Chapter 5: Physiological Structure and Life Process (Blood Circulation)

This chapter explains how blood, the heart, and blood vessels work together to transport substances throughout the human body, and describes common heart-related and blood-related health problems.

The blood circulatory system is one of the nine organ systems of the human body. Its main job is to transport nutrients, oxygen, hormones, and wastes to and from every cell of the body.

The heart, blood vessels, and blood are the three main parts of the circulatory system. The heart pumps blood, which flows through blood vessels and reaches all cells of the body.

SEE Focus: Blood transports nutrients and oxygen to cells and carries carbon dioxide and wastes away from cells.

Human blood circulatory system

Blood is a red-coloured, fluid connective tissue. An average healthy adult has about 5.5 litres of blood in the body.

Blood is alkaline in nature, with a pH ranging from 7.35 to 7.45. It is made up of 55% plasma and 45% blood cells (corpuscles).

Composition of blood (microscopic view)

Plasma is the pale-yellow, water-like liquid part of blood. It makes up 55% of total blood volume and is composed of 80-90% water and 10-20% dissolved substances such as carbohydrates, proteins, fats, and salts.

Three important proteins found in plasma are albumin, globulin, and fibrinogen.

Very Important for SEE: When fibrinogen is removed from plasma, the remaining liquid is called serum. Serum is used to diagnose various diseases.

Blood contains three types of blood corpuscles: Red blood cells, White blood cells, and Platelets.

Red blood cells are red, biconcave, round, and without a nucleus. They are also called erythrocytes.

RBCs contain a red pigment called haemoglobin, made of iron and a protein called globin. Iron in haemoglobin carries oxygen to the cells, and haemoglobin also carries carbon dioxide back to the lungs.

Red blood cells

White blood cells have no definite shape, are larger than RBCs, possess a nucleus, but do not contain haemoglobin. They are also called leucocytes.

WBCs are of two types based on granules in their cytoplasm:

Types of white blood cells

SEE Focus: WBCs fight disease-causing germs and increase the body's immunity, so they are called the soldiers of the human body.

Platelets are the smallest, round, non-nucleated blood cells. About 2 to 4 lakh platelets are present per cubic millimetre of blood.

Platelets combine with fibrinogen to clot blood during cuts and injuries.

Platelets

Blood transports carbon dioxide, oxygen, nutrients, enzymes, hormones, and other essential substances between different parts of the body.

Blood regulates body temperature and keeps the amount of water and salts constant.

White blood cells protect against infections, platelets stop bleeding, and antibodies produced in blood maintain immunity.

Antigens A and B are found on the surface of red blood cells. Blood grouping is based on the presence or absence of these antigens.

Test of human blood group

Blood also contains, or lacks, the D antigen (Rhesus/Rh factor). If Rh factor is present, the blood group is positive (+ve); if absent, it is negative (-ve). Combining ABO groups with Rh factor gives 8 blood groups in total: A+, A-, B+, B-, AB+, AB-, O+, O-.

Very Important for SEE: Blood group must always be confirmed before blood transfusion, because mismatched blood can cause fatal intravenous clumping.

The heart is the central, muscular organ of the circulatory system that pumps blood to every cell and tissue of the body. It is made of cardiac muscle and lies in the thoracic cavity between the two lungs, tilted slightly to the left.

The heart is enclosed by a double-layered protective membrane called the pericardium, with pericardial fluid between the layers that protects the heart from shocks and injuries.

The heart is about the size of the owner's closed fist and weighs about 300 grams on average. It is triangular or conical in shape.

The coronary artery supplies oxygenated blood to the heart itself, while the coronary sinus vein carries deoxygenated blood away from the heart to the right auricle.

External structure of the human heart

The human heart has four chambers: two upper auricles (atria) and two lower ventricles. A thick muscular septum divides the heart into left and right halves and prevents mixing of oxygenated and deoxygenated blood.

Auricles receive blood from the body, while ventricles pump blood out to the body. Ventricle walls are thicker than auricle walls because ventricles must pump blood with great pressure. The left ventricle wall is thicker than the right ventricle wall because the left ventricle pumps blood to the whole body, while the right ventricle only pumps blood to the lungs.

The superior vena cava (from the upper body) and inferior vena cava (from the lower body) bring deoxygenated blood into the right auricle. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary vein carries oxygenated blood from the lungs to the left auricle.

Very Important for SEE: The pulmonary artery is the only artery that carries deoxygenated blood, and the pulmonary vein is the only vein that carries oxygenated blood.

After the left auricle receives pure (oxygenated) blood, it passes to the left ventricle, which pumps it to the whole body through the aorta — the largest artery in the body.

Internal structure of the human heart

Both auricles contract together first, then both ventricles contract together. As each ventricle contracts, its inflow valve closes and its outflow valve opens.

A heart attack (also called myocardial infarction) happens when the flow of blood to the heart tissue is suddenly reduced or blocked, so the heart cannot work properly. It is a serious emergency condition.

It occurs when the coronary artery becomes narrow due to excess deposition of fat or cholesterol (called plaque) on its inner wall. Sometimes plaque breaks down and forms a clot (thrombus) that blocks blood flow and damages heart tissue.

Condition of a heart attack

Diagnosis includes checking blood pressure, pulse, and temperature, along with tests such as Electrocardiogram (ECG), blood test, Echo, Angiography, CT Coronary Angiogram, and MRI.

Angiography is a fluoroscopy X-ray technique used to detect blockages in blood vessels. A special dye called a contrast medium is injected into the blood to clearly highlight blood flow. The resulting X-ray image is called an angiogram.

This procedure (also called percutaneous coronary intervention, PCI) opens a clogged coronary artery. A cardiologist inserts a thin, flexible tube called a catheter through a blood vessel in the hand or leg and guides it to the narrowed artery. A stent (small metal mesh tube), often coated with medicine, is placed to widen the vessel and maintain blood flow.

Also called open-heart surgery, this emergency procedure creates a new pathway for blood using a healthy blood vessel taken from the leg, so that heart tissue receives nutrient- and oxygen-rich blood again.

Coronary artery bypass surgery

A heartbeat is the sound produced by the regular, rhythmic contraction and relaxation of heart muscle. A stethoscope is used to measure heartbeat.

At rest, the heartbeat of a healthy adult is 60-100 times per minute — this is called the heart rate.

The pressure exerted by flowing blood on the artery walls, felt from outside, is called the pulse. Heart rate and pulse rate are normally equal and used synonymously. Pulse can be felt at the radial artery (wrist) or carotid artery (neck).

Measuring the pulse rate

Blood vessels are flexible, muscular tubes that carry blood to and from cells and tissues. There are three types: arteries, veins, and capillaries.

Types of blood vessels (artery, vein, capillary)

An artery carries blood away from the heart to different parts of the body. Arteries are highly muscular to withstand blood pressure and have no valves. The aorta is the largest artery in the human body.

Arterioles divide to form a fine network of thread-like capillaries. Capillaries supply nutrients, oxygen, hormones, and enzymes to cells, and collect wastes such as carbon dioxide and urea from cells.

A vein carries blood from body organs back to the heart. Vein walls are thinner than artery walls. Because blood flows in veins with less speed and pressure, veins contain valves at intervals to prevent backflow of blood.

Blood circulation is the process by which blood is transported from the heart to body parts and back to the heart. The heart pumps about 5-6 litres of blood per minute in an adult. Blood circulation is of two types: systemic and pulmonary.

In systemic circulation, oxygenated blood from the left ventricle flows through the aorta and its branches to all parts of the body, and deoxygenated blood returns from these organs to the right auricle through veins.

Pathway: Left ventricle → aorta → arteries/arterioles → capillaries (deliver oxygen/nutrients to cells) → venules → veins → venacava → right auricle.

In pulmonary circulation, deoxygenated blood flows from the right ventricle through the pulmonary artery to the lungs, where it releases carbon dioxide and picks up oxygen. Oxygenated blood then returns to the left auricle through the pulmonary vein.

Pathway: Right ventricle → pulmonary artery → lungs (gas exchange at alveoli) → pulmonary vein → left auricle.

Systemic and pulmonary blood circulation

Blood pressure is the pressure created by blood on the walls of blood vessels, especially arteries, when the left ventricle contracts and pumps blood. It is measured in mmHg using a device called a sphygmomanometer.

Normal blood pressure of an adult is written as 120/80 mmHg.

If blood pressure exceeds 140/90 mmHg, it is called high blood pressure or hypertension.

Diabetes (hyperglycemia) is a condition where insulin hormone is insufficient, causing blood glucose (sugar) level to rise above normal.

Uric acid is produced when purine-containing foods are broken down during digestion and metabolism. It is mainly produced in the intestine and liver and removed by the kidney. Excess uric acid in the blood is called hyperuricemia.