Excretion — The Excretory System — Practice

Question 1

Name the three steps of urine formation. Where does each step occur in the nephron?

Accepted Answer

Step 1 — Glomerular Filtration.
  Occurs in the GLOMERULUS + BOWMAN'S CAPSULE. High blood pressure forces water, glucose, urea, amino acids, and salts from the blood into Bowman's capsule. Blood cells and proteins remain in the blood (too large to filter).

Step 2 — Tubular Reabsorption.
  Occurs in the PCT (mainly), LOOP OF HENLE, and DCT. Useful substances — ALL glucose, ALL amino acids, about 99% of water, and most salts — are reabsorbed back into the surrounding blood capillaries.

Step 3 — Tubular Secretion.
  Occurs in the DCT and COLLECTING DUCT. Additional waste products (H⁺, K⁺, creatinine, drugs, toxins) are actively secreted from the blood INTO the filtrate for elimination.

✦ Answer: (1) Filtration — glomerulus/Bowman's capsule. (2) Reabsorption — PCT/Loop of Henle/DCT. (3) Secretion — DCT/collecting duct.

Question 2

Why is glucose present in the blood but absent from normal urine? Under what condition would glucose appear in urine? What is this condition called?

Accepted Answer

Step 1 — Why glucose is filtered.
  During GLOMERULAR FILTRATION, glucose (a small molecule) IS filtered from the blood into Bowman's capsule and is present in the GLOMERULAR FILTRATE.

Step 2 — Why glucose is absent from normal urine.
  In the PROXIMAL CONVOLUTED TUBULE (PCT), ALL glucose is ACTIVELY REABSORBED back into the blood via carrier proteins. This process is so efficient that under normal blood glucose levels (70–110 mg/dL), no glucose reaches the final urine.

Step 3 — When glucose appears in urine.
  In DIABETES MELLITUS, blood glucose levels are very HIGH (hyperglycaemia) because insulin is absent (Type 1) or ineffective (Type 2). The concentration of glucose in the filtrate EXCEEDS the RENAL THRESHOLD — the maximum capacity of the carrier proteins to reabsorb it. The excess glucose remains in the filtrate and is excreted in urine.

Step 4 — Name of condition.
  This is called GLUCOSURIA (glucose in urine). It is one of the classic diagnostic signs of diabetes mellitus. A simple urine dipstick test can detect glucosuria.

✦ Answer: Glucose is filtered but normally 100% reabsorbed in the PCT. Glucosuria occurs when blood glucose exceeds the renal threshold — sign of diabetes mellitus.

Question 3

What is dialysis? Explain how a dialysis machine works.

Accepted Answer

Step 1 — What is dialysis?
  DIALYSIS is a medical procedure that artificially performs the FILTRATION function of the kidneys in patients with kidney failure (chronic kidney disease — CKD).

Step 2 — How it works.
  The patient's blood is pumped through a DIALYSIS MACHINE. Inside, blood flows through THIN TUBES made of SEMI-PERMEABLE MEMBRANE. These tubes are surrounded by DIALYSIS FLUID (with the correct composition of water, salts, and glucose — but no urea). By DIFFUSION, urea and excess salts move from the blood (high concentration) into the dialysis fluid. By OSMOSIS, excess water moves out. Clean blood is returned to the patient.

Step 3 — Limitation.
  Dialysis is NOT a cure — it is life-sustaining. Each session lasts 3–4 hours, done 3 times a week. It replaces only the filtration function, not the hormonal functions of the kidneys.

✦ Answer: Dialysis filters blood through a semi-permeable membrane; wastes diffuse out. Life-sustaining but not a cure — only transplant cures kidney failure.

Question 4

How do plants excrete their waste products? Give THREE methods.

Accepted Answer

Step 1 — Plants produce less nitrogenous waste than animals (they don't have protein as the main energy source). Their waste is primarily gaseous and mineral.

Step 2 — Three methods of excretion in plants.
  (1) STOMATA: CO₂ (from respiration) and O₂ (from photosynthesis) are released through stomata. Excess WATER is released as vapour through transpiration.
  (2) OLD LEAVES: Waste products (calcium oxalate crystals, tannins) are stored in old leaves. When leaves FALL (autumn), this waste is shed.
  (3) VACUOLES/BARK: Waste materials (resins, gums, tannins, latex) are stored in the VACUOLES of cells or deposited in BARK (dead cells), where they cause no harm.

✦ Answer: Plants excrete through stomata (gaseous waste), falling leaves (stored waste crystals), and bark/vacuoles (resins, gums, latex).

Question 5

Why do desert animals have longer loops of Henle than aquatic animals?

Accepted Answer

Step 1 — Function of the Loop of Henle.
  The LOOP OF HENLE creates a SALT CONCENTRATION GRADIENT in the kidney medulla. The longer the loop, the greater the gradient — which allows MORE WATER to be reabsorbed from the filtrate. This produces MORE CONCENTRATED urine.

Step 2 — Desert animals.
  Desert animals (like kangaroo rats) have VERY LONG loops of Henle → they reabsorb maximum water → produce extremely concentrated urine → conserve water → can survive without drinking.

Step 3 — Aquatic animals.
  Aquatic animals live in a water-rich environment and need to EXCRETE excess water (especially freshwater fish). Their loops of Henle are SHORTER → less water reabsorption → more dilute, larger volumes of urine.

✦ Answer: Longer loop = more concentrated urine = better water conservation. Desert animals evolved longer loops for survival in arid conditions.

Question 6

Draw a labelled nephron. Describe the function of each part.

Accepted Answer

Step 1 — Parts of the nephron (describe for the diagram).
  (1) BOWMAN'S CAPSULE: Cup-shaped structure surrounding the glomerulus. The site of GLOMERULAR FILTRATION — filtrate is collected here.
  (2) GLOMERULUS: A knot of capillaries under HIGH PRESSURE (afferent arteriole wider than efferent). Forces small molecules into Bowman's capsule.
  (3) PROXIMAL CONVOLUTED TUBULE (PCT): First reabsorption site. Reabsorbs ALL glucose, ALL amino acids, and ~70% of water and salts back into blood. MOST IMPORTANT for reabsorption.
  (4) LOOP OF HENLE: U-shaped tube extending into the medulla. Creates a salt concentration gradient that enables CONCENTRATED URINE to form. Longer in desert animals.
  (5) DISTAL CONVOLUTED TUBULE (DCT): Fine-tuning site. Regulated by ALDOSTERONE (increases salt reabsorption) and ADH (increases water reabsorption). Also site of some secretion.
  (6) COLLECTING DUCT: Collects urine from many nephrons. Further water reabsorption if ADH is present. Leads to RENAL PELVIS → URETER.

Step 2 — Flow of filtrate.
  Blood in glomerulus → filtrate in Bowman's capsule → PCT → Loop of Henle → DCT → collecting duct → renal pelvis → ureter → bladder → urethra → expelled.

✦ Answer: Nephron diagram with six labelled parts as above; each part's function clearly stated.

Practice — Excretion — The Excretory System

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