The Circulatory System Class 8 ICSE (2023

To carry out the various life processes of humans/animals, the role of transport of substances through the body is very essential. The food we eat, the air we inhale, the water we drink, the energy we receive through these nutrients are brought to different parts of the body, and even the removal of waste products generated from these are carried through circulatory system.

In human beings, the circulatory system comprises of Blood Vascular System and also lymphatic system. First of all, we will get to know about the Blood Vascular System in detail and later in brief about the lymphatic system.

1. The Blood Vascular System (Click to read in Detail)

The blood Vascular System comprises blood, blood vessels and the heart. These three constituents of our body make our blood vascular system.

1.1 Blood

It is a liquid connective tissue.

Its constituents are plasma and blood cells, blood cell comprises of Red Blood Corpuscles (RBCs), White Blood Corpuscles (WBCs) and blood platelets. They are suspended in the plasma.

RBCs help in transport of oxygen to the body and carbon dioxide to the lungs. WBCs help in fight against the infections by engulfing and destroying pathogens. Platelets help in the clotting of blood which helps in preventing the excessive blood flow in case of any injury.

1.1.1 Uses of blood

Blood helps in transportation of oxygen and carbon dioxide to body.

Blood helps in transport of nutrients and hormones to different parts of body and removal of urea, uric acid and salts through excretory system.

It helps in regulating the temperature of the body.

It also maintains the water balance of the body as well.

1.2 Blood groups and transfusion of blood (Click to read in detail)

Blood has been classified by Karl Landsteiner in 1900 in to four groups. These blood groups are differentiated on the basis of antigen they produce. Antigens are complex chemical substances found on the surface of RBCs.

Blood Groups

The four blood groups are group A, group B, group AB and group O. If antigen present on a person RBCs is A, then his/her blood group will be A. If antigen present on a person is B, then his/her blood group will be B. If antigen present on a person is AB, then his/her blood group will be AB. And lastly if there is no antigen present on a person, then his/her blood group will be O.

Blood Groups and Antigen

Blood Transfusion

The process in which an injured person or the person in need of blood is transfused with healthy blood of healthy person is known as Blood Transfusion.

Donor: The person who gives the blood is known as donor.

Recipient/receiver: The person who receives the blood is known as recipient/receiver.

[A person can receive blood only from people who have certain blood groups. Similarly, a person can donate blood only to people of certain blood group. This is because the recipients blood rejects the blood of opposite antigen which is not present in it. In this case the recipient's blood considers another blood as foreign substances and recipients blood produce antibodies against these. Blood transfusion of different blood groups can cause to death because of clotting of blood vessels.]

The transfusion between the people having the same blood group can be done. Like the people having blood group A can donate to or receive from people having only blood group A.

People with blood group AB have both antigens A and B, and therefore it does not produce any antibodies against any of them. They can receive blood from all groups and therefore are called universal recipients.

In another case, people with blood group O do not have any antigen neither A nor B, but it do produce antibodies. They cannot receive blood from all groups but can donate to all and therefore are called universal donors.

A detailed table representing blood group, antigen and antibody they produce, to whom they can donate blood to and by whom they may receive blood from:

Chart representing blood group, antigen and antibody, donor and recipient

Rh Factor (Rhesus factor)

Another factor that need to be checked is Rh factor which has been discovered originally in a type of monkey known as the Rhesus monkey. It is of two types, people who possess these antigens are called Rh-positive (Rh+), and those who do not are called Rh-negative (Rh-). Donor or recipient must be Rh+ or Rh-.

1.3 Blood Vessels

Blood vessels are closed network for the transport of blood from heart to all parts of the body and vice-versa. These are of three main types - (i) Arteries, (ii) Capillaries, and (iii) Veins.

(i) Arteries

Arteries generally carry oxygenated blood from heart to various parts of the body while the arteries which carry deoxygenated blood are exception.

These are thick and elastic muscular walls which can withstand the pressure of the blood being pumped by the heart.

Arteries are further divided into smaller branches called arterioles. which further divided into smaller branches called capillaries.

(ii) Capillaries

The smaller branches of arteries i.e. arterioles are further divided into smaller and smaller branches which are called Capillaries.

Capillaries are very thin walled made up of single layer of epithelial cells. The small size of capillaries make them possible to reach all parts of the body.

It is also responsible for transfer of oxygen and nutrients at cellular level and transfer of removal of carbon dioxide and other materials from cells.

These capillaries in upward movement combines to form venules which ultimately join to form veins.

(iii) Veins

Most veins carry deoxygenated from the various parts of the body to the heart. The veins that carry oxygenated blood from lungs to the heart are the exceptions.

Veins have a wider lumen than arteries which help in taking deoxygenated blood from the cells and have thinner walls.

Blood flows through the veins with less pressure than the arteries. There are valves present in veins which prevents the back flow of blood.

1.4 Heart

Heart is a muscular organ located between the lungs with its tip pointing towards left. Its shape and size of almost shape and size of a closed fist.

Heart is surrounded by a protective double membraned bag called the pericardium. The space between the membrane is filled with pericardial fluid, which acts as a shock absorber.

An adult's heart measures about 12 cm in length and 9 cm in width, and weighs about 300 g.

Heart is made up of cardiac muscles which works without the rest and keep the heart functional throughout the life.

1.4.1 Structure of heart

The heart consists of four chambers. The two upper chambers have thin walls and are called right auricle and left auricle*. The two lower chambers have thick walls and are called right ventricle and left ventricle.

*The auricles are also called atria (Singular: atrium).

Fibrous walls separate the two sides of the heart to prevent the mixing of bloods. The fibrous walls that separate the two auricles is called interatrial septum and are thin. The fibrous walls that separate the two ventricles are called interventricular septum and are thicker than the interatrial septum.

Oxygenated and deoxygenated blood enter from different sides of the heart and are prevented from mixing by these fibrous walls.

The right auricle and right ventricle are separated by a tricuspid valve, it has three cups that close to prevent the back flow of blood from the right ventricle into the right auricle.

Similarly, the left auricle and left ventricle are separated by a bicuspid valve, which has two cups that prevent the back flow of blood.

2. Blood Circulation

The heart regulates the circulation of blood and the constant pumping actions of the cardiac muscles keep the blood circulation going.

In easy words the heart pumps oxygenated blood received from the lungs to the rest of the body and deoxygenated blood from the rest of the body to the lungs. Therefore, blood goes twice through the heart to complete one cycle. This type of circulation is called double circulation. It has two parts - systematic circulation and pulmonary circulation.

A systematic circulation carries blood to and from the rest of the entire body, while pulmonary circulation carries blood to the lungs and back to the heart.

In systematic circulation, oxygenated blood from the left auricle passes into the left ventricle and is pumped out of the heart through a major artery, called the aorta. At the entry of aorta, there is aortic valve, which prevents backflow of blood into the left ventricle. Two coronary arteries arise from the aorta just after it leaves the left ventricle. These supply oxygenated blood to the cardiac muscles. These oxygenated blood flows into the smaller arteries which are coming out of the aorta and ultimately in to the capillaries supplying oxygenated blood to various parts of the body.

The deoxygenated blood after the respiration process at cellular level flows back into the heart after passing through network of capillaries and veins. It enters the right auricle through two major veins - superior vena cava and inferior vena cava. Deoxygenated blood from the cardiac muscles is carried into the right auricle by the cardiac veins. These deoxygenated blood from the right auricle passes into the right ventricle.

In pulmonary circulation, deoxygenated blood from the right ventricle is carried to the lungs by pulmonary artery, which branches into two. The pulmonary valve guards the entrance of the pulmonary artery. Blood carrying carbon dioxide reaches the alveoli of the lings, where it is oxygenated. The oxygenated blood enters the left auricle through the pulmonary veins and this cycle keeps repeating.

Heart anatomy image for blood circulation in body J K Online Classes

Image from Freepik (J K Online Classes)

3. Heartbeat and its control

In heart the auricle contracts first to pump blood into the ventricles which relax at the same time to let the blood in. The ventricle contracts with great force and almost immediately and push blood into the aorta and the pulmonary artery. Next the auricles relax to let in blood from the superior and inferior vena cava, and the pulmonary veins. This whole process occurs in less than one second and is called the cardiac cycle.

The rhythmic contraction of the heart muscles is called heartbeat.

3.1 Sinoatrial node and atrioventricular node

The cardiac muscles generate electrical signals that stimulates cardiac muscles to contract. These electrical signals are generated by a specialised group of cardiac muscle cells called the sinoatrial node (SA node) present in the upper wall of the right atrium. These cardiac muscle cells electrically stimulate other cardiac muscles to contract in a continuous rhythm. The sinoatrial node is called the primary pacemaker.

The electrical signal spreads from the sinoatrial node to the walls of the atria and stimulate them to contract. From there, the signal travels in the atrioventricular node (AV node) at the interatrial septum.

The atrioventricular node is another group of specialised cardiac muscle cells that can generate electrical signals to control the heartbeat. It is called the secondary pacemaker of the heart. The signal is delayed a bit at the atrioventricular node to allow the ventricles to fill with blood.

If the sinoatrial node cannot function, the atrioventricular node takes up the function. Together, these and other structures of the heart constitute the electrical conduction system of the heart.

3.2 Artificial Pacemakers

If the natural pacemakers are unable to work properly, the heart may beat too fast, tooslow or irregularly. An artificial pacemaker is then fitted in the heart to restore the normal rhythm of the heart.

4. Heart Condition

The abnormal functioning of the pacemakers is one of the several heart conditions that one can develop. Let us learn some of these conditions:

4.1 Hypertension

While measuring blood pressure, we actually measure arterial pressure, which is the pressure of the blood on the walls of the arteries. Blood pressure or arterial pressure is expressed as the value of the maximum pressure (during a heartbeat) over the minimum pressure (between two heartbeats). The maximum pressure is called the systolic pressure, and the minimum pressure is called the diastolic pressure. The normal range of systolic pressure is about 100-140 millimetres of mercury (mm Hg), while the normal range of diastolic pressure is about 60-90 mm Hg.

When a person's systolic pressure exceeds 140 mm Hg, and diastolic pressure exceeds 90 mm Hg, the person is said to have High blood pressure, or hypertension.

Causes of Hypertension

Hypertension may be caused by kidney diseases, genetic factors or certain medications.

Women may develop hypertension due to pregnancy.

Elderly people are more prone to developing hypertension due to the loss of elasticity of their arterial walls.

High body weight, smoking, excessive consumption of fatty foods and alcohol, an unhealthy lifestyle, and emotional stress can also lead to hypertension.

Effect of Hypertension

Hypertension can damage the inner lining of the arterial walls, promoting the deposition of fatty materials, particularly cholesterol, and other substances. This leads to the narrowing arteries and may even block arteries completely. Blockage may rupture arteries, causing the formation of bloods clots. A blood clot in an artery of the brain or the heart can be fatal (lead to death).

Hypertension can also damage the eyes, kidneys and other organs of the body.

Remedies of Hypertension

A person suffering from hypertension should exercise regularly, have a healthy diet and avoid smoking, consuming tobacco in any form and drinking alcohol.

4.2 Heart attack

If a coronary artery is blocked, the muscles of the heart do not receive enough oxygen and get weakened. This can cause severe problems, depending on the extent of the blockage. The complete blockage of blood supply to any part of the heart results in a heart attack.

A heart attack causes chest pain and may cause permanent damage to the heart. in some cases, the heart continues to pump blood to the rest of the body. However, in other, the heart may stop functioning altogether. This condition is called cardiac arrest.

4.3 Cardiac arrest

Cardiac arrest occurs when the heart stops beating suddenly and blood stops flowing around the body.

A person who has cardiac arrest usually becomes unconscious, has no pulse, and may stop breathing or breathe abnormally. If not treated quickly, cardiac arrest results in death.

Causes of cardiac arrest

Cardiac arrest may result from a lack of oxygen, severe blood loss, heart attack or malfunctioning of the electrical conduction system.

Treatment of cardiac arrest

A cardiac arrest is treated with external cardiac massaging (chest compression), and blowing air into the patient's lungs at intervals to keep oxygenated blood circulating through the body. This process is called cardiopulmonary resuscitation (CPR).

A device called defibrillator is ultimately used to give electric shocks through the chest to restart the heart or restore its natural rhythm.

Difference between heart attack and cardiac arrest

Heart attack and cardiac arrest are different. While a heart attack is caused by a lack of blood flow to the heart muscles, a cardiac arrest can be caused by many conditions, including a heart attack.

4.4 Palpitations

It is a condition when the heart beats too hard or too fast or skips a beat. Palpitations can be felt in the chest, throat or neck, and can range from a fluttering feeling to intense discomfort.

In some cases they may be associated with chest pain, dizziness, breathing trouble or nausea.

Causes of palpitations:

Palpitations may occur due to noncardiac causes. These include physical exertion, emotional stress, low blood sugar, low blood pressure, anaemia, fever, dehydration, hormonal changes and deficiencies of minerals such as potassium.

It can also occur due to the excessive consumption of coffee, alcohol, tobacco and certain drugs.

Diseases of the cardiac muscles and valves can also cause palpitations.

5. The Lymphatic System

The lymphatic system plays a major role in the working of the immune system of the body. It consists of lymph, lymph vessels and lymph nodes. The lymphatic system also includes the thymus gland and the spleen. Bone marrow also forms a part of the lymphatic system.

Blood flow of human heart with lymphatic system (J K Online Classes)

The lymphoid organs and bone marrow produce a type of WBC called lymphocyte, which produces antibodies in response to foreign bodies entering the blood. Lymphocytes also recognise and kill harmful cells such as cancer cells and virally infected cells.

Lymph is a straw-coloured liquid formed from blood. Blood flows through the arterioles and capillaries under high pressure. The high pressure forces a clear fluid to pass through the thin walls of capillaries and accumulate in the space between the neighbouring cells. These fluids are called tissue fluid. Tissue fluid supplies oxygen and nutrients to the surrounding cells and collects carbon dioxide and waste products from them.

The accumulation of tissue fluid in the intercellular spaces would cause swelling within our body, hence it is regularly removed from the intercellular spaces and flows into lymph capillaries as lymph. Lymph capillaries join to form larger lymph vessels. The lymph vessels ultimately drain into the bloodstream.

Lymph also carries digested fats from the small intestine into the blood vessels. These fats are transported to the liver for processing before being sent to the cells of the body via the systematic circulation.

The lymph nodes are oval or kidney-shaped masses of lymphatic tissue. They are found throughout the body, particularly in the neck, armpits, groin and abdomen. A large number of lymphocytes are present in the lymph nodes. Lymph nodes filter and destroy bacteria and other harmful cells from the lymph that passes through them. The lymph nodes become swollen in response to infections and serious diseases like cancer.

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