Normal values
- HR
- Respiratory rate
- BP
- Temp
- Blood glucose
- HR = 60 - 80 beats/min
- Respiratory rate = 12 breaths/min
- BP = 120/80 mmHg
- Temp = 36.5 -37oC
- Blood glucose = 4 - 5.5 mM
Dehydration and resting blood volume
Dehydration reduces resting blood volume
- Blood volume is 15% lower in dehydrated state
Dehydration and body temperature
Dehydration increases body temperature more rapidly during exercise in the heat
Dehydration and skin blood flow and heat loss
Dehydration dampens the rise in skin blood flow and heat loss
- Dehydration reduces the increase in skin blood flow
- Lower skin blood flow leads to lower rate of heat loss, greater gain in body heat content and a faster increase in body temperature.
Fluid restriction during exercise and the effects on temperature
- Fluid restriction leads to dehydration and fall in blood volume
- Results in faster rise in body temperature
Why is the rise of skin blood flow dampened when we are dehydrated during exercise
So the increase in vascular resistance – partly in the skin – and fall in MAP contribute to the fall in skin blood flow, reduction in heat loss and increase in body temperature.
A fall in stroke volume contributes to the decline in cardiac output
- To maintain CO, heart rate increases
- The fall in stroke volume is linked to reductions in blood volume and filling of the heart (preload).
Ventilation and cerebral blood flow
- Alveolar PCO2 is proportional to arterial PCO2
- Excessive breathing blows off CO2 and lowers arterial PCO2
- CO2 alters cerebral vascular tone: decrease arterial CO2 levels (PCO2 ) leads to vasoconstriction and lowers cerebral blood flow.
- Therefore, hyperventilation can reduce cerebral blood flow.
ventilation is increased by hyperthermia
- A rise in body temperature of 1oC or more results in an increase in ventilation
- And MAP falls, due to a fall in stroke volume and cardiac output
Fall in MAP and increase in ventilation cause cerebral blood flow to fall
- Cerebral blood flow falls progressively during ‘hyperthermia’ test by ~25 %, enough to cause symptoms (e.g., dizziness and nausea).
- Approximately half of the fall in cerebral blood flow (“MCA Vmean”) is due to the fall in MAP.
- The remainder of the fall is attributed to the fall in arterial PCO2 induced by hyperventilation
