Blood: Type, Definition,Composition, Functions, Components

Blood: Definition and Importance

Blood is a liquid connective tissue that consists of cells surrounded by a liquid extracellular matrix. The extracellular matrix is called blood plasma, and it suspends various cells. Blood courses in the arteries and veins throughout the body, carrying oxygen and nutrients to metabolic wastes. Its part in homeostasis assists in performing the functions of the body. Some of the physical characteristics of blood are:

• Blood is denser and more viscous than water and feels slightly sticky.

• The temperature of blood is 38°C (100.4°F), and it has a slightly alkaline pH ranging from 7.35 to 7.45.

• The colour of blood varies with its oxygen content. When saturated with oxygen, it is bright red. When unsaturated with oxygen, it is dark red.

• The blood volume is 5 to 6 litres in an adult male and 4 to 5 litres in an adult female.

Blood Composition: Plasma, RBCs, WBCs, Platelets

The two major components of blood are: blood plasma, a watery liquid extracellular matrix that contains dissolved substances, and blood cells. Blood is about 45% formed elements and 55% blood plasma.

Plasma

When the blood cells are removed from the blood, a straw-colored liquid called blood plasma remains. Blood plasma is about 91.5% water and 8.5% solutes, most of which are proteins. Plasma proteins include the albumins (54%), globulins (38%), and fibrinogen (7%). Besides proteins, other solutes in plasma include electrolytes, nutrients, regulatory substances, gases, and waste products.

Blood Cells

The blood cells include three principal components: red blood cells, white blood cells, and platelets. Red blood cells (RBCs) or erythrocytes transport oxygen from the lungs to body cells and deliver carbon dioxide from body cells to the lungs. White blood cells (WBCs) or leukocytes protect the body from invading pathogens and other foreign substances. Platelets release chemicals that promote blood clotting.

Blood Cells and Their Functions

Blood has three main types of cells: Red Blood Cells (RBCs), White Blood Cells (WBCs), and Platelets. Each type of blood cell has a special role.

Red Blood Cells (Erythrocytes)

Red Blood Cells (RBCs) or erythrocytes are biconcave discs. Mature red blood cells have a simple structure. Their plasma membrane is both strong and flexible, which allows them to squeeze through narrow blood capillaries.

RBCs lack a nucleus and other organelles and can neither reproduce nor carry on extensive metabolic activities. The cytosol of RBCs contains haemoglobin molecules and constitutes about 33% of the cell’s weight.

White Blood Cells (Leukocytes)

Unlike red blood cells, white blood cells (WBCs) or leukocytes have nuclei, but they do not contain haemoglobin. WBCs are classified as either granular or agranular, depending on whether they contain cytoplasmic granules that are visible by staining when viewed through a light microscope. Granular leukocytes include neutrophils, eosinophils, and basophils, and agranular leukocytes include lymphocytes and monocytes. WBCs represent one of the most important components of immunity and carry out host defence against various attacking infections.

Platelets (Thrombocytes)

Platelets are described as small, irregular cell fragments playing a crucial role in hemostasis. When a blood vessel is injured, platelets are the first to adhere to the damaged site. Various substances, known as clotting factors, are released, which activate other platelets. This whole activation mechanism results in platelet plugs, for sealing small breaks in the walls of the blood vessels, hence decreasing the risk of bleeding.

Blood Vessels: Arteries, Veins, Capillaries

Blood vessels are the channels that carry blood through the body. These are the vessels through which blood is distributed to all the parts of the body from the heart and back to the heart.

1. Arteries

Arteries distribute blood from the heart to various regions in the body. They are thick and muscular to withstand high pressure as blood is being pumped from the heart.

2. Veins

Veins return blood from different body organs back to the heart. They have thinner walls compared to arteries and have valves that prevent backflow of the blood, thus returning blood.

3. Capillaries

Capillaries are microscopic, thin-walled vessels in which gas exchange and nutrient exchange occur between blood and tissues. They connect arteries to veins and allow the exchange of oxygen, carbon dioxide, nutrients, and waste materials.

Functions of Blood in Transport, Regulation, and Protection

The various critical functions of blood are necessary to maintain health and homeostasis. Blood has three general functions:

1. Transport

Oxygen and Carbon Dioxide: Red blood cells take up oxygen from the lungs to the tissues and, in an exchange manner, get out with carbon dioxide from the tissues and back to the lungs to be exhaled.

Nutrients and Waste Products: Nutrient absorption by the digestive system is transported by the blood into the different cells of the body, while metabolic waste from cell activities is carried to places such as the kidneys for excretion.

Hormones: The blood distributes hormones that are secreted from endocrine glands to target organs/tissues, which regulate many of the body's physiological processes.

2. Regulation

Temperature: By transport and redistribution of heat generated from metabolically active tissues to the skin surface, blood helps in the elimination of body heat.

pH Balance: Blood behaves as a buffer, regulating the hydrogen ion concentration, thus stabilising pH to very narrow limits, optimal for the functioning of enzymes and other cellular actions.

Fluid Balance: Blood maintains the fluid balance in the body through the exchange of water and electrolytes between the blood and tissues.

3. Protection

Immune Response: The white blood cells and antibodies contained in blood protect the body from infections; it identifies the pathogen and kill it.

Blood Clotting: Platelets and clotting factors in blood form clots, which seal off the vessel if injured, preventing a profuse loss of blood from the body.

Blood Formation (Hemopoiesis) in Bone Marrow

Most blood cells of the blood last only hours, days, or weeks, and must be replaced continually. Negative feedback systems regulate the total number of RBCs and platelets in circulation, and their numbers normally remain steady. The WBCs vary in response to invading pathogens and other foreign antigens.

The process of making new blood cells is called hemopoiesis. Hemopoiesis happens in the red bone marrow of flat bones (ribs, skull), long bones (humerus, femur), vertebrae, and pelvis. Stem cells called hemocytoblasts divide to form blast cells. These blast cells grow into different blood cells.

Erythropoiesis

The production of erythrocytes, or erythropoiesis, starts with the development of proerythroblasts from hemopoietic stem cells. Several phases of development occur for three to five days as ribosomes multiply and haemoglobin is produced.

Leukopoiese

Colony-stimulating factors (CSFs), which are hormones made by mature white blood cells, encourage leukopoiesis, the process of creating leukocytes. The division of the hemopoietic stem cells initiates the creation of each kind of white blood cell.

Thrombopoiesis

Megakaryoblasts, which are the precursors of thrombopoiesis, the production of platelets, are created from hematopoietic stem cells. Megakaryocytes, which are enormous cells with a massive, multilobed nucleus, are created when megakaryoblasts divide without cytokinesis. The plasma membrane then infolds into the cytoplasm, causing the megakaryocytes to split into pieces.

Importance of Blood in Biology

All of the body's components can continue to function by receiving oxygen and nutrition through the blood. The lungs, kidneys, and digestive system use the blood to transport carbon dioxide and other waste products for elimination from the body.

Blood protects the body by clotting at injury sites. Blood also carries hormones to target organs. Blood fights infections with WBCs and antibodies. Blood keeps all systems working together.

Blood NEET MCQs (With Answers & Explanations)

This is an important topic and carries significant weightage in the NEET exam. Types of questions asked about this topic are:

• Types of blood cells and their functions

• Functions of blood

• Process of blood formation in bone marrow

• Role of blood in immunity and clotting

Practice Questions for NEET

Q1. Globulins contained in human blood plasma are primarily involved in:

1. osmotic balance of body fluids

2. oxygen transport in the blood

3. clotting of blood

4. defence mechanisms of the body

Correct answer: 4) defence mechanisms of the body

Explanation:

The blood plasma consists of 8-10% of water-soluble proteins. Fibrinogen is involved in the clotting of blood; globulins are involved in defence mechanisms and immunity, whereas albumins maintain osmotic balance.

Hence, the correct answer is option 4) defence mechanisms of body .

Q2. There is no DNA in:

1. Mature RBCs

2. Mature spermatozoan

3. Hair Root

4. An enucleated ovum

Correct answer: 1) Mature RBCs

Explanation:

RBCs are formed in the red bone marrow in adults. RBCs lack nuclei in most mammals and are biconcave in shape. They have a red-coloured, iron-containing complex protein called haemoglobin, hence the colour and name of these cells. A healthy individual has 12-16 gms of haemoglobin in every 100 ml of blood.

Mature RBCs are devoid of a nucleus, and DNA is absent.

Hence, the correct answer is Option (1) mature RBCs

Q3. Which of the following changes (a-d) usually tend to occur in the plain dwellers when they move to high altitudes (3,500 m or more)?

1. Increase in red blood cell size

2. Increase in red blood cell production

3. Increased breathing rate

4. Increase in thrombocyte count

Correct answer: 2) Increase in red blood cell production

Explanation:

When plain dwellers move to high altitudes (3,500 m or more), they typically experience an increase in red blood cell production to cope with lower oxygen levels. Additionally, their breathing rate increases to compensate for the reduced oxygen availability. However, there is no evidence to suggest that red blood cell size or thrombocyte count changes significantly in response to high altitudes.

Hence, the correct answer is Option 2) Increase in red blood cell production

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