Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Blood
Respiratory Function of Blood
Various Forms of Hypoxia

Hypoxia (oxygen deficiency) is a pathological condition that occurs when body Tissues receive an inadequate supply of oxygen or when its utilization in Biological Oxidation is impaired. According to the Classification proposed by I.R. Petrov, hypoxia is divided into 2 groups:

1. Hypoxia caused by a decrease in P_O2 in the inspired air (exogenous hypoxia).

2. Hypoxia resulting from pathological processes that disrupt tissue oxygen supply despite a normal oxygen level in the environment. This category includes the following types: a) respiratory (pulmonary); b) cardiovascular (circulatory); c) hemic (Blood); d) tissue (histotoxic); e) mixed.

Hypoxia due to a decrease in the partial pressure of oxygen in the inspired air. This type of hypoxia primarily occurs during ascent to high altitudes. It can also be observed when the total barometric pressure is normal, but P_O2 is reduced—for example, during mine accidents, malfunctions in aircraft oxygen supply systems, in submarines, and similar situations, as well as during surgery due to anesthetic equipment failure. Exogenous hypoxia leads to The Development of hypoxemia, i.e., a decrease in arterial blood P_O2 and reduced Hemoglobin oxygen saturation.

Hypoxia caused by pathological processes that impair the supply or utilization of oxygen by tissues. The respiratory (pulmonary) type of hypoxia is associated with alveolar hypoventilation, which may result from airway obstruction (inflammatory processes, foreign bodies, bronchospasm) or a reduction in the functional respiratory surface of the Lungs (pulmonary edema, Pneumonia, etc.). In such cases, alveolar P_O2 and blood oxygen tension decrease, leading to reduced hemoglobin oxygen saturation. Typically, carbon dioxide elimination is also impaired, resulting in hypercapnia accompanying hypoxia.

The cardiovascular (circulatory) type of hypoxia occurs in Circulatory Disorders that lead to inadequate Blood supply to Organs and tissues. The blood gas profile in typical cases of circulatory hypoxia is characterized by normal oxygen tension and content in arterial blood, decreased values of these parameters in venous blood, and a widened arteriovenous oxygen difference.

The hemic (blood) type of hypoxia results from a reduced oxygen-carrying capacity of the blood in anemias caused by a significant loss of red blood Cell mass or a sharp decline in hemoglobin concentration within erythrocytes. In these cases, venous blood P_O2 is sharply reduced.

Hemic hypoxia is also observed in Carbon monoxide poisoning (due to carboxyhemoglobin formation), methemoglobin-forming agents (methemoglobinemia), and certain genetic hemoglobin abnormalities. When carboxyhemoglobin and methemoglobin are formed, oxygen tension in venous blood and tissues drops significantly, while the arteriovenous oxygen difference decreases simultaneously.

The tissue (histotoxic) type of hypoxia is typically caused by impaired tissue capacity to extract oxygen from the blood. Tissue oxygen utilization can be hindered by the inhibition of biological oxidation via various toxins, impaired enzyme synthesis, or damage to Cellular Membrane Structures. A classic example of tissue hypoxia is cyanide poisoning. Upon entering the body, CN^- ions actively bind to ferric iron, thereby blocking cytochrome c oxidase—the terminal enzyme of the Respiratory Chain—which suppresses cellular oxygen consumption. In other words, during histotoxic hypoxia, tissues are unable to extract oxygen from tissue capillaries even when P_O2 is high.