Blood is a body fluid in humans and other animals.

Blood performs many important functions within the body, including:

Supply of oxygen to tissues (bound to hemoglobin, which is carried in red cells).

Supply of nutrients such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma proteins (e.g., blood lipids)).

Removal of waste such as carbon dioxide, urea, and lactic acid.

Immunological functions, including circulation of white blood cells, and detection of foreign material by antibodies.

Coagulation, the response to a broken blood vessel, the conversion of blood from a liquid to a semisolid gel to stop bleeding.

Messenger functions, including the transport of hormones and the signaling of tissue damage.

Regulation of core body temperature.

Hydraulic functions.

In vertebrates, it is composed of blood cells suspended in blood plasma. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume) and contains proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), and blood cells themselves.

Albumin is the main protein in plasma, and it functions to regulate the colloidal osmotic pressure of blood. The blood cells are mainly red blood cells, white blood cells and platelets. The most abundant cells in vertebrate blood are red blood cells.

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