A urine test finds protein. What does that indicate?

Proteins are too large to pass through a healthy filter. Their presence means something in the filtering process is not working properly. A fault in the filtration mechanism, which is why the test is followed up

Someone drinks a litre of water. What will their urine look like over the next hour?

The water content of the blood has risen. The body reabsorbs less water along the tubule. More water continues on to the bladder. Plentiful and pale

Sugar passes through the filter but is absent from a healthy person's urine. How?

Sugar is small enough to pass through the filter. Along the tubule it is almost entirely reabsorbed. It was filtered and returned. It appears in urine only when its level in the blood is too high

The Human Body and Its Systems · Grade 8

How does a kidney decide what to remove from the blood and what to keep?

It does not decide in advance. First it filters an enormous volume of liquid out of the blood, carrying everything small enough to pass, and only then reabsorbs what the body still needs — water, sugar and salts. Whatever is not reabsorbed leaves as urine. Filtering everything and returning the useful is simpler than choosing in advance.

Learning objectives

Describe how a kidney filters the blood and then reabsorbs what the body still needs
Explain what urea is and in which organ it is produced
Explain how the body adjusts the urine on a hot day and after drinking a litre of water
Define homeostasis and give two examples of it from the human body

Filter everything, then give it back

Blood enters the kidney and reaches a knot of capillaries. The pressure there is high and liquid is pushed out into a tubule. Water, sugar, salts and urea pass through; blood cells and proteins are too large and stay behind.

Along the tubule, reabsorption takes place. Almost all the sugar is taken back, and so is most of the water and most of the salt. What remains in the tubule carries on to the bladder.

Every day the kidneys filter about a hundred and eighty litres and return nearly all of it; about a litre and a half leaves. It looks wasteful, and it was chosen because it is simple: filtering everything and choosing what to return is easier than identifying each waste molecule inside the blood in advance.

Where urea comes from

The body cannot store surplus protein. Amino acids it does not need are broken down in the liver, and the nitrogen in them becomes urea — far less toxic than the ammonia it is made from.

The liver sends the urea into the blood, and the kidney is what removes it. This division of labour is worth remembering precisely: urea is made in the liver and removed by the kidney.

Homeostasis: keeping the inside steady

Homeostasis is holding conditions inside the body constant despite changes outside. Body temperature, blood sugar and water content are three examples.

On a hot day you sweat and lose a lot of water. The body responds by reabsorbing more water, and the urine is small in volume and dark. After drinking a litre of water the opposite happens: less is reabsorbed, and the urine is plentiful and pale.

In both cases the water content of the blood has barely changed. That is the whole idea — what changes is the urine, so that the blood can stay as it is.

Worked examples

A urine test finds protein. What does that indicate?
1. Proteins are too large to pass through a healthy filter
2. Their presence means something in the filtering process is not working properly
Answer: A fault in the filtration mechanism, which is why the test is followed up
Someone drinks a litre of water. What will their urine look like over the next hour?
1. The water content of the blood has risen
2. The body reabsorbs less water along the tubule
3. More water continues on to the bladder
Answer: Plentiful and pale
Sugar passes through the filter but is absent from a healthy person's urine. How?
1. Sugar is small enough to pass through the filter
2. Along the tubule it is almost entirely reabsorbed
Answer: It was filtered and returned. It appears in urine only when its level in the blood is too high

Common mistakes

Saying the kidney produces urea: Urea is made in the liver from surplus protein. The kidney only removes it, and the two happen in different organs.
Thinking the kidney selects in advance what to remove: It filters first by size alone, and only then reabsorbs what is useful. The choosing happens at the second stage.
Describing homeostasis as nothing changing: Everything changes all the time; what is held is the range. The body corrects deviations rather than preventing them.

What to remember

Filter by size first, reabsorb what is needed second.
Urea is made in the liver and removed by the kidney.
The urine changes so that the blood can stay constant.
Homeostasis is correction, not the absence of change.