applied physiology altitude

5 categories of altitude

1. Near sea-level

2. Low

3. Moderate

4. High

5. Extreme

Near sea-level

<500m

Low

500-2000m

Moderate

2000-3000m

High

3000-5500m

Extreme

>5500m

Acute mountain sickness (MS)

When travelling quickly from ‘low’ altitude to above 2500m without sufficient acclimatisation - from reduced O2 availability and hypoxaemia at altitude

PO2 at 2500m is ~15% (compared to ~21% at sea level)

AMS symptoms

headaches, shortness of breath, nausea, dizziness

What does reduced PO2 cause from MAS?

Hypoxaemia and inadequate O2 delivery to tissues, particularly the brain

What happens to Hb saturation at altitude?

Decreases due to reduced PO2

Why does rapid ascent increase AMS risk?

The body has insufficient time to acclimatise

What detects hypoxia? What does it do?

Peripheral chemoreceptors in carotid bodies detect hypoxia and stimulate the respiratory centres to increase pulmonary ventilation = hypoxic drive

Pulmonary ventilation

Increases quickly at high altitude through increased tidal volume and respiratory rate as chemoreceptors noticed drop in PO2

Respiratory volume stabilises 2-3 litres/min higher

Takes days to return to normal

Respiratory alkalosis

Increased ventilation = decrease in CO2 and H+

Shifts oxyhaemoglobin saturation curve to left → enhance O2 loading in lungs

Prevents ventilation from rising further = short term benefit to maintain O2 delivery when environmental O2 availability is reduced

Leads to compensation mechanisms: 2,3-DPG production

2,3-DPG

Produced within RBCs and increases during acclimatisation to altitude → counteract leftward shift is O2 HB curve by respiratory alkalosis → promotes right shift = reduce Hb affinity to O2 = O2 unloading to tissues (for exercise, high altitude in O2 demand)

Cardiovascular response to altitude exposure

Increase HR - sympathetic innervation, circulating adrenaline, B1-adrenergic receptor stimulation

Decrease SV - loss of plasma volume (reduced venous return and left ventricular filling) or increased BP via increased systemic vascular resistance

Why does plasma volume decrease at altitude?

Respiratory water loss (low humidity with altitude)

Increased urine production

metabolic responses to altitude

BMR increases by 6 to 27% over 48 hrs at altitude

This increase is proportional to altitude gain

Remains elevated, but at reduced rate → increased thyroxin and catecholamine secretion

Food intake must be increased as increased BMR increases energy expenditure

But appetite is often suppressed at high altitude

Andeans

Live in the Andes (South America)
Adapt to high altitude by increasing red blood cells
• can lead to chronic mountain sickness
• polycythemia (too many red blood cells)
• afflicted also suffer from pulmonary hypertension
• increased risk of heart attack and stroke
• treatment involves moving to a lower altitude

Tibetans

Live in Tibet (Himalayas); 4000m
Mutations reduce biological response to low oxygen
PHD2 (part of HIF response regulator) and HIF2A; both instigate bodies response
to hypoxia
• Higher lung capacity, and efficient use of energy
Population was in isolation for 3,500 years
• Sufficient time for evolution to select characteristic
Do not suffer chronic mountain sickness

High altitude training

Body compensates for lower oxygen availability by increasing rbc (and other adaptations)
Benefits with higher blood oxygen carrying capacity when returning to compete at sea level
However, hypoxia at altitude prevents high-intensity aerobic training
Living and training high leads to dehydration, low blood volume, low muscle mass
Value of living at high altitude cancelled by reduced training capability
Solution: live high, train low