Hypoxia refers to inadequate oxygen supply to the cells and tissues of the body or a local part of the body. There are multiple types of hypoxia:
- Hypoxaemic hypoxia
- Anaemic hypoxia
- Circulatory hypoxia
- Histotoxic hypoxia
When the pO2 levels are abnormally low the condition is actually called hypoxaemia and not hypoxia. However, these terms are often used interchangeably.
Hypoxaemic hypoxia occurs when the pO2 levels are so low that tissues of the body become hypoxic. We usually define hypoxaemia as a pO2 below 60 mmHg. At this point the peripheral chemoreceptors will be activated, stimulating ventilation.
Anaemic hypoxia occurs when the haemoglobin levels are so low that tissues of the body become hypoxic.
Circulatory hypoxia, also called ischaemia occurs when a part of the body doesn’t receive enough blood flow, causing it to become hypoxic.
Histotoxic hypoxia occurs when tissues are poisoned so that they cannot use oxygen normally. This occurs in the case of cyanide poisoning.
Acclimatization is the process where a person adapts to chronic hypoxia, for example when living in high altitude. The body has multiple mechanisms to make the best of the oxygen it has available.
There are four major mechanisms involved in acclimatization:
- Changes in the blood
- Changes in the respiration
- Changes in the circulation
- Changes in the tissue metabolism
Changes in the blood:
During chronic hypoxia the kidney will be stimulated to produce more erythropoietin, a hormone which stimulates the bone marrow to produce more red blood cells. The increased red blood cell count in the blood means that there is more haemoglobin in the blood, which increases the utilization of the oxygen available.
RBCs will also start to produce more 2,3-DPG. This molecule causes a right shift of the haemoglobin-oxygen dissociation curve, which means that haemoglobin gives off oxygen to tissues more easily.
Changes in the respiration:
Chronic hypoxia stimulates the breathing centre, causing the respiratory frequency and tidal volume to increase. This increases the amount of oxygen available for gas exchange, but it might cause respiratory alkalosis due to washing out of CO2.
Changes in the circulation:
The heart will increase its cardiac output, which provides more blood to all tissues.
Changes in the tissue metabolism:
Cells will produce more mitochondria, which increases the utilization of oxygen. Glycolysis (anaerobic metabolism) will also be stimulated.
45. Chemical control of respiration. Acidosis, alkalosis
56. Dynamics of glomerular filtration. Glomerular filtration rate. Plasma clearance