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Systems and Sub-systems

Class 8
May 8, 2023

Systems and sub-systems – Circulatory system


The human circulatory system functions to transport blood and oxygen from the lungs to the various tissues of the body.

The circulatory system circulates food, water, and air throughout the body. The most important means of transport in the circulatory system is the blood. Blood is a fluid connective tissue present in our body that circulates food, water, and air to different parts of the body.

Circulatory System

Three main components of the circulatory system:



Blood vessels




Size: Approximately the size of the clenched fist of a person.

Nature: Highly muscular, made of cardiac muscles, and richly supplied with blood vessels.

The heart has four chambers — two on top and two on the bottom.


The two bottom chambers are the right ventricle and the left ventricle. These pump blood out of the heart. A wall called the interventricular septum is between the two ventricles.

The two top chambers are the right atrium and the left atrium. They receive the blood entering the heart. A wall called the interatrial septum is between the atria. The atria are separated from the ventricles by the atrioventricular valves.

The tricuspid valve separates the right atrium from the right ventricle. In addition, a bicuspid valve (mitral valve) is present between the left auricle and left ventricle.

There are two valves that separate the ventricles from the large blood vessels that carry the blood leaving the heart.

  1. The pulmonic valve between the right ventricle and the pulmonary artery carries blood to the lungs.
  2. The aortic valve between the left ventricle and the aorta carries blood to the body.

A double-layered membrane called pericardium covers the heart. The two sides of the heart are like 2 pumps that work together to supply blood from the heart to all parts of the body.


Blood Vessels

Blood vessels form a network through the body that allows for the transportation of blood, food, waste, and other materials. The three major types of blood vessels are arteries, veins, and capillaries.

  • Arteries

Arteries carry blood away from the heart (to either the body or the lungs), while veins carry blood to the heart. The largest artery is the aorta, which connects to the heart and picks up oxygenated blood from the left ventricle. The only artery that picks up deoxygenated blood is the pulmonary artery, which runs between the heart and lungs.

  • Capillaries

The arteries eventually divide down into the smallest blood vessel, the capillary.

  • Veins

Veins have one-way valves instead of muscles to stop blood from running back the wrong way.


It is composed of plasma, red blood cells, white blood cells, and platelets. Blood is responsible for the transportation of oxygen, hormones, and nutrients. Blood also functions as a component of the immune system to protect against infection, repair body tissue, and maintain pH.

Component of BloodDescription of ComponentFunction of Component
PlasmaThe liquid part of blood: mainly waterCarries the blood cells around the body; carries dissolved nutrients, hormones, carbon dioxide, and urea; distributes heat around the body.
Red Blood Cells (erythrocytes)Biconcave, disc-like cells with no nucleus; millions in each mm3 of blood.Transport of oxygen contains mainly hemoglobin, which loads oxygen in the lungs and unloads it in other regions of the body.
White Blood Cells:
LymphocytesAbout the same size as red cells with a large spherical nucleus.Produce antibodies to destroy microorganisms. Some lymphocytes persist in our blood after infection, giving us immunity to specific diseases.
PhagocytesMuch larger cells with a large spherical or lobed nucleus.Engulf bacteria and other microorganisms that have infected our bodies.
PlateletsThe smallest cells are really fragments of other cells.Release chemicals to make blood clot when we cut ourselves.

Circulation of Blood

Blood flows through the vessels and heart in the following pattern:

All veins of the body → vena cava → right auricle → right ventricle → pulmonary artery → lungs → pulmonary vein → left auricle → left ventricle → aorta → arteries → capillaries in all organs → veins (the cycle is repeated).

Blood is pumped once through the left auricle, left ventricle, and the lungs (pulmonary circulation) and once through the right auricle, right ventricle, and aorta (systemic circulation). Thus, blood enters the heart twice during each cycle. This is called double circulation.

The pulmonary vein is the only vein to carry pure blood.

The pulmonary artery is the only artery to carry impure blood.

Functions of Blood

Transports nutrients from the site of digestion and absorption to all parts of the body.

Transports oxygen from lungs to tissues and carbon dioxide from tissues to lungs.

Transports wastes from all parts of the body (ammonia, uric acid, urea, carbon dioxide) to sites of removal (kidneys and lungs).

Transports hormones and enzymes.

WBCs protect the body from diseases and infection-causing organisms.

The heat produced in our body is transported to all parts. Thus, the constant temperature of 37°C is maintained.

Platelets in the blood contain a clotting mechanism to prevent blood loss.

It maintains the levels of necessary elements like sodium and potassium.

Heartbeat and heart rate

One complete contraction and relaxation of the ventricles is called a heartbeat. During a heartbeat, blood flows from the auricles → ventricles, → arteries.

The number of times a heart beats in a minute in a person is called the heart rate (normal heart rate – 70- 72/min for adults; about 92-100/min for children).

Instrument to measure heartbeat- stethoscope.

Blood pressure is measured using an instrument called a sphygmomanometer. The normal BP is 120/80 mm Hg.

Heart rate increases during physical activity or when we are frightened, angry, stressed, or excited.

Common problems

Some common problems of the circulatory system include:

Aneurysm: A weak spot in the wall of an artery.

Atherosclerosis: A narrowing of the arteries caused by plaque deposits.

Heart disease: Lack of blood supply to the heart because of narrowed arteries.

High blood pressure: Can be caused by obesity (among other things).

Varicose veins: Problems with the valves that stop blood from running backward.

Circulatory System


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