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Biology: Human Physiology, Health, and Response - Excretion and Internal Balance

Grade 8IB

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

🔑Concepts

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Homeostasis: The maintenance of a constant internal environment (e.g., body temperature at 37∘C37^\circ\text{C}, blood glucose levels, and water potential) through negative feedback loops.

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Excretion: The removal of toxic materials and metabolic waste products (substances made in chemical reactions within the body), such as CO2CO_2 from respiration and urea from deamination.

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The Urinary System: Consists of the kidneys (filtration), ureters (transport to bladder), bladder (storage), and urethra (release of urine).

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Nephron Structure: The functional unit of the kidney. It includes the Glomerulus for ultrafiltration, Bowman's Capsule, and the Convoluted Tubules for selective reabsorption.

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Ultrafiltration: High-pressure filtration in the glomerulus where small molecules (water, glucose, salts, urea) are forced into the capsule, while large molecules (proteins, blood cells) remain in the blood.

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Selective Reabsorption: The process in the Proximal Convoluted Tubule where useful substances like 100%100\% of glucose and some salts are pumped back into the blood via active transport.

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Osmoregulation and ADH: The control of water balance. When the body is dehydrated, the pituitary gland releases Anti-Diuretic Hormone (ADH), making the collecting ducts more permeable to water for reabsorption.

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Deamination: Occurs in the liver where excess amino acids are broken down. The nitrogen-containing part is converted into ammonia (NH3NH_3) and then into urea (NH2)2CO(NH_2)_2CO.

📐Formulae

2NH3+CO2→(NH2)2CO+H2O2NH_3 + CO_2 \rightarrow (NH_2)_2CO + H_2O

Total Water Input=Total Water OutputTotal\ Water\ Input = Total\ Water\ Output

Excretion Rate=Mass of WasteTime\text{Excretion Rate} = \frac{\text{Mass of Waste}}{\text{Time}}

💡Examples

Problem 1:

During a laboratory analysis, a patient's urine sample was found to contain glucose. Explain why this is abnormal based on the function of the nephron.

Solution:

In a healthy individual, 100%100\% of glucose filtered into the Bowman's capsule should be reabsorbed back into the blood in the Proximal Convoluted Tubule (PCT).

Explanation:

If glucose appears in the urine, it suggests that the blood glucose concentration is so high (often due to Diabetes) that the active transport pumps in the nephron are saturated and cannot reabsorb all of it.

Problem 2:

Calculate the volume of water reabsorbed by the kidneys if 180 L180\text{ L} of fluid is filtered into the nephrons daily, but only 1.5 L1.5\text{ L} is excreted as urine.

Solution:

180 L−1.5 L=178.5 L180\text{ L} - 1.5\text{ L} = 178.5\text{ L}

Explanation:

The kidneys are highly efficient; they reabsorb approximately 99%99\% of the filtered volume to maintain the body's water balance.

Problem 3:

Predict the effect of drinking a large volume of water on the levels of ADH in the blood and the concentration of urine.

Solution:

ADH levels will decrease, leading to dilute (hypotonic) urine.

Explanation:

High water intake increases blood water potential. The hypothalamus detects this and signals the pituitary gland to release less ADH. This makes the collecting duct walls less permeable, so less water is reabsorbed and more is excreted.