Water Balance & Excretion (Kidneys)

Watch first

FreeScienceLessons · AQA BiologyThe Kidneys and Excretion

Key facts

Kidneys maintain the water balance of the blood and remove urea (toxic waste from protein breakdown).
Urea is made in the liver when excess amino acids are broken down (deamination).
Kidneys filter blood by: filtration (everything small passes into tubule — glucose, urea, water, ions) then selective reabsorption (glucose and needed water/ions reabsorbed; urea and excess water pass out as urine).
Urine passes to bladder via ureter, stored, then excreted via urethra.

Dialysis — blood passes through machine, toxins removed by diffusion across a partially permeable membrane, filtered blood returned to body. Done 3× per week, 4+ hours per session.
Kidney transplant — permanent solution. Donor kidney transplanted. Patient takes immunosuppressant drugs to prevent rejection.
Advantages of transplant: more freedom, feels normal, not tied to machine. Risks: rejection, side effects of immunosuppressants, limited donors.
ADH — antidiuretic hormone. Controls water reabsorption. More ADH → more water reabsorbed → more concentrated urine.

Exam questions — 3 questions · 9 marks

1 markUrea productionAQA 8461 P2 style

Where is urea produced and from what?

A In the kidneys from the breakdown of red blood cells

B In the liver from the deamination (breakdown) of excess amino acids

C In the small intestine from the digestion of proteins

D In the kidneys from filtering nitrogenous compounds from food

1 markDialysis principleAQA 8461 P2 style

How does a dialysis machine remove waste from the blood?

A It filters blood using high pressure to force waste through a membrane

B Waste products diffuse across a partially permeable membrane from the blood (high concentration) into dialysis fluid (low concentration)

C It uses enzymes to break down waste products in the blood

D It removes all small molecules from the blood and replaces them with clean fluid

4 marksCompare dialysis and transplantAQA 8461 P2 style

Compare dialysis and kidney transplant as treatments for kidney failure. Consider advantages and disadvantages of each. (4 marks)

Hint: Cover advantages AND disadvantages of BOTH treatments. At least two points per treatment.

+40 XP