Physiology of Hypoxia part 1
Hypoxic hypoxia
1. Part 2 Altitude illnesses
2. Part 3 venous to arterial shunts
1. Causes of
hypoxic hypoxia
1. Inadequate
oxygenation of blood in the lungs due to extrinsic causes.
a. Deficiency of
O2 in the atmosphere
- High altitudes,
mines
b. Hypoventilation
-Neuromuscular
disorders-Fatigue-Depression of the
respiratory center-Causes of hypoxic
hypoxia cont.
2. Lung disease
a. Failure of gas
exchange apparatus
-Congenital heart
disease causing V-A shunts
b. Failure of
respiratory pump
-Pulmonary fibrosis-Ventilation-perfusion
imbalance-Collapse of lung-Pneumothorax-Asthma-Emphysema
High altitudes
& hypoxia
- With increasing altitude:
- Composition of air
stays the same
- But the total
barometric pressure falls
- Therefore, the PO2 falls
- Physiological
changes start taking place due to hypoxia
At sea level –
- Atmospheric
pressure: 760 mmHg
- Partial pressure
of oxygen (PO2) in inspired air: 160 mmHg (760 x 21%)
- Partial pressure
of oxygen (PO2) in alveolar air: 105 mmHg
At 3000m (10,000
ft) above sea level –
- Alveolar PO2: 60
mmHg
- Hypoxia stimulates
chemoreceptors
- Increased
ventilation
If one ascends
higher:
- Alveolar PO2 falls
rapidly
- PCO2 declines due
to hyperventilation
- Respiratory
alkalosis results
- In unacclimatized
subjects:
At 3700 m (12,000
ft):
Irritability,
drowsiness, nausea, headache
At 5500 m (18,000
ft):
Severe hypoxic
symptoms, fits
At 6100 m (20,000
ft):
Loss of
consciousness
At 7000 m (23,000
ft):
Coma and death
Altitude illnesses
1. Mountain
sickness
- Transient syndrome
on first arriving at a high altitude
- Starts 8 – 24 hrs
after arrival, lasts 4 – 8 days
- Headache,
irritability, insomnia, breathlessness, vomiting
- Cause – unknown
- Maybe associated
with more serious altitude illnesses – cerebral & pulmonary oedema
2. High altitude
cerebral oedema
- Cause – unknown
- Assumed – low PO2
causes cerebral arteriolar dilatation
- Increased
capillary pressure (if cerebral autoregulation does not compensate)
- Increased
transudation
- Leads to cerebral
oedema
- May give rise to:
ataxia, disorientation, coma, death due to herniation of the brain through the
tentorium
3. High altitude
pulmonary oedema
- Marked pulmonary
hypertension leading to patchy oedema of the lungs
- Real cause –
unknown
- ? Because some
pulmonary arteries do not have enough smooth muscles to constrict in response
to hypoxia
- In capillaries
supplied by those arteries – increase in pressure
At high altitude…
Those who do not
develop mountain sickness – have diuresis
Those who develop
mountain sickness – decrease in urine output
Labels: RS, RS Revision