Excretion in Humans
How the body gets rid of carbon dioxide, urea, and excess water and ions.
Excretion
Many chemical reactions take place in the body, and some make waste products. These waste products are removed by excretion.
| Excretory product | Excreted through | Notes |
|---|---|---|
| Carbon dioxide | The lungs | Breathed out; see Chapter 11 |
| Urea | The kidneys | In the urine |
| Excess water | The kidneys | In the urine |
| Excess ions | The kidneys | In the urine |
The organs of excretion
The lungs excrete carbon dioxide. The kidneys excrete urea and excess water and ions, which leave the body in the urine. Only substances that the body has made as waste, or has in excess, are excreted.
Worked example (naming the organ): Name the organ that excretes carbon dioxide and the organ that excretes urea. Step 1. Carbon dioxide leaves the body through the lungs. Step 2. Urea is excreted by the kidneys, in the urine. Answer: Lungs; kidneys.
Excess water and ions
Water and ions are needed by the body, but any excess must be removed. The kidneys excrete the excess in the urine, so the amount of water and ions in the body does not rise too far.
Worked example (classifying a substance): Is a drink of water that the body does not need an excretory product? Name the organ that removes the excess. Step 1. Water in excess of requirements is excreted. Step 2. The kidneys remove the excess water in the urine. Answer: Yes; the kidneys.
The Urinary System
The kidneys, ureters, bladder and urethra make up the urinary system. Urine is made in the kidneys and leaves the body through the urethra.
| Part | Description |
|---|---|
| Kidneys | Two organs that remove urea and excess water and ions from the blood |
| Ureters | Tubes that carry urine from the kidneys to the bladder |
| Bladder | A muscular bag that stores urine |
| Urethra | The tube that carries urine from the bladder to the outside |
Worked example (tracing the urine): Give the order of the structures through which urine passes after it is made. Step 1. Urine leaves the kidney in the ureter. Step 2. It is stored in the bladder. Step 3. It leaves the body through the urethra. Answer: Kidney, ureter, bladder, urethra.
Structure of the Kidney
ExtendedKidney in section
A section through a kidney shows two regions: an outer cortex and an inner medulla. Blood enters in the renal artery and leaves in the renal vein. Urine leaves in the ureter.
ExtendedTwo regions
| Region | Position |
|---|---|
| Cortex | The outer part of the kidney |
| Medulla | The inner part of the kidney, inside the cortex |
Worked example (identifying the region): A section of a kidney shows an outer region and an inner region. Name the inner region. Step 1. The cortex is the outer layer of the kidney. Step 2. The region inside it is the medulla. Answer: Medulla.
The Nephron
ExtendedStructure of a nephron
Each kidney contains many nephrons. A nephron is a tube with blood vessels closely associated with it. At one end is a ball of capillaries called the glomerulus.
ExtendedFunction of a nephron
| Stage | What happens |
|---|---|
| Filtration | The glomerulus filters water, glucose, urea and ions from the blood |
| Reabsorption | The nephron reabsorbs all of the glucose, some of the ions and most of the water back into the blood |
| Urine formation | Urine forms, containing urea, excess water and excess ions |
Worked example (explaining the absence of glucose): Explain why urine from a healthy person contains no glucose. Step 1. The glomerulus filters glucose from the blood into the nephron. Step 2. The nephron reabsorbs all of the glucose back into the blood. Answer: All glucose is reabsorbed.
The Liver and Urea
ExtendedAssimilation of amino acids
Amino acids absorbed from digested protein are carried to the liver. The liver converts amino acids to proteins: this is the assimilation of amino acids.
ExtendedDeamination and urea
Amino acids that are in excess are not stored. In the liver the nitrogen-containing part of each is removed. This is deamination, and it forms urea. Urea is formed in the liver from excess amino acids. It passes into the blood, is carried to the kidneys, and is excreted in the urine.
Worked example (where is urea made?): State where urea is formed and what it is formed from. Step 1. Urea is formed in the liver. Step 2. It comes from excess amino acids, by deamination. Answer: In the liver, from excess amino acids.
Putting It Together
Each waste product has a source and an organ that removes it from the body.
| Product | Excreted by | Leaves the body |
|---|---|---|
| Carbon dioxide | Lungs | Breathed out |
| Urea | Kidneys | In urine, through the urethra |
| Excess water | Kidneys | In urine, through the urethra |
| Excess ions | Kidneys | In urine, through the urethra |
ExtendedWhy excretion is important
Urea is toxic: if it built up in the body it would be harmful. Excretion by the kidneys removes the urea from the blood, so its concentration stays low.
ExtendedTracing urea
Excess amino acids are deaminated in the liver, which forms urea. The urea is carried in the blood to the kidneys. The glomerulus filters it from the blood, and it leaves in the urine.
Worked example (explaining the importance): Explain why it is important that urea is excreted. Step 1. Urea is toxic to the body. Step 2. Excretion removes it, so it does not build up. Answer: Urea is toxic.
Exam advice
Common mistakes
Model answer
Recall checklist
- Define excretion.
- State which organ excretes each product.
- Identify the parts of the urinary system.
- Trace the path of urine out of the body.
- Identify the cortex and medulla (Extended)
- Outline the function of a nephron (Extended)
- Describe assimilation and deamination (Extended)
- Explain why urea must be excreted (Extended)
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