Question

A backpacker weighs 180 pounds. Assume his blood weight is about 7% of his body weight. His hematocrit at rest is 40%, and his total plasma proteins 6 g%. After hiking on a hot day, the backpacker loses a net 4 L of body fluid by sweating and drinking, where the drinking is insufficient to match his sweating. Assume that the plasma makes up 8% of his total body water and that the fluid lost from each compartment is in proportion to the size of each fluid compartment.

(A) Assuming no loss of blood, what is his hematocrit after his hot hike?

(B) What is his total plasma protein concentration after hiking?

(C) What effect would these changes have on his blood viscosity?

(D) Describe how the body reacts to losing water due to sweat, and to taking too much water from a meal. In general, how is the water balance reached?

Answer 1

(A) GIVEN,

weight of the backpacker = 180 pounds i.e. 81.6kg

weight of his blood is 7% of his body weight i.e. 7/100 X 81.6 = 5.7 kg

so, volume of blood = mass of blood/ density of blood i.e. 5.7 kg/1056.5 kg /m3 = 5.3 L

he loses 4L of body fluid by sweating and plasma makes up 8% of his total body water.

So, 8/100 X 4 = 0.32L

Therefore, the blood volume now becomes = 5.3 - 0.32 = 4.98L

( Hematocrit = packed cell volume / Total volume)

Now, the packed cell volume earlier was, 40% of 5.3 i.e. 40/100 X 5.3 = 2.12 L

But the blood volume has now changed to 4.98 L

so, the new hematocrit ratio = 2.12/ 4.98 = 0.42

It is mentioned that the fluid lost from each compartment is in proportion to the size of each fluid compartment and assuming that 8% of RBCs fluid also gets lost, then the packed cell volume decreases from 2.12 L to 0.92 X 2.12 =1.95 L.

Therefore, the backpacker's Hematocrit after this hot hike = 1.95/4.98 = 0.39 or 39%

(B) His initial plasma protein concentration = 6g

His initial blood volume = 5.3 L

After hiking the blood volume becomes 4.98L

so, the plasma concentration after hiking = 6/5.3 X 4.98 = 5.6 g

(C) The blood viscosity will increase. we can infer this from the above calculations.

we can see that initially, the packed cell volume was 2.12L in 5.3L of blood. But after hiking the blood volume of that person decreases to 4.98L. so a packed cell volume of 2.12L in 4.98L of blood will definitely increase the blood viscosity.

Apart from this, many studies on high altitude hikers, backpackers have suggested that the RBC and packed cell volume of such person increase as it has to meet the high oxygen and energy requirements of the body.

(D) The requirement of water in the heat is dependent on fluid loss (sweating). Evaporative cooling is the primary determinant for checking water balance and maintaining thermal balance during exercise or other activities. In warm or hot environments cutaneous sweat secretion majorly cools the body as respiratory water loss contributes very little to evaporative cooling. Sweat rate is proportional to the metabolic rate of a person and can range from 3/4 liters per hour to around 10 liters per day. so drinking water ad libitum can certainly maintain the water balance and the excess water in the body is removed as urine. if dehydration is not treated it can even lead to shock.

Taking excess of water aka overhydration is very dangerous to the body and can lead to water intoxication. It can dilute the essential salts and electrolytes in the body. it might lead to hyponatremia (low level of sodium ions in the blood) and hypokalemia ( decreased potassium ions) which could be fatal.