| Core | Supplement |
|---|---|
| 1 State that carbon dioxide is excreted through the lungs | |
| 2 State that the kidneys excrete urea and excess water and ions | |
| 3 Identify in diagrams and images the kidneys, ureters, bladder and urethra | 4 Identify in diagrams and images the structure of the kidney, limited to the cortex and medulla |
| 5 Outline the structure and function of a nephron and its associated blood vessels, limited to: (a) the role of the glomerulus in the filtration from the blood of water, glucose, urea and ions (b) the role of the nephron in the reabsorption of all of the glucose, some of the ions and most of the water back into the blood (c) the formation of urine containing urea, excess water and excess ions (details of these processes are not required) | |
| 6 Describe the role of the liver in the assimilation of amino acids by converting them to proteins | |
| 7 State that urea is formed in the liver from excess amino acids | |
| 8 Describe deamination as the removal of the nitrogen-containing part of amino acids to form urea | |
| 9 Explain the importance of excretion, limited to toxicity of urea |
Excretion in humans
IGCSE Biology · Topic 13
13.1
Excretion
Syllabus
Source: Cambridge International syllabus
Excretion 排泄 is the removal of the waste products of metabolism, and of substances the body has in excess. Two main wastes are removed:
- carbon dioxide 二氧化碳 — made during respiration, and breathed out through the lungs.
- urea 尿素 — made in the liver 肝脏, and removed by the kidneys together with excess water and ions 离子.
Explore
Excretion and the kidneys
The kidneys clean the blood, removing urea and adjusting water, and make urine.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| excretion | 排泄 | pái xiè |
| carbon dioxide | 二氧化碳 | èr yǎng huà tàn |
| urea | 尿素 | niào sù |
| liver | 肝脏 | gān zàng |
| ions | 离子 | lí zi |
13.1
The urinary system
The kidneys filter the blood and make urine.
The kidneys 肾脏 clean the blood and make urine 尿液. The urine then passes through:
- the ureters 输尿管 — tubes from the kidneys to the bladder.
- the bladder 膀胱 — stores the urine.
- the urethra 尿道 — carries the urine out of the body.
The kidneys make urine, which flows down the ureters to the bladder and out of the urethra
If the kidneys stop working, a dialysis 透析 machine can do their job. The patient's blood flows through the machine, where waste and excess water pass out across a thin membrane, and the clean blood returns to the body.
A dialysis machine cleans the blood when the kidneys stop working
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| kidneys | 肾脏 | shèn zàng |
| urine | 尿液 | niào yè |
| ureters | 输尿管 | shū niào guǎn |
| bladder | 膀胱 | páng guāng |
| urethra | 尿道 | niào dào |
| dialysis | 透析 | tòu xī |
| renal cortex | 皮质 | pí zhì |
13.1
Inside the kidney (Supplement)
A kidney has two regions: an outer cortex 皮质 and an inner medulla 髓质.
The nephron
Each kidney holds millions of tiny tubes called nephrons 肾单位. A nephron cleans the blood in two steps:
- Filtration 过滤: at the glomerulus 肾小球 (a knot of capillaries), high pressure forces water, glucose 葡萄糖, urea and ions out of the blood and into the nephron.
- Reabsorption 重吸收: as the liquid flows along the nephron, the blood takes back all of the glucose, some of the ions, and most of the water.
What is left behind — urea, excess water and excess ions — becomes the urine.
A nephron filters the blood, then reabsorbs the useful substances; what is left is urine
Worked example. A healthy person's urine contains urea and ions but no glucose, even though glucose is forced out of the blood at the glomerulus. Explain. Filtration does not choose: high pressure pushes water, glucose, urea and ions into the nephron together. The choosing happens at the next step - during reabsorption the blood takes back all of the glucose, so none is left to reach the urine. Glucose does appear in the urine of someone with untreated diabetes, because there is more of it than the nephron can reabsorb. Filtration removes; reabsorption selects.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| medulla | 髓质 | suǐ zhì |
| nephrons | 肾单位 | shèn dān wèi |
| filtration | 过滤 | guò lǜ |
| glomerulus | 肾小球 | shèn xiǎo qiú |
| glucose | 葡萄糖 | pú táo táng |
| reabsorption | 重吸收 | zhòng xī shōu |
13.1
The liver, amino acids and urea (Supplement)
The liver carries out the assimilation 同化 of amino acids 氨基酸: it joins them together into the proteins 蛋白质 that the body needs.
The body cannot store extra amino acids. The liver breaks the excess down by deamination 脱氨基作用 — it removes the nitrogen 氮-containing part of each amino acid. This part is then turned into urea.
The liver makes urea from excess amino acids; the kidneys excrete it
Urea must be excreted because it is toxic 有毒 (poisonous) if it is allowed to build up in the blood.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| assimilation | 同化 | tóng huà |
| amino acids | 氨基酸 | ān jī suān |
| proteins | 蛋白质 | dàn bái zhì |
| deamination | 脱氨基作用 | tuō ān jī zuò yòng |
| nitrogen | 氮 | dàn |
| toxic | 有毒 | yǒu dú |
13.1
Exam tips
- Excretion removes metabolic wastes. Carbon dioxide leaves through the lungs; urea, excess water and excess ions leave through the kidneys.
- Urine path: kidney → ureter → bladder → urethra.
- Nephron: the glomerulus filters the blood; the nephron reabsorbs all the glucose, some ions and most of the water. What is left is urine.
- Urea is made in the liver from excess amino acids by deamination.
- We must excrete urea because it is toxic to the body.