Revision: Human Anatomy and Physiology >> The Circulatory System Biology (English Medium) ICSE Class 10 CISCE

An open blood circulatory system is found in animals like insects, where blood flows freely through body cavities without confined blood vessels.

A closed blood circulatory system is one in which blood flows entirely within blood vessels, as seen in humans and other vertebrates.

Blood is a fluid that circulates in the heart, arteries, capillaries, and veins of vertebrate animals. It transports nutrients and oxygen throughout the body and removes waste.

The plasma from which the protein fibrinogen has been removed is called serum. 

Phagocytosis is a process in which most WBCs, particularly the neutrophils, engulf particle-like solid substances, especially bacteria.

The enzyme released by injured tissues and disintegrating platelets that initiates clotting is called thrombokinase (also known as Thromboplastin or Factor X or Stuart factor).

The solid mass left behind after clotting, formed by fibrin and trapped blood cells, is called clot or thrombus.

Diapedesis is the movement of white blood cells from lymph capillary walls to neighbouring tissues, often for immunological defence.

Rhesus factor is a hereditary protein present in red blood cells. If present, the individual is Rh-positive; if absent, they are Rh-negative.

Blood transfusion is the process of introducing blood from a healthy donor into the bloodstream of a patient, typically through a vein, often during surgery or after heavy blood loss.

A Blood Recipient is a person who receives blood during a transfusion, requiring compatibility with the donor's blood group.

Antigens are specific proteins present on the surface of red blood cells that determine an individual’s blood group (e.g., Antigen A or Antigen B).

Antibodies are proteins present in blood plasma that react against specific antigens not found on the individual's own red blood cells, playing a key role in blood group compatibility.

A person with blood group AB is called a universal recipient because they can receive blood from all major blood groups (A, B, AB, and O) without risk of incompatibility.

Arteries are the blood vessels in which oxygenated blood is going away from the heart to all parts of the body.

Veins are the blood vessels that carry deoxygenated (impure) blood from the body cells to the heart.

The circulatory system is a transport system moving substances throughout our body with the help of blood.

The heart is a hollow muscular vertebrate organ that pumps blood through rhythmic contractions.

The complete sequence of events that occurs during one heartbeat, including contraction and relaxation of the atria and ventricles, is called the cardiac cycle.

The smallest or the final branch of an artery is called an arteriole.

An ARTERY is a vessel that carries blood away from the heart towards any organ.

A VEIN is a vessel that carries blood away from an organ towards the heart.

A venule is the smallest branch of a vein formed by the union of capillaries. It has a thin muscular coat and gradually joins with other venules to form larger veins.

Double circulation is the process in which blood passes through the heart twice during one complete cycle—once for pulmonary circulation (to and from the lungs) and once for systemic circulation (to and from the body).

The hepatic vein is the blood vessel that carries blood away from the liver and drains it into the posterior vena cava, completing the hepatic portal system.

Pulse is the rhythmic expansion and elastic recoil of an artery caused by the contraction of the heart during ventricular systole.

A pulse is the rhythmic expansion and contraction of arteries (especially the radial artery at the wrist) due to the pumping action of the heart.

The sound or movement of the heart as it sends blood around the body is called heartbeat. Every heartbeat causes a pulse in the arteries.

A sphygmomanometer is the instrument used to measure blood pressure in the arteries.

Blood pressure is the pressure that the blood flowing through the arteries exerts on their walls.

Diastolic pressure is the lower limit of blood pressure, recorded when the heart is at rest and the pressure in the arteries drops after the pulse wave has passed.

Hypertension is a condition in which the blood pressure consistently remains above 140/90 mm Hg.

As the blood flows in the capillaries of the tissues, the plasma and the leukocytes "leak out" through their walls. This fluid bathes the cells and is called the tissue fluid or the intercellular or extracellular fluid.

Lymph is the fluid formed from tissue fluid that enters lymph vessels. It helps in returning excess fluid to the blood and plays a role in immunity.

1. Nutrients from the digestive system are transported to all body cells.
2. Oxygen from the lungs is delivered to tissues, while carbon dioxide is carried back for exhalation.
3. Excess water, salts, and nitrogenous wastes like urea are transported to excretory organs.
4. Hormones from endocrine glands are circulated to target organs via the blood.
5. Blood and lymph not only transport substances but also perform other essential functions in the body.

1. RBCs are biconcave, disc-shaped cells without a nucleus, mitochondria, or endoplasmic reticulum in their mature form.
2. They contain haemoglobin, which transports oxygen as oxyhaemoglobin and a small amount of CO₂ as carbaminohaemoglobin.
3. RBCs are produced in the bone marrow of long bones and have an average lifespan of about 120 days.
4. Old RBCs are destroyed in the spleen, liver, and bone marrow; their iron is retained, and the rest forms bile pigment.
5. RBC count is higher in newborns, during physical activity, and at high altitudes, and lower during sleep.
6. Abnormalities include polycythaemia (increased red blood cells) and erythropenia (decreased red blood cells).

1. WBCs are produced in red bone marrow, lymph nodes, and sometimes in the liver and spleen; their average lifespan is about two weeks.
2. Most WBCs are amoeboid and squeeze through capillary walls into tissues by diapedesis to fight infections.
3. They are classified into granular (neutrophils, eosinophils, basophils) and non-granular (lymphocytes, monocytes) types.
4. Neutrophils and monocytes perform phagocytosis, engulfing germs and damaged cells.
5. Lymphocytes produce antibodies to neutralise specific germs, forming the basis of immunity and vaccination.
6. WBC count increases during infection, stress, or leukaemia (leukocytosis) and decreases in viral illness or bone marrow disorders (leukopenia).

1. Platelets have a short lifespan of 3 to 5 days and are mainly destroyed in the spleen.
2. At the site of injury, platelets disintegrate and release thrombokinase (also called thromboplastin or Factor X), initiating clot formation.
3. Thrombokinase, in the presence of calcium ions, converts inactive prothrombin in the plasma into active thrombin.
4. Vitamin K is essential for the synthesis of prothrombin in the liver.
5. Thrombin converts soluble fibrinogen into insoluble fibrin, forming sticky threads at the wound.
6. The fibrin mesh traps blood cells, forming a clot that seals the wound and stops bleeding.
7. The clot contracts, squeezing out serum, and leaves behind a solid mass called a thrombus (clot).

1. Blood transfusion involves transferring blood from a healthy donor to a patient and requires strict blood group compatibility.
2. The ABO system divides blood into four types—A, B, AB, and O—based on antigens present on RBCs and corresponding antibodies in plasma.
3. Blood group O is known as the universal donor, while group AB is the universal recipient due to compatibility with all types.
4. The Rh system is based on the presence (Rh⁺) or absence (Rh⁻) of the Rh factor (D antigen) on RBCs, discovered in Rhesus monkeys.
5. Rh incompatibility in pregnancy may lead to the mother's sensitisation, causing complications for future Rh⁺ pregnancies, including foetal death or abortion.

1. The tricuspid valve (right atrio-ventricular valve) is located between the right atrium and right ventricle; it has three cusps held by chordae tendineae attached to papillary muscles.
2. The bicuspid or mitral valve (left atrio-ventricular valve) lies between the left atrium and left ventricle and has two cusps.
3. The pulmonary semilunar valve is located at the opening of the pulmonary artery from the right ventricle; it has three pocket-shaped cusps.
4. The aortic semilunar valve is found at the origin of the aorta from the left ventricle and also has three pocket-shaped cusps.

1. The cycle begins with atrial contraction (atrial systole) while the ventricles are relaxed; blood flows easily from atria into ventricles through open cuspid valves.
2. As ventricles contract (ventricular systole) and atria relax, pressure closes the cuspid valves, preventing blood from flowing back into the atria.
3. Chordae tendinae, attached to papillary muscles, hold the cuspid valve flaps in place and prevent their inversion during ventricular contraction.
4. Blood is pushed out of the ventricles into the pulmonary artery and aorta through semilunar valves, which open under pressure from below.
5. When ventricles relax again, blood tends to return, but the semilunar valves fill and close, preventing backflow into the heart.

1. Lymph is formed from tissue fluid and flows through lymph vessels due to the contraction of nearby muscles.
2. Lymph contains only WBCs (mainly lymphocytes) and is free from RBCs and platelets; it is made up of 94% water and 6% solids like proteins and fats.
3. It supplies nutrients and oxygen to areas where blood cannot reach, especially in deep tissues.
4. Lymph plays a role in draining excess tissue fluid and returning proteins and waste products to the bloodstream.
5. It helps in absorbing fats from the intestine and defending the body by removing bacteria and housing immune cells like lymphocytes.

1. The spleen stores blood and releases it during emergencies like haemorrhage or stress.
2. It produces lymphocytes to help in immune defence.
3. The spleen helps destroy old or damaged red blood cells, along with the liver.
4. In the embryo, the spleen functions as a site for RBC production.