from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 5 mark short note on hypoxia- define, classify;describe hypoxic hypoxia
Reading File
Finding Sources
Searching the Web
Guyton Hall hypoxia classification hypoxic hypoxia types of hypoxia
Reading File
Hypoxia
(Guyton and Hall Textbook of Medical Physiology, 4th South Asia Edition)
Definition
Hypoxia means deficiency of oxygen at the tissue level, i.e., inadequate availability of O2 to the cells to support normal aerobic metabolism, regardless of the cause. It differs from hypoxemia (low O2 in arterial blood) and anoxia (total absence of O2), though hypoxemia is one important cause of hypoxia.
Classification of Hypoxia
Guyton and Hall gives a descriptive classification of the causes of hypoxia, which corresponds to the four classical physiological types:
| Guyton's descriptive category | Classical type |
|---|---|
| 1. Inadequate oxygenation of blood in the lungs due to extrinsic reasons (deficiency of O2 in atmosphere, hypoventilation from neuromuscular disorders) | Hypoxic hypoxia |
| 2. Pulmonary disease (hypoventilation from increased airway resistance/decreased compliance, abnormal ventilation-perfusion ratio, diminished respiratory membrane diffusion) | Hypoxic hypoxia |
| 3. Venous-to-arterial (right-to-left) cardiac shunts | Hypoxic hypoxia |
| 4. Inadequate O2 transport to tissues by blood (anemia/abnormal hemoglobin, general or localized circulatory deficiency, tissue edema) | Anemic hypoxia / Stagnant (circulatory) hypoxia |
| 5. Inadequate tissue capability to use O2 (poisoning of cellular oxidative enzymes, diminished metabolic capacity from toxicity, vitamin deficiency, etc.) | Histotoxic hypoxia |
Thus the four classical types are: hypoxic hypoxia, anemic hypoxia, stagnant (circulatory) hypoxia, and histotoxic hypoxia.
Hypoxic Hypoxia
This is the type of hypoxia in which the O2 pressure (PO2) of blood going to the tissues is too low to keep the hemoglobin adequately saturated with O2, so oxygen delivery to tissues is reduced even though blood flow and hemoglobin content may be normal.
Causes:
- Low atmospheric O2 - deficiency of O2 in the inspired air (e.g., high altitude).
- Hypoventilation - from neuromuscular disorders (e.g., poliomyelitis, drug-induced respiratory depression) reducing alveolar ventilation.
- Pulmonary disease -
- Hypoventilation due to increased airway resistance or decreased pulmonary compliance (e.g., emphysema, asthma, fibrosis).
- Abnormal alveolar ventilation-perfusion ratio, including increased physiological dead space or increased physiological shunt.
- Diminished diffusion across the respiratory membrane (e.g., pulmonary fibrosis, pulmonary edema).
- Venous-to-arterial (right-to-left) cardiac shunts - unoxygenated venous blood bypasses the lungs and mixes directly with oxygenated arterial blood (e.g., congenital heart disease such as tetralogy of Fallot), diluting the O2 content of systemic arterial blood.
Effect: In all these conditions, arterial PO2 is reduced, hemoglobin saturation falls, and O2 delivery to the tissues becomes inadequate to sustain normal cellular oxidative metabolism.
Response to O2 therapy: Hypoxic hypoxia due to low atmospheric O2, hypoventilation, or diffusion defects generally responds well to O2 therapy because raising alveolar PO2 increases arterial PO2 and hemoglobin saturation. However, in cases with a large right-to-left shunt, O2 therapy is of only limited value, since the shunted blood never contacts ventilated alveoli.
- Guyton and Hall Textbook of Medical Physiology, p. 547
This is a shared conversation. Sign in to Orris to start your own chat.