Physiological Changes at High Altitude

Question 1

acclimatization at high altitude

Answer 1

when a person ascends to high altitude and stays there for longer periods he slowly gets adapted to the new environment. this process of physiological adaptation to the new altitude is called acclimatization

• starts with in 12 hours and may take several days or weeks to complete
• maximum height to which it occurs - 18000 ft
• primary objective is to increase oxygen supply to the tissues

1) respiratory changes
a) hypoxia stimulates peripheral chemoreceptors
- this inc pulmonary ventilation by inc rate and depth of Respiration
- Initial stimulator effect is less because of breaking effect of low pCO2 which results from CO2 washout.
- After few days, this inhibitory effect disappears due to adjustment in pH of CSF and brain fluid, and ventilation
increases 5 to 7 times.
- Increased ventilation increases alveolar pO2 and thus compensates hypoxia.

b) increased diffusing capacity for oxygen: Results from
a) increased pulmonary blood volume which distends pulmonary capillaries that are already open.
b) opening up of capillaries that normally remain collapsed ( in apical parts of lungs) in response to pulmonary hypertension
c) more stretching of alveoli due to greater depth of respiration. All these factors increase surface area of respiratory membrane and hence diffusing capacity for O2 from resting value of 21 ml/min/mm Hg to
60-65 ml/min/mm Hg.
- Thus O2 diffusion across alveolocapillary membrane increases.

2) changes in blood
a) There is increase in RBC count (polycythemia) and content of blood

• Hypoxia stimulates Kidney to produce
  erythropoietin which stimulates the bone marrow leading to formation of more RBCs.
• Increased Hb content increases the quantity of O2 transported to the tissues.

b) Increased production of 2,3-
diphosphoglycerate in RBCs: Hypoxic conditions favour the production of 2,3 DPG in RBCs.
- This compound reduces the affinity of Hb for O2 (Shifts the Oxygen-Hb dissociation curve to right) and promotes release of O2 to tissues from Hb.

3) cardiovascular changes
There is increase in cardiac output and blood volume by 20 to 30% initially, but with rise in Hb content, it comes back to normal.
b) There is constriction of splanchnic and cutaneous vessels, thus diverting blood to vital organs.
c) Pulmonary hypertension develops from vasoconstriction in response to hypoxia and the right ventricle shows hypertrophy to overcome pulmonary pressure.
d) Chronic hypoxia leads to increased capillarity of active tissues, that is capillary density increases and this reduces the distance through which O2 has to diffuse to reach the tissue

4) Changes in urine: Urine becomes alkaline due increased excretion of HCO3- ions to prevent respiratory
alkalosis.

5) Cellular Changes: Occur at altitudes between 13000 to 17000 feet.
a) There is increase in the number of mitochondria.
b) there is increase in enzyme cytochrome oxidase. These changes ensure efficient utilization O2 by cells.
c) there is also increase in myoglobin content of muscle cells which can extract O2 in extreme hypoxic conditions

Natural acclimatization: is seen in permanent residents of high altitude and consists of

1) large size of chest
2) small body size
3) right ventricular hypertrophy
4) increased Hb content

Applied Physiology : Sometimes the acclimatization fails to occur or disappear after few days leading to ventilatory depression or right ventricular failure, increased viscosity of blood etc. This is called chronic mountain sickness.