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—Blood Vessels. Its intercellular substance is in a liquid state and is referred to as plasma. Suspended in it are formed elements: erythrocytes (red Blood Cells), leukocytes (white blood cells), and thrombocytes (Blood Platelets). Plasma accounts for 55–60% of the total circulating blood volume, while formed elements make up 40–45%. The hematocrit is The ratio of the volume of cellular elements to the total blood volume.

The volume of circulating blood is one of the homeostatic constants of the body's internal environment, varying according to age, sex, and the individual's functional state. In adults, blood accounts for 6–8% of 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 higher than in adults, making up 15% of body weight in newborns and 11% in infants under one year of age. Physically trained individuals have significantly more blood than sedentary people. Under normal conditions, 75% of all blood circulates actively within The Vascular System, while the remaining 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 the circulating blood volume.



Last update: 08/08/2026

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