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Definition, Composition, and Functions of Blood


Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.
Blood consists of cellular elements (Red blood cells, White blood cells, and Platelets) as well as plasma, the fluid in which the blood cells are suspended.
An average adult has about 5.5 liters of blood, with a temperature of approx. 38℃ and a pH of 7.35-7.45.

Functions of Blood

Blood has many functions, including the following:
  • It carries oxygen (O2) from the lungs to the tissues and carries carbon dioxide (CO2) from the tissues to the lungs.
  • It carries nutrients from the digestive system to the tissues.
  • It carries waste products from the tissues to the excretory organs. It maintains the pH of the body.
  • It maintains constant body temperature.
  • It carries hormones from the endocrine glands to wherever they are needed.
  • It transports infection-fighting blood cells (leukocytes) and antibodies.

Composition of Blood

Blood made up of
1. Plasma and
2. Formed cellular elements (red and white blood cells, and platelets).

Plasma makes up 55% of blood volume. Cellular elements make up the other 45%.


The fluid portion of the blood, the plasma, accounts for 55 to 60% of total blood volume and is about 90% water. The remaining 10% contains proteins (8%) and other substances (2%) including hormones, enzymes, nutrient molecules, gases, electrolytes, and excretory products. All of these substances are dissolved in the plasma (e.g., oxygen) or are colloidal materials (dispersed solute
  • Materials that do not precipitate out, e.g., proteins). The three major plasma proteins include: Albumin (about 55%)
  • Globulins (about 38%)
  • Fibrinogen

Types of Blood Cells

Blood cells are made up of:
A. Red blood cells (Erythrocytes)
B. White blood cells (Leucocytes)
C. Platelets.

Erythrocytes/ Red Blood Cells (RBCs)

Red blood cells (RBCs), also known as erythrocytes. The major function of red blood cells is to transport hemoglobin, which in turn carries oxygen from the lungs to the tissues. One cubic millimeter of blood contains about 5million Red blood cells. They are circular, biconcave disc without nuclei. They are located or found in the bone marrow after birth. It has an average life- span of about 120 days.
It contains hemoglobin that is responsible for the transport of oxygen in the blood. They give the blood its characteristic red color.

White Blood Cells (WBCs)

White blood cells have nuclei but no pigment. They produce antitoxins to neutralize bacteria toxins. In the embryo, they are manufactured in the liver and the spleen. In the adult, they are formed in the bone marrow, spleen and lymphatic glands. They have a very short lifespan. One cubic millimeter of blood contains about 7,000 white blood cells (WBCs). They are larger than red blood cells (RBCs) in size.

Blood Platelets

Blood platelets are tiny irregular cell fragments that are non-nucleated. They may be round or oval in shape. They are concerned with clotting.