Response to High Altitude

You need to know.......

• the environmental conditions prevailing at high altitude
• the physiological adaptations associated with life at high altitude i.e. the adaptations found in a native
• the physiological changes experienced by a non-native during acclimatization
• the symptoms of high altitude stress

The high altitude environment

The physical conditions (or abiotic factors to an ecologist) present at high altitude and their significance to humans at high altitude are:

• at high altitudes air is "thinner" i.e. there are fewer molecules per unit of volume so the atmospheric pressure is lower. The number of oxygen molecules per unit of volume reduces so the pressure exerted by oxygen (the pO₂ value) also decreases. From your study of oxygen dissociation curves you should recall that a lower pO₂ value corresponds to lower oxygen saturation of haemoglobin and hence less oxygen transport to the tissues.
• This is a situation known as hypoxia - where there is insufficient oxygen for haemoglobin to become fully saturated.
• But note that haemoglobin has near maximal saturation at pO₂ values as low as 8kPa which reduces the problem somewhat.

• temperature drops linearly with height (at a rate of approximately 1^oC per 150 metres). Potential loss of body heat is thus a problem for mountaineers

• humidity is typically lower at altitude - the rate of sweating is negatively correlated with level of humidity, so low humidity equates to high sweating rate and hence loss of body heat and water

• winds are often stronger at higher altitude so there is a wind chill effect reducing body heat

• molecules in air absorb the sun's radiation, because there are fewer molecules per unit volume in air at high altitude there is less absorption of the [high energy and ionizing] solar radiation. The increase in solar radiation provides heat energy to mountaineers but can cause health problems (e.g. snow blindness, which is damage to the cornea caused by the high energy, and cancer caused by the ionizing radiation)

Adaptations found among groups living at high altitude

• a relatively high lung volume

• a relatively high red blood cell count and hence an increase in haemoglobin concentration

• note that natives of very high altitude areas (4000m) also possess haemoglobin with a greater affinity for oxygen (oxygen dissociation curve shifted to left) so the Hb becomes more readily saturated BUT this means that it does not release its oxygen as readily

• natives of areas of lesser altitude (3000m) show a curve shifted to the right. This means that the oxygen is readily dissociated at respiring tissue but there is less saturation in the lungs

Acclimatizations

• increased breathing rate (hyperventilation)
• deeper breathing rate

• BUT hyperventilation has some serious negatives: 
  • remember that CO₂ in blood combines with water to make carbonic acid and lowers the pH
  • remember that breathing rate is adjusted according to the CO₂ level only (not the O₂) value

• hyperventilation causes the intake of more O₂ as desired, but it also results in more expiration of CO₂

• so the blood becomes alkalaemic (alkalaemia = alkaline blood = blood of high pH)
• this causes the nausea and vomiting associated with mountain sickness (see below)

• The drop in CO₂ associated with hyperventilation causes the breathing rate to decline
• so the CO₂ level increases
• so the breathing rate increases again
• in other words a cycle of irregular breathing is established

• At high altitude the visitor's heart rate increases.
• but the stroke volume remains constant for a few days then falls 
• so, after the first few days of higher cardiac output, the cardiac output is similar to that at sea level but is the result of a faster heart rate but lower stroke volume

• If a visitor remains at altitude their blood cell counts change - the number of erythrocytes increases.
• Erythrocyte production is stimulated by the hormone erythropoietin
• the production of erythropoietin is stimulated by low pO₂
• obviously, an increase in red blood cells results in an increase in haemoglobin and hence of oxygen transported. 

The effects of hypoxia: acute mountain sickness

Hypoxia causes hyperventilation (as already discussed). It can also lead to acute mountain sickness which is a large collection of symptoms:

• headaches
• lack of concentration
• giddiness
• coughing
• difficulty with breathing
• palpitations
• fast heart rate
• loss of appetite
• nausea and vomiting
• muscular weakness
• exhaustion
• poor coordination
• decreased urine output
• oedema

In extreme cases mountain sickness can result in unconsciousness and death

The causes of some these symptoms have been discussed. The mental symptoms (lack of concentration, giddiness etc) can be accounted for by a lack of oxygen to the brain.

The last two symptoms (decreased urine output and oedema) can be explained by changes in the level of the hormone ADH (anti-diuretic hormone) which causes the reabsorption of water from urine as it is made in the kidneys reducing its volume.

In mild hypoxia the level of ADH falls so there is a greater urine output = diuresis.

In severe hypoxia more ADH is made so more water is retained and urine volume falls.

• The retained fluid tends to collect, as tissue fluid, in the lungs and brain.
• Fluid in the lung (pulmonary oedema) makes breathing difficult
• Fluid in the brain (cerebral oedema) causes pressure on the brain resulting in headaches and, if untreated, can be fatal. 

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