Excretion in humans and plants

What is excretion?

Every cell makes wastes as it works, and some of them are harmful. When the body breaks down extra protein, for example, it makes nitrogenous wastes such as urea and uric acid. If they stayed in the blood, they would poison the body. Removing these harmful wastes is called excretion.

Single-celled organisms let their wastes diffuse out through the cell surface into the water around them. Many-celled organisms need special organs.

The human excretory system

The excretory system has:

• a pair of kidneys, at the back of the abdomen, one on each side of the backbone;
• a pair of ureters, tubes that carry urine from the kidneys;
• the urinary bladder, a bag of muscle that stores urine;
• the urethra, through which urine leaves the body.

Urine is made in the kidneys. Urea itself is made in the liver, and the blood brings it to the kidneys.

The nephron

Each kidney has a very large number of tiny filtering units called nephrons, packed close together.

A nephron has two main parts.

1. The filter. At its start is a cup-shaped Bowman's capsule. Inside the cup sits a ball of very thin capillaries, the glomerulus. Blood flows into the glomerulus under pressure. Water and small molecules, such as glucose, amino acids, salts and urea, are pushed out of the blood into the capsule. This is filtration. Blood cells and large proteins stay in the blood.
2. The tubule. The filtered liquid then flows along a long, coiled tubule. Useful substances, such as glucose, amino acids, salts and a major amount of water, are taken back into the capillaries around the tubule. This is reabsorption.

What is left in the tubule is urine. The tubules of many nephrons join a collecting duct, and the urine flows on to the ureter.

How much water is taken back depends on two things: how much extra water there is in the body, and how much dissolved waste must be removed. After much sweating on a hot day, more water is taken back, so the urine is less in amount and darker.

A huge filter

The kidneys filter the blood again and again. In an adult, about $180$ L of filtrate is formed every day. Yet only a litre or two of urine is passed. So about $99\%$ of the filtrate goes back into the blood.

You should find that a small change in the share taken back makes a big change in the urine. Taking back $98\%$ instead of $99\%$ doubles the urine.

Storing and passing urine

The ureters carry urine to the urinary bladder. The bladder is muscular and stretches as it fills. It is under nervous control, so we can usually hold the urine until we choose to pass it out through the urethra.

When the kidneys fail

Infection, injury or a poor supply of blood can damage the kidneys. Then wastes build up in the blood, and this can be fatal. An artificial kidney can help, by haemodialysis.

• The patient's blood flows through long tubes with a semi-permeable lining.
• The tubes sit in a tank of dialysing fluid. It has the same osmotic pressure as blood, but no nitrogenous wastes.
• The wastes pass out of the blood into the fluid, and the cleaned blood goes back to the patient.

Unlike a real kidney, the artificial kidney has no reabsorption. It is usually used a few times a week, for some hours each time.

Excretion in plants

Plants have no excretory organs. They get rid of wastes in other ways.

• Oxygen made in photosynthesis is a waste for the plant. It diffuses out through the stomata.
• Extra water is lost by transpiration.
• Many wastes are stored in the vacuoles of the cells.
• Some wastes go into the leaves, which later fall off.
• Some are stored as resins and gums, especially in old xylem.
• Plants also give out some wastes into the soil around their roots.

Try it in Python

The program finds the filtrate made in a day, and the urine for four different shares taken back.

Practise with new numbers

Common mistakes

• "Urine is made in the bladder." It is made in the kidneys. The bladder only stores it.
• "The kidneys make urea." Urea is made in the liver. The kidneys remove it from the blood.
• "Glucose is not filtered." Glucose is filtered, then taken back in the tubule. That is why the urine of a healthy person has no glucose.
• "An artificial kidney works just like a real one." It filters, but it takes nothing back.
• "Plants do not excrete." They do, by diffusion, by storing wastes and by shedding leaves.

Remember these

• Excretion is the removal of harmful nitrogenous wastes such as urea.
• Kidneys → ureters → urinary bladder → urethra.
• In a nephron, the glomerulus inside Bowman's capsule filters the blood. The tubule takes back glucose, amino acids, salts and much water.
• About $180$ L of filtrate a day gives only a litre or two of urine.
• Dialysis cleans the blood when the kidneys fail, but it has no reabsorption.
• Plants remove wastes by diffusion, store them in vacuoles, drop them with old leaves, and store them as resins and gums.

Check yourself

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

1. Which part of the nephron filters the blood?

2. Name three useful substances that are taken back into the blood from the tubule.

3. After a long football match on a hot day, Arjun passes only a little dark urine. Explain why.

4. What does the dialysing fluid lack, and why?

5. How does a neem tree get rid of the oxygen it makes in the day?