what is blood

Blood is a specialized bodily fluid (technically a tissue) that is composed of blood cells suspended in a liquid called blood plasma. Plasma, which comprises 55% of blood fluid, is mostly water (90% by volume), and contains dissolved proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), platelets and blood cells themselves. The blood cells present in blood are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells, including leukocytes and platelets (also called thrombocytes).

The most abundant cells in blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates transportation of oxygen by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is almost entirely transported extracellularly dissolved in plasma as bicarbonate ion. Mammals have red blood, which is bright red when oxygenated, due to hemoglobin. Some animals, such as the horseshoe crab use hemocyanin to carry oxygen, instead of hemoglobin. White blood cells help to resist infections and parasites, and platelets are important in the clotting of blood. [1]

Blood is circulated around the body through blood vessels by the pumping action of the heart. Arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.