MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 4. THE BLOOD SYSTEM

Blood is a fluid tissue of the body that circulates within a system of closed tubes—the Blood Vessels. Its intercellular matrix is in a liquid state and is known as plasma. Suspended in the plasma are formed elements: erythrocytes (red Blood Cells), leukocytes (white blood cells), and thrombocytes (Blood Platelets). By volume, plasma accounts for 55–60% of total circulating blood, while formed elements make up 40–45%. The hematocrit is The ratio of cellular element volume to total blood volume.

The volume of circulating blood is one of the homeostatic constants of the body's internal environment and varies depending on a person's age, sex, and functional state. In adults, blood accounts for 6–8% of total body weight. For instance, a young adult weighing 70 kg has approximately 5–6 liters of circulating blood. In children, the relative blood volume is greater than in adults, comprising 15% of body weight in newborns and 11% in infants up to one year of age. Physically trained individuals have significantly more blood than those with a sedentary lifestyle. Under normal conditions, 75% of all blood circulates within the active vascular bed, while 25% is held in blood depots (the Spleen, Liver, and Skin). From these reservoirs, blood can enter the bloodstream whenever There is a need to increase circulating blood volume.



Last update: 08/08/2026

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