Nitrogenous Wastes and the Human Excretory System: Quick Revision

A brief note on Nitrogenous Wastes and the Human Excretory System. Read it before a test. Try each question at the end before you open its answer.

Nitrogenous wastes

• Excretion: removal of metabolic wastes. The main nitrogenous wastes are ammonia, urea and uric acid.
• Ammonotelism: ammonia, very toxic, needs much water, lost by diffusion across gills or body surface. Bony fishes, aquatic amphibians, aquatic insects.
• Ureotelism: urea, made in the liver, removed by the kidney. Mammals, many terrestrial amphibians, marine fishes (NCERT).
• Uricotelism: uric acid, least toxic, a pellet or paste, little water. Reptiles, birds, land snails, insects.
• Other organs (not in the NEET syllabus text): flame cells, nephridia, Malpighian tubules, green glands.

The kidney

• System: two kidneys, two ureters, one bladder, one urethra.
• Kidney: bean shaped, between the last thoracic and the third lumbar vertebra. 10 to 12 cm long, 120 to 170 g.
• Layers: capsule, cortex, medulla (pyramids), renal columns, hilum, renal pelvis, calyces.
• Blood: 1100 to 1200 mL a minute, about a fifth of the cardiac output.

The nephron

• Nephron: glomerulus and renal tubule. About one million in each kidney.
• Order: Bowman's capsule, PCT, descending limb, ascending limb, DCT, collecting duct.
• Regions: corpuscle, PCT and DCT in the cortex. Loop of Henle in the medulla.
• Vessels: afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries and vasa recta.
• Types: cortical nephrons have short loops. Juxtamedullary nephrons have long loops.

A worked example

A person excretes 30 g of urea in a day. How much nitrogen is that?

1. A urea molecule has a mass of 60 u and holds 2 nitrogen atoms of 14 u, which is 28 u.
2. The nitrogen is $\dfrac{28}{60} \times 30 = 14$ g.

Practice with new numbers

A program to try

The program finds the nitrogen in a day's urea.

Mistakes to avoid

• Calling every land animal ureotelic. Birds, reptiles and insects are uricotelic.
• Putting the loop of Henle in the cortex. It is in the medulla.
• Mixing up afferent and efferent. The afferent arteriole brings blood to the glomerulus.
• Saying that the renal columns are medulla. They are cortex between the pyramids.

Check yourself

1. Which waste is the least toxic?

2. Where is urea made?

3. Which part of the nephron dips into the medulla?

4. Which vessel leaves the glomerulus?

Sources

• NCERT (2023). Biology, Textbook for Class XI, chapter 16.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 25.