The Kidney (11.3)

Question 1

Define excretion.

Answer 1

The removal of waste products of metabolic pathways from the body.

Question 2

What are the functions of an excretory system?

Answer 2

• To remove nitrogenous waste that may be toxic to the body in large concentrations.- To remove excess water to maintain a suitable osmolarity within the tissues and cells.

Question 3

Define osmoregulation.

Answer 3

The control of the water balance of the blood, tissue or cytoplasm of a living organism.

Question 4

How are waste products lost in plants?

Answer 4

Gases are eliminated through the stomata, pores or surface of the leaves.

Question 5

How are waste products lost in aquatic animals?

Answer 5

Their nitrogenous waste is excreted as ammonia directly into the external environment. Ammonia is toxic but also highly soluble and there is ample water available for dilution.

Question 6

How are waste products lost in terrestrial animals? (Birds, insects and mammals).

Answer 6

Terrestrial animals have less access to water so their nitrogenous waste must be packaged in less toxic forms. Birds excrete uric acid (a white paste). This is metabolically more expensive but is relatively non-toxic and takes less water to flush.Insects use malpighian tubules to excrete metabolic waste as uric acid.Mammals excrete urea, which is combined with water and other ions to form urine.

Question 7

What are the main excretory processes in humans?

Answer 7

The formation of urine (urea, water, excess salts and minerals) in the kidneys. Expelled in urination.The formation of carbon dioxide as a result of respiration. Exhaled by the lungs (exhalation).Perspiration is another excretory process which removes salts and water from the body but its primary purpose is cooling.

Question 8

Why is defecation not considered an excretory process?

Answer 8

Because faeces are undigested food remnants and not metabolic waste products.

Question 9

What is the order of toxicity for nitrogenous compounds?

Answer 9

Ammonia > Uric acid > Urea

Question 10

Define osmoconformers and osmoregulators

Answer 10

Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment. This minimises water movement into and out of cells.Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions. This is a more energy intensive process.

Question 11

What is the function of the kidney?

Answer 11

To remove waste products and excess fluid from the body by producing urine.

Question 12

Define the nephron.

Answer 12

The functional unit of the kidney. Filters the blood and eliminates waste as urine.

Question 13

What are the three processes by which blood is filtered in the kidneys and urine is formed?

Answer 13

1) Ultrafiltration - nonspecific filtration of the blood.2) Selective reabsorption - reuptake of useful substances from the filtrate.3) Osmoregulation - the control of the water balance in the blood, tissues and cytoplasm.

Question 14

Describe ultrafiltration.

Answer 14

Ultrafiltration occurs in the glomerulus and the Bowmans Capsule. The capillaries in the glomerulus are fenestrated. Fluid can pass through these gaps. On the outside of the capillary wall there is a basement membrane. This is a mesh-like structure which acts as a sieve, allowing small molecules to pass through. Large molecules like protein remain in the blood. Blood in the glomerulus is at a high pressure. This forces small molecules out of the blood into the Bowmans Capsule. The filtrate produced flows on to the proximal convoluted tubule.

Question 15

Describe reabsorption.

Answer 15

As the filtrate passes along the nephron many substances (around 90% of the filtrate) are reabsorped back into the blood. This occurs mainly in the proximal convoluted tubule but also in the loop of Henle and the distal convoluted tubule.

Question 16

Describe osmoregulation.

Answer 16

Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm. It occurs in the medulla of the kidney. Firstly, the loop of Henle establishes a salt gradient (hypertonicity) in the medulla. The anti-diuretic hormone (ADH) regulates the level of water reabsorption in the collecting duct. When the blood water content is low, it is secreted from the pituitary gland and stimulates water reabsorption in the collecting tubule. So the amount of urine production is controlled (less water in filtrate = concentrated urine + less frequent urination, more water in the filtrate = dilute urine + frequent urination). The urine will then drain to the renal pelvis.

Question 17

Describe the function of the renal pelvis.

Answer 17

All kidney nephrons drain in the renal pelvis. It connects to the ureter which transports urine to the bladder.

Question 18

How does the composition of blood in the renal artery differ from that in the renal vein?

Answer 18

Blood in the renal vein (i.e. blood flowing out of the kidney) has less urea (as large amounts are removed via the nephrons to form urine), less water and ions (depending on the hydration status of the individual), less glucose and oxygen (used by the kidney to generate energy and fuel metabolic reactions) and more carbon dioxide (produced by the kidneys as a by-product of metabolic reactions).

Question 19

How can urine be used to detect kidney disease?

Answer 19

The kidneys prevent the excretion of blood cells and proteins (during ultrafiltration) as well as glucose (selective reabsorption). The presence of these materials in urine can be used as an indicator of kidney disease.

Question 20

What are the consequences of dehydration and overhydration?

Answer 20

Dehydration:- Thirst.- Small quantities of heavily concentrated urine (to minimise water loss).- Blood pressure will drop (less water in plasma) and heart rate will increase to compensate for this.- Physical and mental deterioration.- In severe cases, dehydration may cause seizures, brain damage and eventual death.Overhydration:- Excessive quantities of clear urine (to remove water from the body).- In severe cases, overhydration may lead to blurred vision, delirium, seizures, coma and eventual death.

Question 21

What is the relationship between water conservation and the Loop of Henle?

Answer 21

The length of the loop of Henle is positively correlated with the degree of water conservation in animals.A longer loop of Henle increases the salt gradient in the medulla so more more water from the filtrate can be reabsorbed by osmosis. Organisms that live in arid environments (eg. the desert) need to conserve as much water as possible so they will have longer loops of Henle.

Question 22

Define kidney dialysis and describe how it works.

Answer 22

The external filtering of blood in order to remove metabolic wastes in patients with kidney failure.Blood is removed from the patient and pumped through a dialyzer. Inside the dialyzer, a porous semi-permeable membrane separates the blood from the dialysis fluid. This allows for small molecules (eg. urea) to diffuse into the fluid. The dialyzer also introduces fresh dialysis fluid and removes wastes to maintain an appropriate concentration gradient. The cleaned blood is then returned to the body. Treatments typically last about 4 hours and occur 3 times a week – they can be effective for years.*Patients are given a drug that prevents the formation of blood clots during dialysis (anticoagulants).

Question 23

What is the problem with kidney dialysis?

Answer 23

While it ensures continued blood filtering, it does not address the underlying issue affecting the kidney function.

Question 24

How does a kidney transplant work?

Answer 24

The transplanted kidney is grafted into the abdomen, with arteries, veins and ureter connected to the recipient’s vessels. Donors must typically be a close genetic match to minimise the potential for graft rejection. Donors can survive with one kidney.

Question 25

Give symptoms of kidney failure.

Answer 25

• Reduced urine.- Swollen legs, ankles and feet.- Weakness and fatigue.- High blood pressure.- Confusion.