High Altitude Physiology Flashcards
First visible response to high altitude?
Hyperventilation
Why does increasing altitude cause hypoxaemia?
At increased altitude have decreased atm pressure: leads to reduced partial pressure of alveli O2.
Sea level O2: 21kPa Env; 14 kPa alveoli
5000m: 10kPa atm; 7kPa alveoli.
Driving force of oxygen transfer due to a lower partial pressure of 2.5 kPa occuring. (7kPa - 4.5 (partial pressure of o2 in mixed venous blood))
Which chemoreceptors detect change in CO2?
Peripheral chemoreceptors in carotid bodies detect pO2 / hypoxiaemia
Central chemoreceptors in medulla detect pH & paCO2, not O2!
Describe initial the physiological response to hypoxaemia occuring at high altitude
Low pO2 detected in peripheral chemoreceptors: causes hyperventilation.
Hyperventilation causes Co2 levels to decrease
This is detected by central chemoreceptors to try and stop hyperventilating.
Still hyperventilate, just less compared to level that would if had high Co2
A conflict of chemoreceptors !!
Describe 3 physiological changes that initially occur (after 3 hours) at high altitude [4]
- Sympathtic NS activated: increase HR, CO, BP. (BP normalise with time)
- Ventilation / perfusion matching mechanism stimulated by low O2: causes pulmonary vasoconstriction: causes increased hypoxaemia (maladaption)
- Plasma volume decreases due to diuresis caused by carbonic anhydrase action: increases hematocrit
- More EPO produced at kidney: increased RBC count
Describe the changes occuring in O2 saturation curve due to increase in altitude [2]
Hyperventilation increases pH and causes respiratory alkalosis: causes a shift to the LEFT
BUT
2-3 DPG is produced when RBC is hypoxic. This shifts the curve to the RIGHT (after a couple of days)
What is the equation for bicarb. buffer system in body? [1] xx
Which enzyme accelerates this reaction? [1]
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
Enyzme: Carbonic anhydrase
How does your body compensate for resp. alkalosis? [3]
Kidney’s respond by
Decreasing excretion of protons (the more protons the more acid)
Decreasing reabsorption of bicarbonate
Increasing excretion of bicarbonate
Describe how the kidney reabsorbs bicarbonate under normal physiological conditions
At PCT of Kidney, body reabsorbs most HCO3-.
- HCO3- filtered through glomerulus.
- Activity of carbonic anhydrase inside AND outside cell is increased.
- Increased intracellular CA activity increases excretion of H+ outside of the cell through the Na/H+ ATPase pump.
- Extracelluar H+ then reacts with HCO3-, via carbonic anhydrase, to make Co2 and H20
- Co2 reabsorbed into cell. CO2 reacts with H20 via CA to make more H+. repeat
- HCO3- absorbed into blood
Describe how the kidney reabsorbs bicarbonate if there is respiratory alkalosis
- Decreased carbonic anhydrase activity within the cell: less CO2 converted to bicarbonate and protons. Less protons in cell means less protons excreted by sodium-proton exchange ATPase in luminal wall into urine to be excreted.
- This results in less bicarbonate is converted to CO2 in tubule and filtered bicarbonate is excreted in urine
- Leads to decreased excretion protons and decreased bicarbonate reabsorption
How does pulmonary vascular resistance acclimatise to high altitude? [1]
How? [1]
During acclimatisation pulmonary vascular resistance due to vasoconstriction falls.
Due to release of nitrous oxide
Name 4 physiological response that occur for acclimatisation to occur? [5]
- Pulmonary vascular resistance falling
- EPO raises - increase in Hb
- blood pH adjustment
- CSF pH normalised
- 2,3 DPG levels increase
What is a key factor in a persons ability to acclimatise to high altitude? [1]
Genetics xxxx
Smaller degree of hypoxic pulmonary vasoconstriction compared with “low landers”
Higher plasma concentrations of nitric oxide
Higher capillary density in muscles
Name the 3 diseases that can occur if ascent is too rapid xx [3]
1. AMS: Acute Mountain Sickness. The first sign that something is wrong. Major determinants of AMS are the altitude attained, individual susceptibility, rate of ascent and degree of pre-acclimatisation
2. HACE: High Altitude Cerebral Edema Can follow on from AMS. A serious neurological condition; fatal if not treated.
3. HAPE: High Altitude Pulmonary Edema. Equally serious pulmonary condition which can follow on from AMS.
Signs and Symptoms of Acute Mountain Sickness (AMS)?
Symptoms
Headache – required for diagnosis
Insomnia
Fatigue
Anorexia, nausea or vomiting
Dizziness or lightheadedness
Signs
Physical exam typically normal in AMS
(all scored 0-3 for severity of symptoms)
score of > 3 for diagnosis of AMS
Explain the MoA behind HACE
In hypoxaemia ATP supply in nerve cells decreases and sodium pumps run down
Sodium leaks into** nerve cell pulling water with it** - brain cells swell.
Raises intracranial pressure (icp) and blocks cerebral veins
Cerebral circulation fails, hypoxia worsens, neurons, starved of oxygen and squashed together, start to die.
Dead neurons release ions and fluids – worsen cerebral oedema
