An average adult human has about five to six litres of blood. Blood is a connective tissue containing 45% cellular components and 55% of a fluid called plasma.
Human blood:
- Cellular components: erythrocytes, leucocytes, platelets
- Plasma: water(90-92%), soluble solutes(dissolved substance)
Erythrocytes (Red Blood Cell)
There are about five million erythrocytes in every cubic millimetre (microlitre) of blood. Erythrocytes are small biconcave disc with a diameter of 8 µm and a thickness of 2 µm. This shape serves to increase the surface area for gaseous exchange through the thin cell membrane.
In human, erythrocytes have no nucleus. As a result, there is space for great quantities of haemoglobin. Haemoglobin is a protein which contains iron. The main function of haemoglobin is to transport oxygen. When haemoglobin combines with oxygen, the cells become bright red. Without oxygen, they are dark red.
When the partial pressure of oxygen is high in the lungs, haemoglobin will combine with oxygen to form oxyhaemoglobin. When the partial pressure of oxygen is low as in respiring tissues, the oxyhaemoglobin dissociates and oxygen is released. A small amount of carbon dioxide can also bind with haemoglobin to form carbaminohaemoglobin to be carried to the lungs.
Erythrocytes are produced in the bone marrow at the rate of about two million cells per second. They circulate in the body for 120 days. After that they are destroyed by the phagocytes in the liver and spleen.
Leucocytes (White Blood Cells)
Leucocytes are responsible for the defence of organisms against diseases. They are different from erythrocytes in several ways. In the blood, they are much less numerous than erythrocytes. There are about 6000 to 10 000 leucocytes in every microlitre of blood. Leucocytes have nuclei but do not have haemoglobin. Leucocytes are larger than erythrocytes and do not have fixed shapes. Phagocytic leucocytes can move by changing body shape.
Leucocytes are manufactured in the bone marrow but may migrate to the thymus gland or lymph nodes for their growth and development stages.
There are two basic types of leucocyte, namely granulocytes and agranulocytes. Granulocytes have granular cytoplasm and lobed nuclei. There are three types of granulocyte: neutrophils, eosiniphils and basophils. The neutrophils are phagocytes. They engulf foreign materials, for example, bacteria by phagocytosis, and destroy them. Eosinophils help tto control allergic responses. Basophils secrete heparin to prevent blood from clotting.
Agranulocytes have relatively clear cytoplasm and their nuclei are not lobed. There are two types of agranulocyte: monocytes and lymphocytes. Monocytes are the largest of the leucocytes. They spend only a few days in the blood and then move to the body tissues to become phagocytic macrophages. They engulf dead cells and bacteria that enters the body. Lymphocytes are the smallest leucocytes. Some lymphocytes produce antibodies to aid in the destruction of pathogens or to neutralise toxins whereas others attack and destroy infected cells.
Platelets
When the partial pressure of oxygen is high in the lungs, haemoglobin will combine with oxygen to form oxyhaemoglobin. When the partial pressure of oxygen is low as in respiring tissues, the oxyhaemoglobin dissociates and oxygen is released. A small amount of carbon dioxide can also bind with haemoglobin to form carbaminohaemoglobin to be carried to the lungs.
Erythrocytes are produced in the bone marrow at the rate of about two million cells per second. They circulate in the body for 120 days. After that they are destroyed by the phagocytes in the liver and spleen.
Leucocytes (White Blood Cells)
Leucocytes are responsible for the defence of organisms against diseases. They are different from erythrocytes in several ways. In the blood, they are much less numerous than erythrocytes. There are about 6000 to 10 000 leucocytes in every microlitre of blood. Leucocytes have nuclei but do not have haemoglobin. Leucocytes are larger than erythrocytes and do not have fixed shapes. Phagocytic leucocytes can move by changing body shape.
Leucocytes are manufactured in the bone marrow but may migrate to the thymus gland or lymph nodes for their growth and development stages.
There are two basic types of leucocyte, namely granulocytes and agranulocytes. Granulocytes have granular cytoplasm and lobed nuclei. There are three types of granulocyte: neutrophils, eosiniphils and basophils. The neutrophils are phagocytes. They engulf foreign materials, for example, bacteria by phagocytosis, and destroy them. Eosinophils help tto control allergic responses. Basophils secrete heparin to prevent blood from clotting.
Agranulocytes have relatively clear cytoplasm and their nuclei are not lobed. There are two types of agranulocyte: monocytes and lymphocytes. Monocytes are the largest of the leucocytes. They spend only a few days in the blood and then move to the body tissues to become phagocytic macrophages. They engulf dead cells and bacteria that enters the body. Lymphocytes are the smallest leucocytes. Some lymphocytes produce antibodies to aid in the destruction of pathogens or to neutralise toxins whereas others attack and destroy infected cells.
Platelets
Platelets are small irregularly shaped fragments of large cells in the bone marrow. They are important in the process of blood clotting. Clots form to reduce blood loss and prevent the entry of pathogens through wounds. Each microlitre of blood contains about 250 000 platelets.
Plasma
Plasma is the pale yellow liquid part of the blood. It is made up of 90% water and 10% dissolved solutes. The dissolved solutes consist of digested nutrients, dissolved gases, minerals, hormones, plasma proteins and excretory wastes. Blood serum is the same as plasma except that clotting factors such as fibrin have been removed.
Plasma
Plasma is the pale yellow liquid part of the blood. It is made up of 90% water and 10% dissolved solutes. The dissolved solutes consist of digested nutrients, dissolved gases, minerals, hormones, plasma proteins and excretory wastes. Blood serum is the same as plasma except that clotting factors such as fibrin have been removed.
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