Composition and Functions of Blood
Introduction
Blood is a specialized fluid connective tissue that circulates throughout the body through blood vessels. It connects different organs and tissues by transporting oxygen, nutrients, hormones and waste products.
Blood also protects the body against infection, prevents excessive blood loss and helps maintain a stable internal environment. In an adult, blood accounts for approximately 7–8% of body weight.
Blood consists of two major components: plasma, the liquid portion, and formed elements, which include red blood cells, white blood cells and platelets.
[IMG 1 — Horizontal educational infographic showing a blood sample separated into plasma and formed elements, with labels for plasma, red blood cells, white blood cells and platelets. Use a clear, textbook-style layout with accurate proportions and concise labels.]
1. Composition of Blood
Blood is composed of two main parts:
Plasma: Approximately 55% of blood volume.
Formed elements: Approximately 45% of blood volume, mainly red blood cells.
These proportions may vary slightly depending on factors such as age, health and hydration status.
2. Plasma
Plasma is the pale-yellow liquid portion of blood in which the formed elements are suspended. It consists mainly of water, along with proteins, electrolytes, nutrients, hormones, gases and waste products.
A. Water
Water forms approximately 90–92% of plasma. It acts as a solvent and provides the medium for transporting dissolved substances throughout the body.
B. Plasma Proteins
The major plasma proteins include albumin, globulins and fibrinogen.
Albumin is the most abundant plasma protein. It helps maintain the osmotic pressure of blood, which supports the movement and distribution of water between blood and tissues.
Globulins include proteins involved in transport and immune defence. Some globulins, known as immunoglobulins or antibodies, help protect the body against pathogens.
Fibrinogen is a clotting protein that is converted into fibrin during blood coagulation. Fibrin forms a mesh that helps stabilize a blood clot.
C. Other Substances in Plasma
Plasma also contains:
Electrolytes: Sodium, potassium, calcium, chloride and bicarbonate.
Nutrients: Glucose, amino acids, lipids and vitamins.
Hormones: Chemical messengers transported to target organs.
Dissolved gases: Oxygen and carbon dioxide.
Waste products: Urea, uric acid and other metabolic wastes.
3. Formed Elements of Blood
The formed elements are the cellular components of blood. They include red blood cells, white blood cells and platelets.
A. Red Blood Cells (Erythrocytes)
Red blood cells, also called erythrocytes, are the most numerous formed elements in blood. In humans, mature red blood cells are biconcave discs and lack a nucleus.
They contain haemoglobin, a protein that binds oxygen and gives blood its red colour.
The biconcave shape provides a large surface area for gas exchange and allows red blood cells to pass through narrow capillaries.
Main functions of red blood cells:
Transport oxygen from the lungs to body tissues.
Carry a portion of carbon dioxide from tissues towards the lungs.
Help maintain acid-base balance through haemoglobin’s buffering action.
Red blood cells are produced in the red bone marrow through a process called erythropoiesis. Their average lifespan is approximately 120 days.
[IMG 2 — Horizontal labelled illustration of a biconcave red blood cell showing its shape, haemoglobin-rich interior and role in oxygen transport.]
B. White Blood Cells (Leukocytes)
White blood cells, also called leukocytes, are involved in the body’s defence and immune responses.
Unlike red blood cells, white blood cells contain a nucleus. They are much less numerous but play an essential role in identifying and responding to infections and other harmful substances.
White blood cells are classified into two main groups:
Granulocytes
Neutrophils: Engulf and destroy bacteria and other microorganisms.
Eosinophils: Help defend against parasitic infections and participate in allergic reactions.
Basophils: Release substances such as histamine that contribute to inflammatory and allergic responses.
Agranulocytes
Lymphocytes: Include B cells, T cells and natural killer cells. They support specific immune responses and the destruction of infected or abnormal cells.
Monocytes: Can enter tissues and develop into macrophages or related cells that engulf microorganisms, dead cells and debris.
C. Platelets (Thrombocytes)
Platelets, also called thrombocytes, are small cell fragments formed from large bone marrow cells called megakaryocytes.
They do not contain a nucleus. Platelets are essential for haemostasis, the process that prevents blood loss after a blood vessel is damaged.
When a vessel is injured, platelets adhere to the damaged area, become activated and aggregate to form a temporary platelet plug. They also release substances that support blood coagulation.
4. Functions of Blood
Blood performs several important functions that support the survival and normal functioning of the body.
A. Transport
Blood transports substances between organs and tissues.
Oxygen: Red blood cells carry oxygen from the lungs to tissues.
Carbon dioxide: Blood carries carbon dioxide from tissues to the lungs for removal.
Nutrients: Blood transports absorbed nutrients to cells.
Hormones: Blood carries hormones from endocrine glands to target organs.
Waste products: Blood transports metabolic wastes to organs such as the kidneys and liver for processing or elimination.
B. Protection
Blood protects the body through immune responses and clotting.
White blood cells and antibodies help identify and destroy pathogens. Platelets and clotting proteins prevent excessive blood loss when blood vessels are damaged.
C. Regulation
Blood helps maintain homeostasis by regulating several internal conditions.
Body temperature: Blood distributes heat throughout the body.
pH: Buffers in blood help maintain acid-base balance.
Fluid balance: Plasma proteins, particularly albumin, help maintain the distribution of water between blood and tissues.
Electrolyte balance: Blood transports electrolytes that are essential for normal cellular activity.
5. Difference Between Plasma and Serum
Plasma and serum are both liquid components obtained from blood, but they differ in the presence of clotting factors.
Feature | Plasma | Serum |
|---|---|---|
Definition | Liquid portion of anticoagulated blood | Liquid remaining after blood has clotted |
Clotting factors | Contains clotting factors, including fibrinogen | Most clotting factors have been removed or consumed during clotting |
Preparation | Obtained by centrifuging blood collected with an anticoagulant | Obtained after blood is allowed to clot and then centrifuged |
Common use | Transfusion and laboratory testing | Many diagnostic laboratory tests |
Remember: Serum is plasma without most of its clotting factors, particularly fibrinogen.
6. Blood Groups
Blood groups are determined by specific antigens present on the surface of red blood cells.
The two major blood group systems commonly discussed are the ABO system and the Rh system.
ABO Blood Group System
The ABO system is based on the presence or absence of A and B antigens on red blood cells.
Blood Group | Antigen on Red Blood Cells |
|---|---|
A | A antigen |
B | B antigen |
AB | Both A and B antigens |
O | Neither A nor B antigen |
Blood group compatibility is important during transfusion because incompatible blood can cause serious immune reactions.
Rh Blood Group System
The Rh system is particularly associated with the D antigen.
Rh-positive: D antigen is present on red blood cells.
Rh-negative: D antigen is absent.
Rh compatibility is important in blood transfusion and pregnancy-related care.
7. Blood Clotting
Blood clotting, or coagulation, is part of haemostasis. It helps prevent excessive blood loss after injury.
The process involves several coordinated steps:
Vasoconstriction: The damaged blood vessel narrows, reducing blood flow to the injured area.
Platelet plug formation: Platelets adhere to the damaged vessel wall, become activated and aggregate.
Coagulation: Clotting factors activate a sequence of reactions that leads to the formation of fibrin.
Clot stabilization: Fibrin forms a mesh that strengthens the platelet plug and helps stabilize the clot.
[IMG 3 — Horizontal flow diagram showing haemostasis: vessel injury → vasoconstriction → platelet plug formation → fibrin clot formation.]
Key Concepts
Blood is a specialized connective tissue made up of plasma and formed elements. Plasma transports dissolved substances and contains proteins that support fluid balance and clotting. Red blood cells transport respiratory gases, white blood cells protect the body against infection, and platelets help prevent blood loss.
Together, these components allow blood to perform its major functions of transport, protection and regulation, helping maintain homeostasis throughout the body.