Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Internal Environments of the Body. Blood
Basic Concepts

The Human Body is about two-thirds Water. It is the primary component of virtually all Tissues, located both intracellularly and extracellularly. Fluid tissues, such as Blood AND Lymph, have the highest water content. In addition to water, tissue fluid contains various organic substances synthesized by Cells.

Blood, lymph, and tissue fluid make up the internal environment of the Organism.

Blood is a fluid tissue, the volume of which in an adult is 5 — 6 l (7 — 8% of body weight). Its relative density is 1.052—1.064. Blood circulates through Blood Vessels. Within the capillary network, it exchanges substances with the intercellular fluid. Nutrients and oxygen pass through the Capillary Wall to the cells, while Metabolic waste products return to the blood.

Lymph is a fluid tissue formed from tissue fluid within blind-ending lymphatic capillaries: excess intercellular fluid enters them through large pores between endothelial cells. This allows protein and fat molecules to penetrate the lumen of the microvessels.

Over the course of a day, 2—4 l of lymph is produced in the body. At the same time, the volume present within the Lymphatic vessels is approximately 0.5 —1.0 l. Lymph contains cellular elements, predominantly immune system cells—lymphocytes—which play a vital role in protecting the body against infectious diseases.

Homeostasis. The internal environment of the body is characterized by its constancy. Temperature, the pH of blood and lymph, and The chemical composition of fluid media are maintained at specific levels within the body.

Despite changing external conditions, the key biochemical parameters of the internal environment remain virtually constant. When any internal environment factor changes, powerful self-regulation systems are activated in the body. They coordinate the functioning of Organs and systems aimed at restoring PHYSIOLOGICAL AND BIOCHEMICAL parameters to levels constant for the individual. This set of mechanisms that ensures the Maintenance of the constancy of the body's internal environments is called

homeostasis.

Thus, during heavy physical exertion, tissues actively consume oxygen. Its amount in the blood and intercellular fluid decreases, while the concentration of carbon dioxide, conversely, increases. The rise in CO₂ concentration is detected by specialized receptors that transmit this information to the respiratory center. In response, the breathing rate increases, allowing significantly more oxygen to enter the blood and more carbon dioxide to be eliminated from the body per unit of time. Simultaneously, BLOOD FLOW IN the tissues intensifies. This accelerates the outflow of blood carrying dissolved carbon dioxide to the Lungs and the inflow of oxygen-rich blood from the lungs to the tissues, thereby maintaining the homeostasis of gas composition. The Main parameters of homeostasis are presented in App. 1.