January 09, 2024 | UR Gate

Blood: Definition, Composition, and Components

Blood: Definition and Introduction

Blood is defined as a tissue that circulates in a closed system of blood vessels, or as a fluid that circulates within the body, pumped from the heart to various tissues of the body. It is composed of plasma and cells suspended in it.

Blood is a red, slightly alkaline, slightly viscous, oily fluid with a distinctive taste and smell. It flows as a suspension of red blood cells (RBCs), white blood cells (WBCs), and platelets (Blood platelets). These cells account for 45% of the volume of blood, while plasma accounts for 55% in an adult. The normal blood volume is about 5-6 liters.

The main function of blood is to carry oxygen from the lungs to the different cells of the body and remove carbon dioxide from them and expel it from the lungs to the outside. Therefore, the content of blood varies depending on its location, whether it is arterial or venous. The appearance of blood also varies due to oxygen. Arterial blood saturated with oxygen is red in color and turns dark red as the oxygen content decreases.

Blood is composed of a basic substance called plasma, which is found in the form of a light yellow fluid. In this substance, blood cells, representing tissue cells, swim.

Blood Composition:

1- Plasma (Liquid Phase)

Water
Electrolytes (Sodium, Calcium, Bicarbonates, Chloride, etc.)
Proteins
Sugars, Vitamins, Fats, and Hormones

2- Solid Phase

Red Blood Cells (Erythrocytes)
White Blood Cells (Leukocytes)
Blood Platelets

Types of Blood Cells:

1- Red Blood Cells (RBC) or Erythrocytes:

Small spherical cells with an average diameter of 7.5 microns and a thickness of about 2.2 microns. They exhibit biconcavity, meaning they can change shape under external pressure but regain their original shape once the pressure is removed. The average count of red blood cells in humans ranges from 4 to 5.5 million cells per milliliter of blood. In males, the count is higher, estimated at 5.4 million cells per cubic millimeter, while in females, it is approximately 4.8 million cells per cubic millimeter.

The membrane of red blood cells consists of proteins linked to a lipid layer (phospholipids and cholesterol). The cell has a concave disc shape with a central depression on both sides, increasing its surface area to facilitate oxygen transport.

The primary function of red blood cells is the transportation of respiratory gases. They carry oxygen from the lungs to various tissues and transport carbon dioxide from tissues to the lungs, facilitated by hemoglobin, which easily binds with these gases.

The liver is the main organ responsible for red blood cell production in most fetuses, with the spleen and lymph nodes following the liver in importance for cell production. After birth, red blood cells are primarily produced in the bone marrow.

The lifespan of red blood cells in humans is very short, not exceeding 120 days. After this period, they break into small fragments and are engulfed by the reticuloendothelial system.

The process of red blood cell formation requires:

Nutrients such as fats, sugars, and amino acids.
Growth factors like folic acid and vitamin B12.
Raw materials such as glycine, cysteine, and succinyl coenzyme A.

2- White Blood Cells (WBC) or Leukocytes:

White blood cells are colorless blood cells that lack hemoglobin, distinguishing them from red blood cells. They contain a nucleus, and their size is generally larger than that of red blood cells, although their count is lower. These cells can be divided into two main groups:

A. Non-granular Cells:

Lymphocytes: Smallest among white blood cells, characterized by a large nucleus that nearly fills the cell, with minimal cytoplasm. They play a crucial role in the immune system by producing antibodies.
Monocytes: Largest naturally occurring white blood cells, with a kidney-shaped nucleus. These cells engulf foreign bodies, such as bacteria and dead cells.

B. Granular Cells:

Neutrophils: The most abundant white blood cells, constituting 40-80% of the total white blood cell count. They are vital in the body's defense mechanism, increasing in number during acute inflammation to phagocytize bacteria. The nucleus of a neutrophil consists of 2-5 lobes.
Eosinophils: Characterized by large granules in the cytoplasm, constituting 1-6% of the total white blood cell count. Their nucleus consists of two lobes. Eosinophils are involved in phagocytosis and can neutralize compounds produced during antibody-antigen reactions. They also absorb histamine produced during allergic reactions and prevent the toxic effects of certain substances entering the body.
Basophils: The least numerous white blood cells, comprising 1-2% of the total white blood cell count. Basophils contain heparin, an anticoagulant, and histamine. Their count is useful in studying allergic reactions, as there is a correlation between basophil count and histamine concentration in the blood.

Granular and non-granular white blood cells differ in origin and function. Granular cells are produced in the bone marrow, while non-granular cells are produced in lymphoid organs such as the spleen and lymph nodes, as well as in the liver. The lifespan of white blood cells ranges from 13 to 20 days, during which they defend the body by phagocytosis (performed by phagocytic cells like neutrophils, monocytes, eosinophils, and lymphocytes) and antibody production (by lymphocytes). Unlike red blood cells, most white blood cells do not perform their functions while circulating in the blood; instead, they carry out their roles in the connective tissues outside the blood vessels.

3- Platelets:

Platelets are the smallest blood components, oval in shape, and characterized by the presence of clear granules in the cytoplasm. They are produced in the bone marrow under the influence of the hormone thrombopoietin. Platelets have a size ranging from 2 to 4 microns, and their lifespan is approximately 8 to 14 days. In a healthy individual, the platelet count ranges from 150,000 to 400,000 platelets per milliliter of blood.

Platelets play a crucial role in the blood clotting process. They contain a substance known as thromboplastin, which is essential for the clotting mechanism.