Blood

Blood is perhaps the most complex fluid found in nature and it is essential to life of humans and other vertebrates. Blood is a suspension of cells in an aqueous medium containing many micro- and macro-molecules, including various proteins, hormones, vitamins, minerals, amino acids, glucose, and nutrients. Blood is circulated throughout the body to deliver nutrients and oxygen to cells of the body, and transport metabolic waste products including carbon dioxide away from the cells. It is circulated through the blood vessels (arteries and veins) by the pumping action of the heart. In some vertebrates, blood carries oxygen from the inhaled air through arteries to tissues and cells that utilize oxygen to carry out metabolic activities, while the veins carry deoxygenated blood containing carbon dioxide, which is a waste product of metabolism, from the tissues to the lungs to be exhaled. The arterial blood is bright red, while the venous blood is dark red in color.

The fluid portion of the blood is called plasma, representing about 55% of the blood volume and the blood cells constitute about 45%. The main protein in plasma is albumin which functions to regulate the colloidal osmotic pressure of blood and helps keep fluid in the bloodstream, so it doesn't leak into other tissues. The most numerous cells of blood are erythrocytes or red blood cells that are non-nucleated in mammals and in human they have the shape of biconcave discs. The other cells in blood include the leukocytes or white blood cells that give protection against invading pathogens and foreign tissues, and platelets (also called thrombocytes) that are essential in blood clotting. Blood is typed on the basis of genetically determined antigens present on the surface of erythrocytes. The erythrocytes contain hemoglobin, an iron-containing protein that binds oxygen and facilitates its transportation through the blood. Some animals, such as crustaceans, arthropods, octopus, use hemocyanin (a copper-containing protein) to carry oxygen instead of hemoglobin to their bloodstream.

Blood has no substitute. Only the generous blood donors can donate their blood for those who need a blood transfusion to save lives. [Laila Noor Islam]

See also blood pressure, blood thinner.