An adult’s body contains about 5 liters of blood.
The bulk of the blood is the liquid intercellular substance – plasma (55-60%), which contains the corpuscles (blood cells): erythrocytes, leukocytes and platelets (40-45%).
Blood plasma is 90% water, 10% is dissolved in it organic substances (proteins, fats, carbohydrates) and inorganic compounds (mineral salts). Some of these substances are nutrients carried by the blood to various organs.
The composition of the plasma does not change, despite the constant intake of many substances into the blood. This is achieved by the work of the lungs and kidneys. In the lungs, the blood is freed from excess carbon dioxide, and through the kidneys, excess water, salts and metabolic products harmful to the body are released.
All blood cells are formed from the stem cells of the red bone marrow, which is in the spongy substance of the bones (its mass in an adult is 1.5 kg).
Corpuscles of blood also develop in other organs: the spleen, lymph nodes, tonsils, etc.
- respiratory – carries oxygen from the lungs to all cells of the body and carbon dioxide – in the opposite direction.
- Nutrient – transports nutrients that are absorbed in the intestines.
- Excretory – removes metabolic products from tissues to the kidneys and liver.
- Thermoregulatory – at a low ambient temperature, the blood, warming up, transfers heat from skeletal muscles and liver to those organs that need to be warmed (skin, brain, etc.).
- Protective – thanks to lymphocytes and antibodies, dangerous microbes and substances that enter the body are destroyed and neutralized; platelets provide blood clotting.
- Regulatory – the blood transports hormones and other substances through the body and provides homeostasis (constancy of the internal environment of the body).
All the listed functions of blood are due to its ability to transfer substances from one organ to another, and therefore they can be combined into one function – transport.