The body needs to maintain equilibrium to function properly in everyday life. The most important substance it must regulate is water; water is everywhere in our body and its balance is essential for proper body function. A very carefully regulated process is solute concentration. If there is a sudden increase in water which enters the extracellular fluid, sodium ions will then contribute less to the extracellular solute concentration as the ratio between water and solute has now changed. Osmolality is the amount of solute in a kilogram, hence the osmolality in the extracellular space has also decreased. Water diuresis is the increase in urinary water excreted with little or no change in the solute excretion.
Excess water needs to be excreted to maintain a good balance of water and solute inside the body. The aim of this practical is to test the effects of strenuous exercise and desmopressin (anti-diuretic hormone) on urine flow and urine sodium concentration. We will be testing the hypotheses that vigorous exercise will decrease urine flow and increase urine sodium concentration, whereas desmopressin will have the opposite effect of decreased urine flow and increased sodium concentration.
Use the class data (refer to the appropriate figures in your discussion). (a) What happens to the rate of urine production (i.e. urine flow) for the three procedures (i.e. control, desmopressin and exercise)? Use your knowledge of statistics to evaluate the evidence that the responses after the treatments (i.e. desmopressin and exercise) differed from that in the control subjects. Explain the following (including the underlying mechanisms): i. why there is a delay in the onset of the diuresis after water loading in the control subject (A); ii. the effects of administered desmopressin on the diuresis (subject B); iii. the mechanisms by which a single session of vigorous exercise affects the diuresis produced by the water load (subject C).
After drinking water, the control and test subjects had gradual increase of urine flow, reaching a peak then decreasing again, whereas the desmopressin subject had decreased urine flow after taking the hormone, thereafter plateauing. According to the Dunnett’s t test between the urine flow of the subjects, the urine flow of the treatment subjects was significantly different to that of the control. There is a delay in the onset of diuresis after loading in the control subject as it takes time for the water to be filtered in the body. Water is absorbed from the gut into the extracellular fluid. Osmoreceptors from the posterior pituitary detect the water through cell stretch and initiate responses that control ADH secretion (Widmaier et al., 2014).
Fluids are filtered through the kidneys and the excess water is transported to the bladder where it will then be excreted. Desmopressin decreases the urine volume excreted. Desmopressin is a synthetic substitute for anti-diuretic hormone (ADH). ADH acts on the kidneys to reabsorb water. Due to increased water reabsorption, diuresis (urine volume) is decreased. A single session of vigorous exercise sharply decreases the urine flow and hence, diuresis. During exercise, you lose sodium and water by sweating so the kidney works to reabsorb the water so you are not dehydrated. This results in more concentrated urine.
(b) If a control subject was dehydrated at the beginning of the practical class, how would you expect this to affect their response to the water load? They would retain some of the water and hence would have decreased urine excretion compared to someone who is well hydrated.
(c) What effect does alcohol have on water diuresis? What is the mechanism of the action of alcohol on a water diuresis? Alcohol inhibits the pituitary secretion of ADH, which acts on the kidneys to reabsorb water. Because ADH levels drop, the kidneys do not reabsorb as much water and hence produce more urine, causing increased water diuresis.
(d) Use the class graphs and statistical analysis of the urine sodium concentration to determine if this is different for the control and desmopressin subjects. Do you think there a relationship between urine flow and the urine sodium concentration? According to the statistical analysis, the difference between the control and desmopressin subjects for sodium urine concentration were significantly different. Looking at the class graphs, this is also true; the graph values vary significantly. There seems to be an inverse relationship between urine flow and urine sodium concentration. When the urine flow is high, the urine sodium concentration is relatively low and vice versa.
(e) Use the class graphs and statistical analysis of the sodium excretion rate to determine if this is different for the control and desmopressin subjects. Do you think there a relationship between urine flow and the sodium excretion rate? According to the statistical analysis, the sodium excretion rate for the control and desmopressin subjects are not significantly different. This can also be seem from the class graphs; they follow similar values. There does not seem to be a high correlation between urine flow and sodium excretion rate. After taking desmopressin, the subjects’ sodium secretion rate is similar to the control’s, however, the urine flow is noticeably decreased after drinking water. Looking at the exercise subjects, their sodium excretion is lower than the other subjects after drinking water, however their urine flow after a few samples is significantly increased.
(f)Complete the following table:
Type/Site of Receptors
Physiological response on Urine Volume
Physiological response on Blood Volume
Increased osmolality (dehydration)
High ADH levels
Low ADH levels
Increased blood volume
Decreased ADH secretion
Decreased blood volume
Increased ADH secretion
We concluded that strenuous exercise decreases urine flow and hence will increase the concentration of urine. Desmopressin decreased urine flow and due to this increased water excretion, also decreased the concentration of urine.
Widmaier, EP, Raff, H & Strang, KT (2014). Vander’s Human Physiology. The Mechanisms of Body Function. MCGraw Hill, Chapter 14, page 499.