Worksheet Solutions: Excretion in Humans | Biology Class 7 ICSE PDF Download

Part A – Multiple Choice Questions (MCQs)

Q1. The structural and functional unit of the kidney is:
a) Alveoli
b) Nephron
c) Bronchi
d) Ureter

Answer: b) Nephron
Explanation: The nephron is the basic unit of the kidney. It filters blood, removes wastes like urea, and forms urine.

Q2. Which organ helps in the removal of carbon dioxide from the body?
a) Kidney
b) Skin
c) Lungs
d) Liver

Answer: c) Lungs
Explanation: The lungs excrete carbon dioxide formed during respiration.

Q3. Which substance is NOT normally present in urine?
a) Urea
b) Salts
c) Glucose
d) Water

Answer: c) Glucose
Explanation: Glucose in urine indicates diabetes. Normally, urine has water, urea, salts, and uric acid.

Q4. Stones in the kidney are formed due to crystallisation of:
a) Proteins
b) Calcium salts and uric acid
c) Vitamins
d) Oxygen

Answer: b) Calcium salts and uric acid
Explanation: Kidney stones are hard crystals formed by calcium oxalate, calcium phosphate, or uric acid.

Q5. The process of maintaining water and salt balance in the body is called:
a) Photosynthesis
b) Osmoregulation
c) Circulation
d) Filtration

Answer: b) Osmoregulation
Explanation: Kidneys regulate water and salt balance to maintain normal blood composition.

Part B – Short Answer Questions

Q6. What is excretion? Why is it important?
Answer:
Excretion is the process of removing waste products like urea, uric acid, excess water, and salts from the body.
It is important because if these wastes accumulate, they can be toxic and may damage organs or even cause death.

Q7. Name the main organs of the human excretory system.
Answer:

• Kidneys

• Ureters

• Urinary bladder

• Urethra
  Together, they filter blood, form urine, store it, and finally expel it.

Q8. How does the skin help in excretion?
Answer:

• Skin has sweat glands that produce sweat.

• Sweat contains water, salts, and traces of urea.

• When sweat evaporates, it removes wastes and also cools the body.

Q9. What happens if both kidneys fail?
Answer:

• Waste products build up in the blood, making it poisonous.

• This condition is life-threatening.

• Patients need dialysis (artificial blood cleaning) or a kidney transplant to survive.

Q10. Why is drinking water important for kidney health?
Answer:

• Water helps dilute urine.

• It prevents crystal formation that may lead to kidney stones.

• Adequate water keeps kidneys functioning smoothly.

Part C – Long Answer Questions with Stepwise Solutions

Q11. Explain the structure and role of a nephron in urine formation.
Answer:
A nephron is the functional unit of the kidney. Each kidney has about 1 million nephrons.

Stepwise explanation:

1. Structure:

   • Starts with a cup-like structure called Bowman’s capsule.

   • Contains a twisted tubule connected to a collecting duct.

   • Collecting ducts join to carry urine to the renal pelvis.

2. Urine formation process:

   • Filtration: Blood enters nephron through renal artery. Wastes like urea, salts, and excess water are filtered.

   • Reabsorption: Useful substances like glucose, sodium, and water are absorbed back into blood.

   • Collection: Remaining waste forms urine, passes into collecting ducts → ureters → urinary bladder → urethra.

Q12. Describe the accessory excretory organs and their roles.
Answer:

Besides the kidneys, other organs also help in removing waste. These are called accessory excretory organs.

(i) Skin:

• Sweat glands present in the skin remove waste in the form of sweat.

• Sweat contains water, salts, and a little urea.

• Sweating also helps in cooling the body.

(ii) Lungs:

• Lungs remove carbon dioxide produced during respiration.

• Carbon dioxide diffuses from the blood into the lungs and is exhaled.

(iii) Liver:

• The liver breaks down extra amino acids into urea, which is sent to the kidneys.

• It also removes bile pigments, excess vitamins, and drugs.

Q13. What is osmoregulation? Explain with examples.
Answer:
Osmoregulation is the process by which kidneys maintain proper water and salt balance in the blood.

Stepwise explanation:

1. In summer:

   • The body loses more water through sweat.

   • Kidneys conserve water by producing less but concentrated urine.

2. In winter:

   • Less sweating occurs.

   • Kidneys excrete more urine which is dilute.

3. Importance:

   • Prevents dehydration.

   • Maintains correct blood composition for proper body function.