marks]

1. The pulmonary artery carries blood away from the heart, so its deoxygenated blood does not change its classification as an artery.
2. Pulmonary veins carry blood towards the heart, so their oxygenated blood does not change their classification as veins.

Q3. Arteries carry high-pressure blood away from the heart and have thick, elastic walls. Veins return lower-pressure blood and contain valves. Capillaries have one-cell-thick walls for exchange. Explain one structure-function link for each vessel type. [3 marks]

1. Arteries have thick, elastic walls suited to carrying blood that has left the heart under high pressure.
2. Veins contain valves that prevent backward flow and help direct their lower-pressure blood towards the heart.
3. Capillaries have walls only one cell thick, allowing materials to pass between the blood and nearby tissue cells.

Q4. The tricuspid valve lies between the right atrium and ventricle, the bicuspid valve between the left atrium and ventricle, and semilunar valves at the ventricular exits. Forward flow goes from atria to ventricles and from ventricles into arteries; valves prevent backflow. The sinoatrial node initiates rhythmic contractions. State the role of each of these four structures. [4 marks]

1. The tricuspid valve permits flow from the right atrium into the right ventricle and prevents blood flowing backwards into that atrium.
2. The bicuspid valve permits flow from the left atrium into the left ventricle and prevents backward flow into that atrium.
3. The semilunar valves prevent blood returning from the outgoing arteries into the ventricles.
4. The sinoatrial node acts as the natural pacemaker by initiating rhythmic contractions.

Q5. Use these route facts: the right ventricle pumps deoxygenated blood into the pulmonary artery; lung capillaries exchange carbon dioxide for oxygen; pulmonary veins return blood to the left atrium; blood passes to the left ventricle; the aorta supplies body tissues; veins and the vena cava return blood to the right atrium, which passes it to the right ventricle. Trace the complete route in six stages. [6 marks]

1. Begin at the right ventricle, which pumps deoxygenated blood out of the heart into the pulmonary artery towards the lungs.
2. At the lung capillaries, carbon dioxide leaves the blood and oxygen enters it, changing the blood's gas content.
3. The pulmonary veins carry this oxygenated blood back to the heart, entering the left atrium from the lungs.
4. Blood passes from the left atrium into the left ventricle, which then pumps it into the aorta.
5. The aorta supplies the body tissues, continuing the route outside the heart before blood is collected into returning veins.
6. Veins and the vena cava return blood to the right atrium; it passes into the right ventricle, completing the route.

Q6. In an ABO donor table, recipients A, B, AB and O accept donor groups A/O, B/O, AB/A/B/O and O respectively. O is termed the universal donor and AB the universal recipient in this comparison. Rh group must also be matched. Explain these two labels and give one qualification. [3 marks]

1. Group O appears in the donor list for each of the four recipient groups, explaining the label universal donor in the ABO comparison.
2. Group AB has all four groups in its donor list, explaining the label universal recipient in that comparison.
3. The Rh group must also be matched, so the ABO labels alone do not remove the need for careful matching before transfusion.

Q7. Some water and small dissolved substances leave blood capillaries, forming fluid around cells. Exchange occurs through this fluid. Lymphatic capillaries collect it; fluid inside lymphatic vessels is called lymph. Larger lymph vessels return it to major veins. Explain this sequence in four stages. [4 marks]

1. Some water and small dissolved substances pass out of blood capillaries into the spaces around tissue cells, forming tissue fluid.
2. This fluid provides a medium through which blood and tissue cells exchange materials.
3. Lymphatic capillaries collect fluid from these spaces; fluid within the lymphatic system is called lymph.
4. Larger lymph vessels carry the collected fluid towards major veins, returning it to the blood circulation.

Q8. Cardiac arrest means the heart stops beating; heart failure means pumping insufficient to meet the body's needs; hypertension means blood pressure above normal. Distinguish these conditions in three separate points. [3 marks]

1. Cardiac arrest refers to the heart stopping its beating, rather than continuing to beat with inadequate pumping.
2. Heart failure refers to pumping that is insufficient to meet the body's needs; the definition does not mean that beating has stopped.
3. Hypertension refers to raised blood pressure, describing pressure in the circulation rather than either of the two pumping states above.

Key takeaways

• The heart pumps blood through a branching vessel network that supplies useful materials and carries wastes away.
• Plasma transports dissolved substances, red blood cells carry oxygen, white blood cells defend, and platelets help clotting.
• Arteries carry blood away from the heart, veins return it, and thin-walled capillaries permit exchange.
• The heart's four chambers, separating septa and valves maintain distinct routes and forward blood movement.
• The natural pacemaker initiates rhythmic contractions, while the cardiac cycle includes contraction and relaxation.
• Pulmonary circulation connects heart and lungs; systemic circulation connects heart and body tissues in double circulation.
• ABO donor relationships explain the universal-donor and universal-recipient labels, but transfusions still require careful matching.
• The lymphatic system returns tissue fluid to blood and helps transport absorbed fat from the intestine.

Test yourself

Which blood component contains haemoglobin?

Red blood cells contain haemoglobin, the red protein that binds and transports oxygen.

What determines whether a vessel is an artery?

An artery carries blood away from the heart, irrespective of whether that blood is oxygenated or deoxygenated.

Which chamber receives blood from the pulmonary veins?

The left atrium receives oxygenated blood returning from the lungs through the pulmonary veins.

Which structure is the natural pacemaker?

The sinoatrial node in the right atrium initiates and maintains the heart's rhythmic contractions.

What prevents blood returning from the aorta into the left ventricle?

The semilunar valve at the aortic opening prevents this backward movement of blood.

Why is circulation in humans described as double?

Blood passes through the heart twice during a complete journey through the pulmonary and systemic circuits.

What is the difference between pulse rate and blood pressure?

Pulse rate counts arterial beats per minute; blood pressure describes the force blood exerts against vessel walls.

Where does collected lymph eventually return?

Lymph vessels return the collected fluid to major veins, bringing it back into the bloodstream.