© SSER Ltd.

Water Balance The concentration and volume of the urine, excreted by mammalian kidneys, is determined by the amount of water reabsorbed from the distal convoluted tubules and collecting ducts The permeability to water of the cells lining the distal tubules and collecting ducts is controlled by the hormone, antidiuretic hormone (ADH) When ADH is present in the blood, the permeability to water of the cells lining the distal tubules and collecting ducts increases; water can therefore be reabsorbed from the urine and a small volume of concentrated urine is produced When ADH levels in the blood are low or absent, the cells lining the distal tubules and collecting ducts are impermeable to water; water reabsorption does not occur and a large volume of dilute urine is produced

When ADH is present, the walls of the distal convoluted tubules and collecting ducts are permeable to water Water passes by osmosis into the hypertonic medulla and is reabsorbed by the blood vessels A small volume of concentrated urine is produced, and water is conserved - antidiuresis

When ADH is absent, or present at low blood levels, the permeability of the distal convoluted tubules and collecting ducts decreases Most of the water passes through the hypertonic medulla without being reabsorbed A large volume of dilute urine is produced, and excess water is eliminated - diuresis

Homeostatic Control of Blood Osmotic Potential The osmotic potential of the blood reflects the water concentration of the blood; the water balance of the tissues is dependent upon the maintenance of a relatively constant blood osmotic potential Blood osmotic potential is constantly monitored by osmoreceptors in the hypothalamus of the brain The hypothalamus manufactures the hormone ADH and secretes this hormone down axons that link the hypothalamus to the posterior lobe of the pituitary gland; the pituitary gland secretes ADH into the bloodstream The hypothalamus is the detector of changes in blood osmotic potential; ADH levels, from the pituitary gland, increase when the blood osmotic potential falls, and decrease when blood water concentration increases

Osmoreceptors in the hypothalamus monitor changes in the blood osmotic potential Neurosecretory cells manufacture ADH and secrete this hormone into neurons that link to the posterior lobe of the pituitary gland; the pituitary gland secretes the ADH into the bloodstream When ADH is present, the walls of the distal convoluted tubules and collecting ducts are permeable to water and water is reabsorbed

increase in blood osmotic potential (water concentration rises) decreased ADH output from the posterior lobe of the pituitary gland restoration of the norm (negative feedback) decrease in blood osmotic potential (water concentration falls) detected by osmoreceptors in the hypothalamus increased ADH output from the posterior lobe of the pituitary gland restoration of the norm (negative feedback) detected by osmoreceptors in the hypothalamus permeability of distal tubule and collecting duct decreases less water reabsorbed; copious dilute urine produced more water reabsorbed; small volume of concentrated urine produced permeability of distal tubule and collecting duct increases

A Study of Osmoregulation in the Human Kidney During an investigation into the functioning of the human kidney, several subjects drank a large volume of distilled water Each subject emptied their bladder (time 0 min) and then drank a litre (1 dm 3 ) of distilled water Urine samples were collected, for each subject, at 30 minute intervals over a period of two hours; the volume of urine was measured and the chloride concentration was determined using the Quantab chloride titrator The data obtained from one of these subjects is displayed in the following table

Time/min Volume of Urine (cm 3 ) Salt (chloride concentration) g/100cm Using the same graph for both sets of data, plot a bar chart for the volume of urine produced and a line graph for the salt concentration

Explain the changes in the volume of urine collected and the salt concentration over the 120 minute period

The Relationship between Medullary Thickness and Kidney Function The thickness of the medulla, and the ability to produce a concentrated urine varies in different mammals The following table provides data obtained from different mammals with respect to medullary thickness and urine concentration

Mammal Relative Thickness of Medulla Urine Concentration (  o C) Beaver Pig Human Cat Rat Kangaroo Rat Using the same graph for both sets of data, plot a bar chart for the relative thickness of the medulla and a line graph for the urine concentration

Explain the relationship between medulla thickness and urine concentration and account for differences between the mammals

The kangaroo rat occupies the deserts of North America and displays a remarkable ability to tolerate drought conditions In its natural habitat, the kangaroo rat rarely, if ever, drinks water and survives on a staple diet of ‘dry’ seeds

Water Balance of the Kangaroo Rat This remarkable water balance is achieved through physiological and behavioural adaptations Water Gain 6g in Food 54g in Metabolic Water (formed during cellular respiration) Total Gain = 60g Water Loss 13.5g in Urine 44g in Expired Air from the lungs (evaporation) 2.5g in Faeces Total Loss = 60g

These are physiological adaptations that promote water reabsorption in the kidney tubules such that the kangaroo rat produces urine that is 17 times as concentrated as its blood Physiological Adaptations Long loops of Henlé High levels of ADH in the bloodstream

The kangaroo rat has no sweat glands and does not lose water through evaporation from the skin Physiological Adaptations The large intestine of the kangaroo rat is adapted for absorbing large amounts of water from waste food; the resulting faeces are dry with a low water content

Behavioural Adaptations The kangaroo rat spends long periods of time in its cool, humid burrow where it remains inactive; inhaled air thus has a similar water content to exhaled air and water loss is minimised Kangaroo rats are nocturnal and forage for food when desert temperatures are at a minimum

Acknowledgements Copyright ©2006 SSER Ltd. and its licensors. All rights reserved. Certain Images © Certain Images ©