Excretory System — Important Questions

Kidneys, Ureters, Urinary bladder, Urethra

The human excretory system is made up of two kidneys, two ureters, one urinary bladder and one urethra, as these are the organs that produce, carry, store and expel urine. The heart is part of the circulatory system, not the excretory system, so it is excluded.

True

Urine formation begins with ultrafiltration, in which blood plasma is filtered through the glomerular capillaries into the Bowman's capsule due to high hydrostatic pressure. Only after this filtrate is formed do selective reabsorption and tubular secretion take place, confirming that ultrafiltration is indeed the first step.

Glucose, Water, Sodium, Amino acids

The proximal convoluted tubule (PCT) reabsorbs glucose, water, sodium, potassium, bicarbonates, phosphate and amino acids back into the blood, restoring their normal concentration. Creatinine, in contrast, is a substance actively secreted into the tubule rather than reabsorbed, so it is not included.

Skin, Lungs, Liver, Large intestine

Besides kidneys, the skin, lungs, liver and large intestine also help in excretion by removing sweat, carbon dioxide/water vapour, urea/bile pigments, and undigested food/bile pigments respectively. The stomach is mainly involved in digestion, not excretion, so it is not included.