人体的排泄

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.

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.

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 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:

1. 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.
2. Reabsorption 重吸收: in healthy kidneys, 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.

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.

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.

Urea must be excreted because it is toxic 有毒 (poisonous) if it is allowed to build up in the blood.

Common mistakes.

• Calling the removal of undigested food (faeces) excretion. That food was never part of the body's metabolism, so it is not excretion; excretion removes waste made by cells, such as carbon dioxide and urea.
• Saying the kidneys make urea. Urea is made in the liver; the kidneys only remove it from the blood.
• Mixing up the ureter and the urethra. The two ureters carry urine from the kidneys to the bladder; the one urethra carries it out of the body.
• Saying the glomerulus filters out only the waste. Filtration is not selective: water, glucose, urea and ions are all forced out. The useful substances are taken back later, by reabsorption.
• Saying "some glucose" is reabsorbed. In a healthy person all of the glucose is reabsorbed, so there is none in the urine.
• Saying deamination makes urea directly from proteins. The liver deaminates excess amino acids; the nitrogen part removed is then turned into urea.

• 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.