White blood cells help to resist infections and parasites. Jawed vertebrates have an adaptive immune system, based largely on white blood cells.

In most insects, this 'blood' does not contain oxygen-carrying molecules such as hemoglobin because their bodies are small enough for their tracheal system to suffice for supplying oxygen. Insects and some mollusks use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed circulatory system. Some animals, such as crustaceans and mollusks, use hemocyanin to carry oxygen, instead of hemoglobin. Vertebrate blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated. In contrast, carbon dioxide is mostly transported extracellularly as bicarbonate ion transported in plasma. These contain hemoglobin, an iron-containing protein, which facilitates oxygen transport by reversibly binding to this respiratory gas thereby increasing its solubility in blood.

The most abundant cells in vertebrate blood are red blood cells. The blood cells are mainly red blood cells (also called RBCs or erythrocytes), white blood cells (also called WBCs or leukocytes), and in mammals platelets (also called thrombocytes). Albumin is the main protein in plasma, and it functions to regulate the colloidal osmotic pressure of blood. 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. īlood is composed of blood cells suspended in blood plasma. Blood in the circulatory system is also known as peripheral blood, and the blood cells it carries, peripheral blood cells. Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells.

Blood & Ashes by Jane Jordan