Blood delivers necessary substances, such as oxygen and nutrients, to the cells and transports carbon dioxide and metabolic wastes away from the cells.
A normal adult has a blood volume of approximately 5 liters.
Blood, as a connective tissue, is composed of the blood cells (red blood cells, while blood cells, platelets) and the plasma (Fig. 16-29).
< Blood cells >
The size of a red blood cell is about 1/3 of that of a common cell (Fig. 16-1).
Red blood cell takes up oxygen in the lung (Fig. 5-25) and releases it into tissue while squeezing through the capillary (Fig. 10-76).
Red blood cell lacks a nucleus (Fig. 16-2) and most cell organelles in order to accommodate maximum space for hemoglobin. Hemoglobin is an iron-containing molecule that can bind oxygen and is responsible for the red color of red blood cells.
Red blood cell is a biconcave disc with large surface area, which is useful in grabbing oxygen and carbon dioxide. The structure is also good for passing the narrow capillary (Fig. 10-76).
White blood cell has a nucleus that a red blood cell (Fig. 11-6) and platelet do not have (Fig. 11-11). White blood cells can be divided into five types: eosinophil, basophil, neutrophil, lymphocyte, and monocyte.
White blood cell (especially, lymphocyte, monocyte) participates in immunity that is involved in protecting the body against both infectious microorganisms and foreign invaders (Fig. 12-2).
White blood cell is larger than red blood cell and platelet (Fig. 11-3), but only white blood cell can pass through the blood vessel wall (Fig. 12-1). Among the white blood cell to leave the bloodstream, monocyte becomes tissue macrophage, which removes undesirable substances.
Platelet has no nucleus; it is a fragment of a large cell cytoplasm in the bone marrow (Fig. 11-15).
The function of platelet is to contribute to clotting.
Platelets gather at the site to plug the hole. They then connect to each other by fibrins.
Red blood cells are the most common in blood, followed by platelets and white blood cells.
The bone marrow in the medullary cavity (Fig. 1) produces innumerous blood cells, which exit to the blood vessel. Red blood cell lives for 120 days. Old red blood cell is degraded in the spleen (Fig. 12-14).
< Plasma >
Plasma is like the ground substance of the other connective tissue; fibrin in plasma (Fig. 11-13) is like the fiber in the ground substance (Fig. 16-29).
In clinics, blood is collected in a tube that is treated with anticoagulant which prevents blood from clotting (Fig. 11-13). The blood cells are removed by centrifugation; then there remains serum that is used in numerous diagnostic tests (e.g., calcium level, sodium level, blood typing).