Free Homeostasis MCQs with Answers

1. Homeostasis is best defined as

• A. the total of all chemical reactions in the body
• B. the removal of metabolic waste from the body
• C. the maintenance of a constant internal environment despite changes outside the body
• D. the growth of an organism to a fixed adult size

Explanation: Homeostasis keeps temperature, water potential, blood glucose and pH within narrow limits, because enzymes work properly only over a small range of these conditions. It operates by negative feedback, in which a departure from the set point triggers a correction that returns the value towards normal. Excretion is one of the processes that contributes to it, not a synonym for it.

2. The structural and functional unit of the kidney is the

• A. nephron
• B. neuron
• C. alveolus
• D. ureter

Explanation: Each kidney contains about a million nephrons, and every one carries out filtration, reabsorption and secretion on its own, so the organ's work is the sum of theirs. A nephron runs from the Bowman's capsule through the proximal tubule, loop of Henle and distal tubule to a collecting duct. The ureter merely carries the finished urine to the bladder, and the neuron belongs to the nervous system.

3. Ultrafiltration in the glomerulus is driven by

• A. active transport using ATP from the capsule cells
• B. the high hydrostatic pressure caused by the efferent arteriole being narrower than the afferent
• C. osmosis from the tubule into the capillary
• D. the beating of cilia in the capsule

Explanation: Blood enters the glomerulus through a wide afferent arteriole and leaves through a narrower efferent one, so pressure builds up and forces water and small solutes through the capillary pores and the basement membrane into the capsule. The process is purely physical and uses no ATP, which is why it is called filtration rather than transport. Roughly 125 cm3 of filtrate is formed every minute.

4. Which of the following is normally absent from glomerular filtrate?

• A. Glucose
• B. Urea
• C. Sodium ions
• D. Plasma proteins

Explanation: The basement membrane acts as the real molecular sieve and holds back anything much larger than about 68,000 relative molecular mass, so plasma proteins and blood cells stay in the capillary while water, glucose, salts and urea pass through freely. Protein appearing in the urine therefore signals damage to the filtration membrane. Glucose is filtered but is then completely reabsorbed further along the tubule.

5. All the glucose in the filtrate is normally reabsorbed in the

• A. proximal convoluted tubule
• B. collecting duct
• C. loop of Henle
• D. Bowman's capsule

Explanation: The proximal tubule cells carry microvilli and many mitochondria because glucose and amino acids are recovered here by active transport, along with most of the water and salts. Reabsorption is described as selective because useful substances are taken back while urea is largely left behind. The collecting duct handles the final water adjustment under the control of ADH, not glucose recovery.

6. A desert mammal can produce very concentrated urine because it has

• A. a larger Bowman's capsule
• B. more glomeruli per kidney
• C. a much longer loop of Henle, which builds a steeper salt gradient in the medulla
• D. no collecting ducts

Explanation: The loop works as a countercurrent multiplier, making the medulla progressively saltier towards the tip, and the collecting duct passing through that region loses water by osmosis. A longer loop maintains a steeper gradient, so more water can be withdrawn and the urine leaves more concentrated, which is exactly the adaptation seen in the kangaroo rat. Filtration rate is unrelated to how concentrated the final urine is.

7. Antidiuretic hormone controls the water content of the blood by

• A. increasing the rate of glomerular filtration
• B. making the walls of the distal tubule and collecting duct more permeable to water
• C. actively pumping water out of the tubule
• D. reducing the reabsorption of sodium ions

Explanation: ADH from the posterior pituitary inserts water channels into the tubule walls, so more water is reabsorbed by osmosis into the salty medulla and a small volume of concentrated urine is produced. It is released when osmoreceptors in the hypothalamus detect that the blood is too concentrated, a textbook negative feedback loop. Alcohol suppresses ADH, which is why it causes dehydration.

8. A patient passes very large volumes of dilute urine but the urine contains no glucose. The most likely condition is

• A. diabetes mellitus
• B. kidney stones
• C. a urinary tract infection
• D. diabetes insipidus

Explanation: Diabetes insipidus results from a lack of ADH or a failure of the tubule to respond to it, so water cannot be reabsorbed and the patient produces litres of dilute, sugar free urine and is constantly thirsty. Diabetes mellitus also causes a large urine volume, but there the cause is unreabsorbed glucose holding water in the tubule, so glucose is present. The word diabetes simply means a large flow.

9. Aldosterone from the adrenal cortex acts on the kidney to increase the reabsorption of

• A. sodium ions
• B. glucose
• C. urea
• D. plasma proteins

Explanation: Aldosterone stimulates the distal tubule and collecting duct to pump sodium back into the blood in exchange for potassium, and since water follows sodium osmotically, blood volume and pressure rise as well. It therefore regulates salt balance, while ADH regulates water directly. Glucose reabsorption is complete without any hormonal control, and urea is a waste the kidney is trying to remove.

10. The chief nitrogenous waste product of humans is

• A. ammonia
• B. urea
• C. uric acid
• D. creatine

Explanation: Surplus amino acids are deaminated in the liver and the toxic ammonia produced is converted at once into far less toxic urea by the ornithine cycle, which the blood then carries to the kidney. Ammonia is too poisonous to accumulate and is excreted directly only by animals surrounded by water, such as bony fish. Uric acid is the main waste of birds, reptiles and insects.