Form 4 · Common mistakes

Homeostasis and Human Urinary System, common mistakes

The mistakes SPM students make on Homeostasis and Human Urinary System, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying glucose is normally present in urine.	Students remember that glucose is filtered and forget the second stage.	Glucose is filtered but then fully reabsorbed by active transport, so healthy urine has none; its presence points to diabetes.
Writing that proteins are filtered into the nephron.	They confuse what enters the filtrate with what stays in the blood.	Large plasma proteins and blood cells are too big to cross the filtration membrane and remain in the blood.
Confusing ultrafiltration with selective reabsorption.	Both move substances, so the direction gets mixed up.	Ultrafiltration forces substances out of the blood at the glomerulus; selective reabsorption takes useful ones back into the blood along the tubule.
Saying the bladder makes urine.	The bladder is where urine is seen to collect, so it is mistaken for the source.	The kidneys make urine; the bladder only stores it, and its composition does not change there.
Stating that ADH produces more urine.	The name hormone is linked loosely with urine without tracking its effect.	ADH increases water reabsorption, so it produces a smaller volume of more concentrated urine.
Saying sweat cools the body simply by leaving the skin.	The cooling is attributed to liquid loss rather than to energy transfer.	Sweat cools the body because evaporation absorbs heat energy from the skin surface.
Mixing up the effects of insulin and glucagon.	The two names sound similar and both come from the pancreas.	Insulin lowers blood glucose by promoting glycogen storage; glucagon raises it by breaking glycogen down.
Describing vasodilation as blood vessels moving to the skin surface.	Dilation is pictured as movement rather than widening.	The arterioles widen so more blood flows through vessels already near the surface, increasing heat loss.
Treating a kidney stone as the same thing as kidney failure.	Both are kidney problems, so their severity is assumed to be equal.	A kidney stone is a localised, often treatable blockage; kidney failure is a widespread loss of filtering function needing dialysis or transplant.
Describing negative feedback as making a change larger.	The word negative is misread as meaning harmful or worsening.	Negative feedback opposes the change and pushes the condition back toward its set point.
Saying the renal vein carries urine away from the kidney.	Any tube leaving the kidney is assumed to carry urine.	The renal vein carries cleaned blood away; the ureter is the tube that carries urine to the bladder.
Writing that the high pressure in the glomerulus comes from the heart alone.	Blood pressure is linked only to the heartbeat.	The pressure is high because the afferent arteriole is wider than the efferent arteriole, narrowing the outflow.

How to avoid these mistakes

Label the nephron and describe ultrafiltration followed by selective reabsorption.
Compare the contents of blood, glomerular filtrate and urine in a table.
Explain how ADH adjusts water reabsorption when the body is short of water.
Learn the negative feedback pattern, detect, respond, correct, and apply it to thermoregulation, osmoregulation and blood glucose control so you are confident with all three examples.
Compare insulin and glucagon in a table of source, trigger and effect on blood glucose.
Learn one urinary health issue with its cause and symptom, and explain how it differs from kidney failure.

Frequently asked questions

What is homeostasis?

Homeostasis is the maintenance of a constant internal environment in the body, such as steady body temperature, blood glucose level, water content and pH, despite changes outside. It keeps conditions ideal for enzymes and cells to work, and is controlled by organs including the kidneys, liver, skin and the nervous and endocrine systems. Almost every homeostatic mechanism in the body follows the same negative feedback pattern of detecting, responding to and correcting a change.

How is urine formed in the nephron?

First, ultrafiltration: high pressure in the glomerulus forces water, glucose, salts and urea out of the blood into the Bowman's capsule, while large proteins and blood cells stay behind. Then selective reabsorption: as the filtrate flows along the tubule, useful substances such as all the glucose, some salts and much of the water are reabsorbed into the blood. What remains, mainly urea, excess salts and water, becomes urine. This urine then flows through the ureter to the bladder, where it is stored before being released.

Why is there normally no glucose in urine?

Glucose is small enough to be filtered out during ultrafiltration, but it is a useful nutrient, so it is completely reabsorbed back into the blood during selective reabsorption. Healthy urine therefore contains no glucose. If glucose appears in urine, it often means blood glucose is too high, as in diabetes.