• Written By Shreya_S
  • Last Modified 24-01-2023

Body Fluids


Body fluids are the body’s mode of transportation. These fluids might be intracellular or extracellular. Potassium ions, phosphate ions, and proteins are abundant in the intracellular fluid. Blood, lymph, cerebrospinal fluid, and other extracellular fluids are examples. Body fluid as blood helps in the transportation of respiratory gases (i.e., O₂, CO₂, etc.), hormones from endocrine glands to target organs, as well as bodily wastes from various body sections to the kidney, are facilitated by blood. Body pH, water, ionic balance, and appropriate body temperature are all maintained by body fluids.

Body Fluids: Overview

Any fluid produced by a living thing is referred to as bodily fluid. The bodily fluid in humans can be divided into two categories based on its location:

A. Intracellular fluid
a. The intracellular fluid is a bodily fluid that exists within a cell(s).
b. In humans, intracellular fluid accounts for 67 per cent of total body water. It is made up of water and dissolved ions.

B. Extracellular fluid
a. Extracellular fluid is a type of bodily fluid that exists outside of the cell(s).
b. In humans, it makes up roughly 26% of total body water composition.
c. Extracellular fluid is made up of intravascular fluid (blood plasma), interstitial fluid, lymph, and transcellular fluid.

The interstitial fluid, which fills the gaps between cells, is the most abundant, while the transcellular fluid, which fills the spaces of chambers produced from epithelial cell linings, is the least abundant.


Blood is a fluid, plasma, and cells, blood corpuscles, that make up a mobile connective tissue. The foundation of life is blood. Blood is the body’s softest tissue. About 30-35 per cent of the ECF (Extracellular fluids) is made up of blood. The blood volume in an adult of 70 kg weighs around 5.5 litres. It has a pH of 7.4 and is mildly alkaline. Blood in arteries has a higher pH than blood in veins.

Components of Blood

Blood Composition

Fig: Blood Composition

The components of human blood are:

I. Plasma: The liquid component of the blood in which the following blood cells are suspended:
II. Blood Cells:
A. Red blood cells (erythrocytes): These carry oxygen from the lungs to the rest of the body
B. White blood cells (leukocytes): These help fight infections and aid in the immune process. Types of white blood cells include:
a. Lymphocytes
b. Monocytes
c. Eosinophils
d. Basophils
e. Neutrophils
C. Platelets (thrombocytes): It helps in blood clotting.

Functions of Blood

1. Respiratory gas transport: Blood transports oxygen from the lungs to the tissues. Carbon dioxide is also transported from the tissues to the lungs via this pathway.
2. Nutrient transport: Nutrients absorbed in the small intestine pass through the capillaries of the bloodstream. These nutrients are carried by the blood and distributed throughout the body.
Waste materials are transported to the excretory organs by blood. Examples include urea, uric acid, and other waste items.
3. Regulation of water content of cells: Blood regulates the water content of cells by absorbing excess cellular water. When cells require water, blood delivers it.
4. Body temperature regulation: Increased body temperature caused by excessive respiration of a specific tissue is balanced by blood circulation.
5. Infection protection: Blood protects the body from infection.
6. Defence against infection: Excess bleeding can be prevented by clotting the blood.
7. Blood pH maintenance: Furthermore, haemoglobin is an efficient acid-base buffer that is largely responsible for blood pH maintenance. Because of the acidity of the blood, haemoglobin carries less oxygen.

Blood Plasma

The crystallo-colloidal mixture, which accounts for 55-60% of blood, comprises 90-92 per cent water and 0.9 per cent salts, is mildly alkaline, and accounts for around 5% of body weight. Plasma is a light-yellowish or straw-coloured liquid that is also known as blood plasma. It acts like a liquid foundation for whole blood. Plasma is made up of whole blood minus erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets).  Water makes up 91 to 92 per cent of plasma, whereas solids make up 8 to 9 per cent.

Composition of Plasma

Blood Plasma Composition

Fig: Blood plasma composition

It primarily consists of:

1. Blood clotting is facilitated by coagulants, particularly fibrinogen.
2. Albumin and globulin are plasma proteins that keep the colloidal osmotic pressure around 25 mmHg.
3. Sodium, potassium, bicarbonate, chloride, and calcium are electrolytes that assist keep blood pH balanced.
4. Immunoglobulins, as well as other minor amounts of enzymes, hormones, and vitamins, aid in the battle against infection.
5. Serum: The fluid and solute component of blood, serum, does not play a role in clotting. It can be defined as blood plasma without clotting factors or blood that has been devoid of all cells and clotting factors. All proteins that aren’t involved in blood coagulation, as well as electrolytes, antibodies, antigens, hormones, and any foreign chemicals, are found in serum (e.g., drugs or microorganisms). White blood cells (leukocytes), red blood cells (erythrocytes), platelets, and clotting factors are not found in serum.

Functions of Plasma Proteins

Plasma and blood have similar functions since plasma is the liquid component of blood. Among the numerous functions are:

1. Coagulation: Fibrinogen, along with other procoagulants such as thrombin and factor X, plays an important part in blood clotting.
2. Defence: Immunoglobulins and antibodies in plasma contribute to the body’s defence against bacteria, viruses, fungi, and parasites.
3. Maintenance of Osmotic Pressure: The colloidal osmotic pressure is maintained at roughly 25 mmHg by plasma proteins such as albumin, which are generated by the liver.
4. Nutrition: Nutrients such as glucose, amino acids, lipids, and vitamins are transported from the digestive system to various regions of the body and serve as a source of fuel for growth and development.
5. Regulation of Acid-Base Balance: Plasma proteins help to maintain acid-base balance by regulating the pH of the blood.
6. Regulation of Body Temperature: this is maintained by balancing heat loss and heat gain in the body.
Role in Erythrocyte Sedimentation Rate (ESR): Fibrinogen, an acute phase reactant, increases during acute inflammatory conditions and contributes to the increase in ESR, which is used as a diagnostic and prognostic tool

Blood Cells

Different Types of Blood Cells

Fig: Different types of blood cells

Blood Cells make up roughly 40 – 45 per cent of the blood. They have a specific gravity of approximately 1.09, making them slightly heavier than plasma. The bone marrow is where blood cells are formed. The spongy material in the middle of the bones that produces all sorts of blood cells is known as bone marrow.

Other organs and systems in our body help us in the regulation of blood cells. The lymph nodes, spleen, and liver all play a role in controlling cell production, destruction, and function. Hematopoiesis is the process of producing and developing new cells in the bone marrow.

Stem cells are the precursors of blood cells generated in the bone marrow. The first phase of all blood cells is the stem cell (or hematopoietic stem cell). Several different cells emerge as the stem cell grows. In blood, there are three types of cellular elements:

1. Red Blood Cells or Erythrocytes
2. White Blood Cells or leukocytes
3. Blood platelets or Thrombocytes

Classification of Blood Cells

Fig: Classification of Blood cells

Characteristic Features of Blood Cells

1. Red blood cells, also known as erythrocytes, have the primary function of transporting oxygen from the lungs to the body’s tissues while also transporting carbon dioxide away from the tissues and back to the lungs as a waste product. Haemoglobin Hgb is a protein found in red blood cells that transports oxygen from our lungs to every part of our bodies.
2. White blood cells, often known as leukocytes, are primarily responsible for fighting infection. White blood cells come in various shapes and sizes, each with its unique function in the battle against bacterial, viral, fungal, and parasitic illnesses.
a. Phagocytosis occurs after a foreign body, such as a bacterial cell, binds to molecules on the phagocyte’s surface known as “receptors.” The phagocyte engulfs a bacterium by stretching itself around it. Human neutrophils take an average of nine minutes to phagocytose microorganisms.
b. Inflammatory mediators stimulate resting phagocytes, causing them to develop surface receptors that improve their capacity to stick to the inner surface of capillary walls, allowing them to squeeze out of the capillary and into the tissue, a process known as diapedesis.
3. Platelets, also known as thrombocytes, are responsible for blood clotting. Platelets are a fraction of the size of other blood cells. They clump together to halt bleeding by forming clumps, or plugs, in the hole of a vessel.

The characteristic features of blood cells that are most crucial for protecting the body is given below:

Characteristics of Blood Cells

Fig: Characteristics of blood cells

Blood Groups 

On the surface of blood cells, there are around 30 antigens that give rise to different blood types. Antigens (agglutinogens) in red blood cells react with antibodies (agglutinin) in blood plasma during agglutination. ABO blood grouping and Rh (rhesus) blood grouping are two types of blood grouping that are commonly used around the world.

1. ABO Blood Groups

Karl Landsteiner was the first to discover the A, B, and O blood types in humans. The presence or absence of two antigens on RBCs, namely A and B, determines the ABO blood group.

Blood Groups

Fig: Blood Groups

2. Rhesus (Rh) Blood Group
It was discovered in the blood of rhesus monkeys by Landsteiner and Wiener. Individuals are classified as Rh positive (Rh) + or Rh negative (Rh) based on the presence or lack of rhesus antigen on the surface of red blood corpuscles. Rh+ is dominant over Rh.

a. Rh Incompatibility During Pregnancy
When the father’s blood is Rh+, and the mother’s blood is Rh, it is visible. Rh+, as a dominating character, manifests itself in the foetus, posing a major concern. The Rh mother’s first child will not be harmed, but the Rh+ blood of the foetus induces the production of anti-Rh factors in the mother’s blood.

b. Rh Incompatibility During Blood Transfusion
Because Rh people generate anti-Rh antibodies in their blood, the first transfusion between Rh+ and Rh blood causes no harm. However, when Rh+ blood is transfused into Rh blood, the anti-Rh antibodies in the latter’s blood kill the donor’s RBCs.

Coagulation of Blood

Clotting of Blood

Fig: Clotting of Blood

1. In the event of an accident or trauma, blood coagulates or clots. This is a process that prevents the body from losing too much blood.
2. A clot, also known as a coagulam, is a network of threads called fibrins that traps dead and injured blood components.
3. The injured tissue cells and the platelets which disintegrate at the site of the wound release a substance thrombokinase (also called thromboplastin). More recently, it is called “Factor X” or Stuart Factor).
4. The thrombokinase acts as an enzyme, and with the help of the calcium ions present in the plasma, it converts a substance called prothrombin (inactive) of the plasma into thrombin (active).
5. Thrombin in the presence of calcium ions reacts with the soluble fibrinogen of the plasma to convert it into insoluble fibrin. Fibrin is a solid substance that forms threads of fibrin that are sticky and form a network (mesh) at the wound.
Blood cells are trapped in the network of the fibrin; the network then shrinks and squeezes out the rest of the plasma, which is in the form of a clear liquid, the serum. The solid mass which is left behind is called a clot ( or thrombus).
6. When platelets in the blood are stimulated by an injury or trauma, they release specific substances that activate the coagulation mechanism.
7. Coagulation can also be triggered by certain substances secreted by the tissues at the site of damage. In clotting, calcium ions play a critical function.

Lymph (Tissue Fluid)

Lymph Capillary

Fig: Lymph Capillary

It is an interstitial mobile connective tissue comprising lymph plasma and lymph corpuscles. It contains little O₂ but a lot of CO₂ and metabolic waste. In fact, when blood flows from the arterial end to the venous end of a capillary, most of its contents move into tissue (at the arterial end). 90% of these constituents return back at the venous end, while the remaining 10% constitute the lymph

Lymphoid Organs

  1. These are the lymph secreting/accumulating organs.
  2. They include lymph nodes, tonsils, thymus, spleen and Peyer’s patches.
  3. The spleen is the body’s largest lymphoid organ.

Functions of Lymph

  1. Its white blood corpuscles help in defence mechanisms, tissue repair and healing.
  2. It is an important carrier for nutrients, hormones, etc.
  3. It helps in the absorption of fats in the lacteals present in the intestinal villi.


Vertebrates use blood, a fluid connective tissue, to transport critical nutrients to their cells and to transport waste away from them. Lymph (tissue fluid) is another fluid that is used to transfer certain substances. A fluid matrix, plasma, and formed components make up blood. The produced elements include red blood cells (erythrocytes), white blood cells (leucocytes), and platelets (thrombocytes). Human blood is divided into four systems: A, B, AB, and O, based on the presence or lack of two surface antigens, A and B, on RBCs. Another blood grouping is based on the presence, or lack of another antigen on the surface of RBCs termed Rhesus factor (Rh). Tissue fluid is a fluid generated from the blood that fills the gaps between cells in the tissues. Except for the protein composition and the synthesized elements, lymph is nearly identical to blood.

Frequently Asked Questions (FAQs)

Q.1. Define blood and lymph.
Ans: The human body’s two vital fluids are blood and lymph. Plasma, blood cells, and platelets compose blood, which is a fluid connective tissue. Lymph is a colourless fluid that circulates within the lymphatic vessels, which are made up of lymph nodes and veins.

Q.2. What is the function of lymph?
Few major functions of lymph are mentioned below:
1. It keeps the body cells moist.
2. It transports oxygen, hormones and nutrients to different parts of the body and removes metabolic waste from the cells. It transports antibodies and lymphocytes to the blood.

Q.3. What is an extracellular fluid called?
Ans: The interstitial fluid, also known as tissue fluid, is the primary component of the extracellular fluid (ECF), which surrounds the cells in the body. Blood plasma, which is an intravascular fluid in the circulatory system, is another important component of the ECF.

Q.4. What is meant by plasma?
Ans: Plasma makes up the majority of your blood. Plasma is a pale yellow liquid that separates from the remainder of the blood. Plasma transports water, ions, and enzymes throughout the body. Plasma’s primary function is to transport nutrients, hormones, and proteins to the regions of the body that require them. The waste products of cells are also deposited in the plasma.

Q.5. What is lymph and tissue fluid?
Ans: The lymphatic system circulates lymph, which is a fluid that flows throughout the body. It is formed when tissue fluids/blood plasma drain into the lymphatic system (mainly water, with proteins and other dissolved compounds). There are a lot of lymphocytes in there (white cells that fight infection).

Learn Everything About Circulatory System Here

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