Response to challenges

Two balances that affect each other

The body needs to keep both its temperature and the concentration of its body fluids within suitable ranges. These are parts of homeostasis: keeping the internal environment relatively stable as conditions change. Water links the two balances. Evaporating sweat helps prevent overheating, but sweating also removes water and dissolved salts from the body.

The kidneys help balance water gained from food and drink against water lost in sweat, breath and urine. They filter the blood and return some water from the fluid in the kidney tubules to the blood. Changing how much water is reabsorbed changes the volume and concentration of urine. Thirst encourages water intake. Together, thirst and kidney function help the body respond to an osmotic challenge—a change in the balance of water and dissolved substances in body fluids.

Hot conditions and heavy sweating

When body temperature rises, more blood flows near the skin surface as skin blood vessels dilate. Sweat glands release sweat; as it evaporates, it transfers heat away from the skin. During exercise or in hot weather, heavy sweating can remove substantial water and some salts. If water loss exceeds intake, the person becomes dehydrated and the blood becomes more concentrated.

This increased concentration stimulates thirst and greater release of ADH. ADH makes kidney tubules and collecting ducts more permeable to water, so more water is reabsorbed into the blood. Less water is then lost in urine: the urine is usually smaller in volume and more concentrated. The kidneys can conserve water, but they cannot replace water already lost; drinking is needed to do that. If sweating is reduced in cooler conditions and fluid intake remains similar, the kidneys can allow more water to leave in a larger volume of paler, more dilute urine. Shivering and reduced blood flow near the skin also help the body respond to cold by generating heat and reducing heat loss, respectively.

Excess water and high salt intake

After drinking more water than the body needs, the blood becomes more dilute. Less ADH is released, so the kidney tubules reabsorb less water. More water leaves as a large volume of dilute urine, helping return the concentration of body fluids towards its usual level.

A high salt intake presents the opposite osmotic challenge: there is more dissolved salt relative to water, so body fluids become more concentrated. Water may move out of cells by osmosis. Thirst encourages drinking, while increased ADH causes the kidneys to conserve water by producing less, more concentrated urine. The kidneys also remove excess salts in urine. Responses depend on the whole situation: a person who has eaten salty food and then drunk plenty of water may not produce the same urine as someone who remains short of water.

In each case, trace the change from the challenge, through the change in body-fluid concentration, to thirst and kidney response. These responses oppose the original change rather than making it larger.

Temperature and water loss

Hot conditions or exercise → sweating → evaporation cools the skin, but water and salts are lost. Heavy sweating can cause dehydration. Cold conditions → less sweating; shivering and reduced skin blood flow help maintain temperature.

Osmotic challenges

Dehydration or high salt intake → body fluids become more concentrated → thirst and more ADH → more water reabsorbed by kidneys → smaller volume of concentrated urine. Drinking replaces lost water.

Excess water intake → body fluids become more dilute → less ADH → less kidney water reabsorption → larger volume of dilute urine.