11.3 The Kidney & Osmoregulation

One effect of an ageing population is the increase in deaths and health complications from dehydration in the elderly, many of whom lack a thirst response when partially dehydrated. 

A study was performed to examine the effect of ethanol ('alcohol') consumption on the urine output of a group of elderly men. 

Ethanol is a known diuretic.

Define the term diuretic and outline its mode of action. 

In the study referred to in part a), a group of 20 elderly men (age range 65-75) were given the same diet and were optimally hydrated before the study. All the men consumed no alcohol for 48 hours prior to the experiment. At the beginning of the study,

• one subgroup was given alcoholic beer, another non-alcoholic beer
• one subgroup was given alcoholic wine, another non-alcoholic wine
• one subgroup was given strong spirits, another plain water
• for the men given alcoholic drinks, an equivalent volume of ethanol was consumed in each case
• for the men given non-alcoholic drinks, the same volume was consumed as for their alcohol-containing counterparts

In each case, the men's average cumulative urine output was measured over the 4 hours following consumption of the drinks. The results are shown below.

Calculate the percentage difference between the urine produced in the first hour by men drinking alcoholic beer versus those drinking alcoholic wine. 

The researchers conducting the study concluded that drinking beer of any variety contributed more to dehydration than any other choice of drink. 

Evaluate this conclusion.

Organisms that live in a saltwater environment face extreme osmoregulatory challenges. 

Suggest why it is so difficult for marine mammals to maintain the correct osmotic balance in their blood.

The table below contains information about several marine mammals and their osmoregulatory mechanisms. Humans have been added for comparison purposes, and seawater sodium concentration has also been included.

Suggest how sea otters are able to drink seawater and still maintain osmotic balance.

Manatees are unusual marine mammals in that they are able to spend time in both saltwater and freshwater environments.

Research into osmoregulation in manatees shows that levels of a hormone called aldosterone change when the manatee’s environment changes. Aldosterone is known to activate a sodium transporter protein in the cells lining the nephron.

Suggest how the saltwater and freshwater environments might affect aldosterone levels and explain how this could help the manatee regulate its ion balance.

Sea birds do not osmoregulate in the same way as marine mammals. They excrete nitrogenous waste via highly concentrated uric acid, and excrete excess salt separately via salt glands.

The diagram below shows the structures within the salt glands, where a countercurrent mechanism involving capillaries and salt secretory tubules removes excess salt to an external secretory duct. 

Explain how the countercurrent system ensures maximum salt excretion from the blood of sea birds.

The glomerulus is a structure in the kidneys responsible for the process of ultrafiltration. Explain how ultrafiltration would be affected by severe dehydration. 

An important role of the kidneys is the removal of urea from the blood. The amount of urea removed from the blood can be used as a measure of the rate of ultrafiltration, also known as the glomerular filtration rate (GFR).

An individual excreted 540 mg of urea from the blood over the course of 1 hour, and has a blood urea concentration of 0.01 mg cm^-3 entering the kidneys. Use this information to calculate this person's GFR.

State your answer in cm³ min^-1. 

After ultrafiltration, the filtrate travels through the kidney nephron. The graph below shows what happens to various components of the glomerular filtrate as they move through the different regions of an individual’s nephron.

Describe and explain the shape of the curve for sodium ions and water as they travel through region 2 of the nephron as shown in the graph.

Explain how the plotted line for water in region 4 of the graph in part (c) would look different if the ADH concentration in this individual's blood were to decrease. 

One mark is available for clarity of communication throughout this question.

Draw and label a diagram of a human kidney.

The diagram below shows a human nephron and its associated blood supply.

Annotate the labels A - F shown to explain briefly the functions of the various parts of the nephron. 

Kidney patients on a course of haemodialysis treatment are advised to increase their protein intake and limit the amount of ions (eg. potassium, phosphorus, sodium) and fluid in their diets. 

Suggest three reasons for this dietary advice. 

The image below shows a kidney nephron.

Identify the structures labelled A-C in the image.

State, with a reason, one feature of structure A from the image in part a) which enables ultrafiltration to take place.

Selective reabsorption takes place after ultrafiltration.

Describe the events of selective reabsorption.

The graph below shows what happens to various components of the glomerular filtrate as they move through the different regions of an individual’s nephron.

Describe and explain the shape of the curve for glucose concentration.

Explain the shapes of the curves for sodium ions and water in the graph in part a) as they travel through the first half of region 2 of the nephron. Note that region 2 in this graph is the loop of Henle.

The image below shows cells lining the collecting duct. ADH released from the pituitary gland binds to the receptor proteins on the collecting duct cell surface membranes. This triggers vesicles containing aquaporins to bind with the plasma membrane next to the lumen. Note that aquaporins are channel proteins that enable the movement of water molecules.

Explain how ADH increases the movement of water from the lumen of the collecting duct into the blood.

Suggest why ADH only affects the cells lining the kidney nephron.

Outline the link between excretion of nitrogenous waste and water loss in mammals.

The table below shows the maximum urine solute concentrations that can be achieved in different species of mammal. Seawater has been included for comparison.

Humans have a blood plasma concentration of roughly 120 mEq L^-1.

Calculate the urine:blood plasma osmolarity ratio in humans.

Use information from part b) and your own knowledge to explain how kangaroo rats are adapted for survival in a dry environment.

A researcher wanted to investigate kangaroo rat kidneys. They fed the kangaroo rats on soybeans for several days to increase their thirst (kangaroo rats normally get all their water from their food), and then either gave the rats access to fresh water, seawater, or no water for the following 16 days, using weight over the experimental period as a measure of rat health. The results are shown in the graph below.

Suggest, with a reason, one possible conclusion about the capabilities of kangaroo rat kidneys that can be drawn from the graph above.  Note that humans cannot drink seawater without becoming severely dehydrated.

One mark is available for clarity of communication throughout this question.

Draw a labelled diagram of the human kidney.

Outline the process of haemodialysis.

Describe how nitrogenous waste is excreted in land insects.