Human Body: Blood Cells

The human body contains three types of blood cells: red blood cells, white blood cells, and platelets.
Red blood cells, also called erythrocytes, play a crucial role in the transportation of oxygen from the lungs to all parts of the body. They contain a pigment called haemoglobin that binds to oxygen.
Red blood cells are small and flexible, allowing them to travel through the smallest blood vessels. They have a biconcave (double-curved) shape to maximise their surface area for effective oxygen exchange.
White blood cells (leucocytes) make up a part of the immune system and ward off disease and infection. They can change shape to consume harmful organisms in a process called phagocytosis.
There are different types of white blood cells that perform various functions. Some produce antibodies, while others engulf and destroy pathogens.
Platelets (thrombocytes) are cell fragments that play a vital role in blood clotting. When a blood vessel is damaged, platelets stick together to form a clot and stop bleeding.
Blood cells are made in the bone marrow in a process known as haematopoiesis.
All blood cells have a finite lifespan and are constantly replaced. Red blood cells live for about 120 days, most white blood cells live for less than a day, whereas platelets live for about a week before being replaced.
Blood groups are determined by surface antigens on the red blood cells. There are four main blood groups – A, B, AB, and O, and each can be RhD positive or RhD negative.
The blood’s composition, including its cellular elements, is kept within a specific range by negative feedback mechanisms that ensure homeostasis.
Blood plays a vital role in the transport of other substances inside the body besides oxygen, including hormones, nutrients, waste products, heat, and cells of the immune system.