What is blood?

Blood is an essential body fluid produced in the bone marrow, the spleen, lymph nodes, and thymus gland.

It is an essential fluid that makes life possible. Blood is made up of four different cells.

  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Platelets (thrombocytes)
  • Plasma.

Red blood cells (erythrocytes)

RBC is the most abundant type of cell in humans, making up about 40-45% of the whole blood.

It is produced in the bone marrow, under the control of a hormone called erythropoietin.

Red blood cells have a biconcave shape and a life span of 120 days.

It lacks a nucleus, especially those in mammals, and is usually destroyed by macrophage at the end of their cycle.

Red blood cells contain hemoglobin, an iron filled protein responsible for their red colour. Hemoglobin also partners with RBC to perform its functions.

Functions of red blood cells.

  • RBC carries oxygenated blood from the lungs to tissues where needed.
  • It returns carbon dioxide to the lungs for expiration.
  • They circle nutrients into all parts of the body.
  • It regulates body temperature.

White blood cells (leukocytes)

WBC is part of the immune system, they offer protection to the body against infections.

It is also produced in the bone marrow, but unlike the RBC, WBC has a nucleus.

Types of white blood cells.

There are five types of WBC, and they all work towards improving the immune system. They include:

  • Neutrophils
  • Monocytes
  • Lymphocytes
  • Basophils
  • Eosinophils

Neutrophils (polymorphonuclear)

These are the most abundant WBCs in humans and are usually the first to respond to microbial infections.

They have a multi-lobed nucleus that gives them the name polymorphonuclear leukocyte.

Neutrophils are part of the WBCs called granulocytes, and help digest pathogens, using granules.


Monocytes are young WBCs produced in the bone marrow. They circulate in the blood and transform into macrophage once it gets to the tissue.

Macrophage plays a vital role in body defense, they engulf and kill microorganisms, remove dead cells, improve immune response, and also ingest foreign bodies.


Lymphocytes are round cells, characterized by a single large and round nucleus.

They are of two types, the B, and T lymphocyte.

Both cells are produced by the stem cells in the bone marrow, but the T lymphocyte travels to the thymus and becomes T cells, while B cells remain in the bone marrow.

The B cells work with the immune system to produce antibodies which fight foreign bodies (antigens) that gain access into the body,

while the T cells help control the immune response, and in killing cancer cells.


Basophil is a WBC produced in the bone marrow. It is the least abundant in humans, but as part of the immune system, they help protect the body from infection.

Basophil contains heparin and histamine. A chemical that helps it function properly.

  • Heparin being an anticoagulant prevents blood clotting and also improves the circulation of other WBCs to infection sites.
  • Histamine widens blood vessels to improve blood circulation to injured tissues.


It is a disease-fighting white blood cell that protects the body from parasitic infections, allergies, and collagen disease.


Platelets are parts of the blood cells produced in the bone marrow. It helps prevent bleeding by forming a clot on any injured site.


Plasma is the liquid part of the blood. It is mostly filled with water, and a smaller amount of hormones, sugar, proteins and salt.

Plasma helps in the transportation of nutrients and water to the whole body.

Blood type determination.

Blood type is determined by the presence or absence of antigen on the surface of the cell.

The ABO blood group is classified according to the antigens on their surface. For example,

  • Blood type A has antigen A on its surface.
  • Type B has antigen B on the surface of the blood.
  • Blood type AB has both antigen A and B on the surface of the blood.
  • Type O has neither A nor B on the surface of the blood.

For more information on blood type determination, visit the blood group and genotype compatibility.

How blood circulates in our body.

Blood moves round the body through the cardiovascular or circulatory system.

This system comprises the heart, blood vessels, and blood. They transport oxygen and other vital nutrients to various tissues and organs and also remove metabolic waste from the body.

The blood circulatory system has two-components;

  • Pulmonary circulation.
  • Systemic circulation.

Pulmonary circulation deals with the transportation of de-oxygenated blood from the heart to the lungs where they are refilled with oxygen.

It also releases carbon dioxide at this point.

Systemic circulation carries oxygenated blood to all parts of the body.

Blood circulation starts from the heart and has two phases. The diastole (relax) and systole (contracts) phase.

During the diastole stage, the heart muscles relax, allowing blood from both atriums to fill the ventricles. This causes the ventricle to expand and thus pump blood into the large arteries.

In the systole stage, the left ventricle contracts and pumps the oxygenated blood into the aorta (main artery). The blood then circulates from the aorta to the smaller arteries and capillaries, to distribute the oxygen and essential nutrients.

They also pick up carbon dioxide and other waste from there to the lungs for expiration.

Terms and their meaning.

Circulatory system.The system is responsible for the distribution of blood, nutrients, and excretion of waste materials from the body.
PulmonaryRelated to the lungs.
CardiacRelated to the heart.
ArteryA blood vessel that carries blood away from the heart.
VeinA vessel that carries blood to the heart.
AortaThe main artery that carries oxygenated blood to the whole system.
CapillaryTiny vessels that aid in nutrient exchange.
AtriumThe upper chamber of the heart.
VentricleThe lower chamber of the heart.