Blood as a system

The Concept of the "Blood System"

In normal physiology, blood is not considered in isolation. The concept of the "blood system" combines three closely interrelated structural and functional components:

1. Blood proper. This is the liquid connective tissue itself. It is important to understand that it is not entirely located within the vascular bed: physiologists distinguish the circulating fraction (which continuously moves through the vessels) and the pooled / depot fraction (held in reserve).
2. Organs of hemopoiesis and hemocatheresis (blood destruction). The sites where formed elements are generated, mature, and where they are utilized after the end of their life cycle.
3. Neurohumoral regulatory apparatus. A complex of neural and humoral mechanisms that sensitively respond to the body's needs and control the processes of synthesis, release into the bloodstream, or destruction of blood cells.

Internal environment and basic functions

Blood permeates absolutely all tissues and organs. Its main feature is constant, continuous substance exchange with the interstitial (tissue) fluid. It is precisely the properties of plasma, cellular elements, and the very fact of continuous circulation throughout the body that determine the manifold functions of blood.

Functions aimed at maintaining basic tissue viability:

• Respiratory. Ensures gas exchange. Oxygen ($O_2$) is transported to tissues as oxyhemoglobin and in small amounts in dissolved form. Carbon dioxide ($CO_2$) is removed from tissues as bicarbonates, as carbaminohemoglobin, and dissolved in plasma.
• Trophic (nutritive). Blood distributes necessary "building materials" and energy sources throughout the body: amino acids, lipids, mono- and disaccharides, as well as vitamins and trace elements.
• Excretory. Metabolic waste products are collected by blood and transported to excretory organs: kidneys, lungs, sweat glands, and the gastrointestinal tract.

Regulation, defense, and homeostasis

In addition to transporting nutrients and gases, blood acts as a global integrator and defender of the organism.

• Thermoregulatory function. Blood possesses high heat capacity and thermal conductivity. Due to this, it efficiently redistributes heat, maintaining a constant temperature in various parts of the body.
• Regulatory function. Blood is the primary pathway for humoral regulation of physiological processes. It carries biologically active substances (e.g., enzymes and vitamins). This transport can occur in a free state, in stable complexes with carrier proteins, or substances can be adsorbed directly onto the membranes of formed elements.
• Homeostatic function. Blood ensures the maintenance of relative constancy of the internal environment parameters, in particular, tightly controlling acid-base balance.
• Protective function. Realized through two powerful mechanisms. First, these are immune responses (humoral and cellular immunity). Second, this is hemostasis — a complex array of procoagulant and anticoagulant mechanisms that prevents blood loss upon vascular injury while simultaneously maintaining blood in a fluid state within the vascular bed.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed composition and physicochemical properties of plasma
• Types and functions of blood-formed elements
• Mechanisms of humoral and cellular immunity
• Coagulation and anticoagulation systems (hemostasis)
• Acid-base balance parameters and buffer systems

Other topics in the "Blood" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.