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The Kidneys and the Excretory System — Answers

Answer sheet with model answers and success criteria for the 9th Grade the human body worksheet.

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Success TutoringWorksheetsAnswer sheet9th Grade · Science · The Human Body

The Kidneys and the Excretory System — Answers

Part A: Excretion

Excretion is the removal of waste products made by chemical reactions in the body's cells. The main wastes are carbon dioxide (from respiration), removed by the lungs, and urea (from breaking down excess proteins in the liver), removed by the kidneys.

The urinary system contains two kidneys, two ureters (tubes to the bladder), the bladder (which stores urine) and the urethra (which carries urine out of the body).

1.Match each waste product to the organ that excretes it.
  • Carbon dioxide
  • Urea
  • Excess heat, water and some salts
  • Kidneys (in urine)
  • Skin (in sweat)
  • Lungs

Answer: Carbon dioxide → Lungs; Urea → Kidneys (in urine); Excess heat, water and some salts → Skin (in sweat)

2.Urea is made in the liver. What substance is broken down to produce it?

Answer: Excess amino acids (from proteins) that the body cannot store.

3.Write the path of urine from where it is made to where it leaves the body, using: bladder, urethra, kidney, ureter.

Answer: kidney → ureter → bladder → urethra

4.Removing undigested food from the body is an example of excretion.

Answer: False

Excretion is the removal of wastes made by the body's cells, such as urea and carbon dioxide. Undigested food was never part of the body's cells, so removing it is called egestion.

Part B: How the kidney works

Each kidney contains about a million tiny tubes called nephrons. Filtration: blood under high pressure is pushed through a knot of capillaries (the glomerulus) into the start of the nephron. Small molecules such as water, glucose, salts and urea pass into the tubule; this liquid is called the filtrate. Blood cells and proteins are too big and stay in the blood.

Reabsorption: as the filtrate moves along the tubule, all the glucose, most of the water and the salts the body needs are taken back into the blood. What is left (water, urea and excess salts) is urine.

Approximate concentrations (g/L) — Plasma: protein 80, glucose 1.0, urea 0.3. Filtrate: protein 0, glucose 1.0, urea 0.3. Urine: protein 0, glucose 0, urea 20.

5.Which substance is found in the blood plasma but not in the filtrate?
  1. AWater
  2. BGlucose
  3. CUrea
  4. DProtein

Answer: Protein

Protein molecules are too large to pass through the filter in the glomerulus, so they stay in the blood.

6.Glucose is present in the filtrate but not in the urine of a healthy person. Explain why.

Answer: All of the glucose is reabsorbed back into the blood from the tubule, because it is useful to the body.

7.Urea is about 0.3 g/L in plasma and about 20 g/L in urine. How many times more concentrated is urea in urine? Give your answer to the nearest whole number.

Answer: About 67 times

  1. 20 ÷ 0.3 = 66.7
  2. ≈ 67 times
8.Explain why urea is much more concentrated in urine than in the filtrate.

Answer: Most of the water in the filtrate is reabsorbed into the blood, but most of the urea is not, so the same urea ends up dissolved in much less water.

9.The kidneys make about 180 L of filtrate a day but produce only about 1.5 L of urine. What volume is reabsorbed, and what percentage of the filtrate is this, to one decimal place?

Answer: 178.5 L; 99.2%

  1. 180 − 1.5 = 178.5 L reabsorbed
  2. 178.5 ÷ 180 × 100 = 99.17
  3. ≈ 99.2%
10.Using a urine concentration of 20 g/L, how much urea is excreted in 1.5 L of urine?

Answer: 30 g

  1. Mass = concentration × volume
  2. 20 × 1.5 = 30 g
11.A urine test shows glucose in a person's urine. Which condition could this be a sign of?

Answer: Diabetes: the blood glucose level is so high that the tubules cannot reabsorb all of it.

Part C: Water balance and dialysis

The kidneys keep the body's water level balanced. When the blood is too concentrated (too little water), the pituitary gland releases more antidiuretic hormone (ADH), which makes the tubules reabsorb more water. When there is too much water, less ADH is released. This is an example of negative feedback.

If the kidneys fail, a dialysis machine can filter the blood, or the person may receive a kidney transplant.

12.On a hot day, a person sweats a lot and drinks little. What happens to their urine?
  1. ALarge volume, pale and dilute
  2. BSmall volume, dark and concentrated
  3. CLarge volume, dark and concentrated
  4. DIt contains glucose

Answer: Small volume, dark and concentrated

More ADH is released, so the kidneys reabsorb more water, producing a small volume of concentrated urine.

13.Describe how ADH helps the body respond when the blood contains too much water.

Answer: Less ADH is released by the pituitary gland, so the kidney tubules reabsorb less water. More water stays in the urine, producing a large volume of dilute urine, and the blood's water level falls back to normal.

14.In kidney dialysis, the person's blood flows past a partially permeable membrane with dialysis fluid on the other side. Why does the dialysis fluid contain glucose and salts at the same concentration as normal blood, but no urea?

Answer: Because there is no concentration difference for glucose and useful salts, they do not diffuse out of the blood. Urea is at a higher concentration in the blood than in the fluid, so it diffuses out of the blood into the fluid and is removed.

Part D: Evaluate treatments

15.Compare a kidney transplant with regular dialysis as treatments for kidney failure. Discuss at least two advantages and two disadvantages of each.[4 marks]

Model answer: Dialysis is available without waiting for a donor and does not require major surgery, and there is no risk of rejection. However, it must be done several times a week for hours at a time, the person must carefully control their diet and fluid intake between sessions, and it is expensive over a long period. A transplant allows the person to live a much more normal life with no dialysis sessions and fewer diet restrictions, and it is cheaper in the long term. However, there is a shortage of donors, so people may wait years; it requires major surgery; and the immune system may reject the new kidney, so the person must take immunosuppressant medicines for life, which increase the risk of infections.

Success criteria:

  • Two advantages of dialysis
  • Two disadvantages of dialysis
  • Two advantages of a transplant
  • Two disadvantages of a transplant, including rejection or donor shortage
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