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Respiration and Transportation in Organisms - Excretion in Animals and the Human Urinary System

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Excretion is the biological process of removing metabolic wastes, such as urea and salts, produced in the cells of living organisms.

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The human excretory system (Urinary System) consists of a pair of kidneys, two ureters, a urinary bladder, and a urethra.

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Kidneys contain a network of blood capillaries that filter the blood. Useful substances are reabsorbed, while harmful wastes are removed as urine.

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An adult human typically passes 11 to 1.81.8 L of urine per day. Its composition is approximately 95%95\% water (H2OH_2O), 2.5%2.5\% urea, and 2.5%2.5\% other waste products.

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Sweating serves two purposes: it removes excess water and salts from the body and helps in cooling the body through the process of evaporation.

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Different animals excrete different nitrogenous wastes: Aquatic animals like fishes excrete NH3NH_3 (ammonia), while birds, snakes, and lizards excrete a semi-solid white compound called Uric acid (C5H4N4O3C_5H_4N_4O_3).

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Dialysis is the procedure used to filter the blood of a person using an artificial kidney when their natural kidneys fail to function.

📐Formulae

Urine=95%H2O+2.5% Urea+2.5% Other WastesUrine = 95\% H_2O + 2.5\% \text{ Urea} + 2.5\% \text{ Other Wastes}

Urea Formula: CO(NH2)2\text{Urea Formula: } CO(NH_2)_2

Ammonia: NH3\text{Ammonia: } NH_3

Uric Acid: C5H4N4O3\text{Uric Acid: } C_5H_4N_4O_3

💡Examples

Problem 1:

Calculate the approximate volume of urea excreted if a person produces 1.61.6 L of urine in a day, given that urea makes up 2.5%2.5\% of the urine volume.

Solution:

Volume of urea=2.5100×1.6 L=0.04 L\text{Volume of urea} = \frac{2.5}{100} \times 1.6 \text{ L} = 0.04 \text{ L} or 40 mL40 \text{ mL}.

Explanation:

Since the concentration of urea in urine is constant at 2.5%2.5\%, we multiply the total volume of urine by 0.0250.025 to find the specific volume of urea.

Problem 2:

Why do white patches appear on clothes in summer, especially in areas like underarms?

Solution:

The white patches are formed by the salts left behind after the water in sweat evaporates.

Explanation:

Sweat contains H2OH_2O and salts. When the body heat causes H2OH_2O to evaporate to cool the skin, the non-volatile salts remain on the fabric.

Problem 3:

Identify the primary nitrogenous waste for a fish and a crow.

Solution:

Fish: Ammonia (NH3NH_3); Crow: Uric acid (C5H4N4O3C_5H_4N_4O_3).

Explanation:

Aquatic animals excrete ammonia which requires a lot of water to dissolve. Birds excrete uric acid as a semi-solid to conserve water.

Excretion in Animals and the Human Urinary System Revision - Class 7 Science CBSE