Blood is a non-Newtonian fluid that is pumped throughout the body to provide nutrients to and remove waste from the cells. It is a mixture of fluid (55%) and cells (45%). Plasma contains proteins, sugars, hormones, and salts. Most of the cells are red blood cells, which contain hemoglobin, a molecule used to transport oxygen. White blood cells are also present, but only at about 1/1000th the rate of red blood cells. White blood cells are part of the immune system and remove old or abnormal cells, pathogens, and other foreign substances. Platelets, which are responsible for blood clotting, are another type of cell within plasma. The transport of oxygen is one of the primary functions of blood. Oxygen from the alveoli is captured by hemoglobin. As the oxygen is bound to the hemoglobin, the partial pressure of oxygen in the blood decreases, enabling more oxygen to diffuse into the blood. Hemoglobin enables the blood to carry 70 times more oxygen than possible through equilibrium alone. Only about 1.5% of the oxygen is carried dissolved in the blood.
CO2 is a form of waste produced by metabolism. This is the same molecule that is produced when burning gasoline in an internal combustion engine. Like an engine, the body produces energy by combining fuel in the form of sugars and other molecules with oxygen, producing CO2 and water. This is a controlled reaction and the waste CO2 produced by the cells diffuses into the blood. Most of the CO2 is combined with bicarbonate in a reversible equilibrium reaction for transport. Bicarbonate serves a crucial biochemical role in the physiological pH buffering system.
The fuel—sugars and other nutrients—is carried within the plasma. The concentration of sugar is carefully regulated within the body. Abnormal blood sugar levels are associated with the disease diabetes mellitus.
Blood also contributes to thermal regulation. When the body’s core temperature rises, blood supply to the skin and extremities, where it is more easily dissipated, is increased. When the body is cold, blood is retained in the core, reducing heat loss.
Because it is a non-Newtonian fluid, the viscosity of blood is not constant. It is shear thinning. This means that the fluid becomes less viscous as shear stress increases. This property is advantageous because blood needs to travel through very small capillaries as it circulates through the body.
