Course 15.3Lesson 2 of 3

Other organs of excretion and the water balance

The kidneys are not the only organs that excrete. The lungs remove carbon dioxide and water, the liver passes bile pigments into the gut, the sweat and sebaceous glands remove salts and oily wastes, and the saliva has a little nitrogen. This lesson lists them, and adds up the water that the body gains and loses in a day.

about 25 min1 simulation1 practice question

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

More than the kidneys

The kidneys are the main organs of excretion, but they are not the only ones. Several other organs take part. Look at the figure and learn what each one removes.

The lungs

The lungs remove large amounts of carbon dioxide, about 200 mL every minute. They also remove significant quantities of water every day, as vapour in the breath.

The liver

The liver is the largest gland of the body. It secretes bile. Bile holds bilirubin and biliverdin, which are bile pigments, and also cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances pass out with the digestive wastes.

The skin

The skin has two kinds of glands that excrete.

Sweat glands make sweat, a watery fluid that holds NaCl, small amounts of urea and lactic acid. The main work of sweat is to cool the body surface. But it also helps to remove some wastes.
Sebaceous glands remove sterols, hydrocarbons and waxes in the sebum. The sebum gives the skin a protective oily covering.

The saliva

Small amounts of nitrogenous wastes can be removed in the saliva too.

Organ	What it removes
Kidneys	urea and other nitrogenous wastes, excess water and ions, in urine
Lungs	carbon dioxide (about 200 mL a minute) and water
Liver	bile pigments, cholesterol, degraded steroids, vitamins, drugs
Sweat glands	water, NaCl, a little urea, lactic acid
Sebaceous glands	sterols, hydrocarbons, waxes
Salivary glands	small amounts of nitrogenous wastes

The balance of water in a day

The body must lose as much water as it gains, or its fluids change. The values in the table are from a physiology book (Guyton and Hall), for a person in a mild climate. They are not from NCERT, and they change from person to person.

Two stacked bars for one day. The left bar, water gained, is 2.3 litres: 2.1 litres from drinks and food and 0.2 litre from metabolism. The right bar, water lost, is also 2.3 litres: 1.4 litres of urine, 0.35 litre from the skin without sweat, 0.35 litre from the lungs, 0.1 litre of sweat and 0.1 litre in the faeces.

Water gained	Litres
Drinks and food	2.1
Water made in metabolism	0.2
Total	2.3

Water lost	Litres
Urine	1.4
Skin, without sweat	0.35
Lungs	0.35
Sweat	0.1
Faeces	0.1
Total	2.3

So the urine is the part that the kidneys can change. On a hot day, the sweat is much more, and the kidneys pass less urine. This is what the control by ADH is for. The kidneys cannot pass less than about 0.5 L of urine a day, because the wastes need that much water.

You should find that a person who sweats a lot and drinks only a little has a very small volume of urine. Below 0.5 L, the body is short of water.

Try it in Python

The program adds up the gains and the losses of water and finds the urine that is needed to keep the balance.

Practise with new numbers

Common mistakes

Saying that the lungs remove only carbon dioxide. They remove water too.
Saying that sweat is made mainly to excrete urea. Its main work is to cool the body.
Saying that the bile pigments leave in the urine. They pass out with the digestive wastes.
Mixing up sweat glands and sebaceous glands. Sweat has salts and a little urea. Sebum has sterols, hydrocarbons and waxes.
Forgetting the saliva. A little nitrogenous waste is lost in it.

Remember these

Lungs: CO $_2$ (about 200 mL a minute) and water. Liver: bile with bilirubin, biliverdin, cholesterol, steroids, vitamins, drugs.
Sweat: water, NaCl, a little urea, lactic acid. The main work is cooling. Sebum: sterols, hydrocarbons, waxes.
Saliva: small amounts of nitrogenous wastes.
Water balance: gains equal losses. The urine is the part that the kidneys adjust.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

How much carbon dioxide do the lungs remove in a minute?

Which substances are in the bile that are removed by the liver?

What does sweat contain, and what is its main work?

What do the sebaceous glands remove?

Why do you pass less urine on a hot day when you sweat much?

References

NCERT (2023). Biology, Textbook for Class XI, chapter 16, Excretory Products and Their Elimination.
Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapter on the body fluid compartments, for the table of the daily water balance.