Blood is not merely a biological fluid, but a complex functional system and a vital component of the internal environment of the body. It represents a specialized liquid type of connective tissue consisting of plasma circulating within vessels and suspended cellular (formed) elements.
Tissue TypeSpecialized liquid connective tissue
CompositionPlasma and suspended formed elements
FractionsDivided into circulating and pooled (depot) fractions
RegulationControlled by the neurohumoral regulatory apparatus
The Concept of the "Blood System"
In normal physiology, blood is not considered in isolation. The concept of the "blood system" integrates three closely interrelated structural and functional components:
Blood proper. This is the liquid connective tissue itself. It is important to note that it is not entirely located within the vascular bed: physiologists distinguish between the circulating fraction (which continuously moves through vessels) and the pooled/depotted fraction (held in physiological reservoirs).
Hemopoietic and hemocatheretic organs. The sites where formed elements are generated, mature, and where they are destroyed (cleared) after completing their life cycle.
Neurohumoral regulatory apparatus. A complex of neural and humoral mechanisms that rapidly respond to the body's metabolic needs and control the synthesis, release, or destruction of blood cells.
Internal Environment and Basic Functions
Blood permeates absolutely all tissues and organs. Its primary characteristic is a constant, continuous exchange of substances with the interstitial (tissue) fluid. The properties of plasma, cellular elements, and continuous circulation determine the diverse functions of blood.
Respiratory. Ensures gas exchange. Oxygen ($O_2$) is transported to tissues as oxyhemoglobin and, to a small extent, in dissolved form. Carbon dioxide ($CO_2$) is removed from tissues as bicarbonates, carbaminohemoglobin, and dissolved in plasma.
Trophic (nutritional). Blood distributes essential building blocks and energy sources throughout the body: amino acids, lipids, mono- and disaccharides, vitamins, and trace elements.
Excretory. Metabolic waste products are collected by blood and transported to excretory organs: the kidneys, lungs, sweat glands, and gastrointestinal tract.
Regulation, Defense, and Homeostasis
Beyond nutrient and gas transport, blood acts as a global integrator and defender of the organism.
Thermoregulatory function. Blood possesses high heat capacity and thermal conductivity, allowing it to efficiently redistribute heat and maintain a stable core body temperature.
Regulatory function. Blood is the primary pathway for humoral regulation of physiological processes, transporting biologically active substances (such as hormones, enzymes, and vitamins). This transport can occur in free form, bound tightly to carrier proteins, or adsorbed directly onto the membranes of formed elements.
Homeostatic function. Blood maintains the relative constancy of the internal environment, notably providing strict control over acid-base balance.
Protective function. Implemented via two major mechanisms. First, immune reactions (humoral and cellular immunity). Second, hemostasis—a complex network of procoagulant and anticoagulant mechanisms that prevents blood loss upon vascular injury while maintaining blood in a fluid state within the vascular lumen.
Mnemonic
To remember the 7 functions of blood, use the mnemonic: "Respiratory Trophics Excrete Thermal Regulation, Homeostasis, and Defense" (Respiratory, Trophic, Excretory, Thermoregulatory, Regulatory, Homeostatic, Protective/Defense).
Frequently asked questions
Which specific organs belong to the hemopoietic and hemocatheretic systems?
The following structures belong to hemopoietic and hemocatheretic organs:
Hemopoietic organs — red bone marrow, thymus, lymph nodes, spleen. During embryonic development, the yolk sac and liver also participate in hemopoiesis.
Hemocatheretic (destruction) organs — spleen, liver, lymph nodes, and bone marrow.
Which blood buffer systems are responsible for homeostatic function and acid-base balance?
Four main chemical buffer systems maintain the homeostatic function and acid-base balance (ABG/pH control):
Bicarbonate buffer — consists of carbonic acid and bicarbonates. The primary buffer system of plasma.
Phosphate buffer — monobasic and dibasic sodium/potassium phosphates.
Protein buffer — relies on the amphoteric properties of plasma proteins.
Hemoglobin buffer — includes reduced and oxyhemoglobin. It is the major blood buffer system, providing about 75% of total blood buffer capacity.
Which cells are classified as formed elements of blood?
The formed (cellular) elements of blood include:
Erythrocytes (red blood cells).
Leukocytes (white blood cells).
Thrombocytes (platelets).
What components make up the blood system?
The system includes three parts: blood proper (circulating and pooled fractions), hemopoietic and hemocatheretic organs, and the neurohumoral regulatory apparatus.
In what forms does blood transport carbon dioxide?
$CO_2$ transport occurs in three ways: as bicarbonates, as carbaminohemoglobin (bound to hemoglobin), and in a simple dissolved form.
What is included in the protective function of blood?
The protective function includes immune reactions (cellular and humoral immunity) and the hemostatic system—the balance of clotting and anti-clotting mechanisms.
How are biologically active substances transported in the blood?
They can be transported in free form, complexed with plasma proteins, or adsorbed onto the membranes of blood formed elements.
Go deeper
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)