稳态

Homeostasis 稳态 means keeping the conditions inside the body steady, even when the outside changes. Keeping things like temperature, water and blood glucose steady lets enzymes and cells work properly all the time.

The principles of homeostasis

Most homeostasis follows the same plan:

• a change (an internal or external stimulus 刺激) is detected by a receptor 受体.
• a coordination system carries the message — either the nervous system 神经系统 (using nerve signals) or the endocrine system 内分泌系统 (using hormones 激素).
• an effector 效应器 (a muscle or a gland 腺体) makes a response that corrects the change.

This works by negative feedback 负反馈: a change away from the normal level triggers a response that pushes it back towards normal.

The body cannot store extra amino acids. In the liver 肝脏, the process of deamination 脱氨基作用 removes the amino group from excess amino acids 氨基酸, and this is turned into urea 尿素. The urea is carried in the blood to the kidneys to be removed.

The kidney 肾脏 cleans the blood and controls its water content. Its parts are:

• an outer fibrous capsule,
• an outer region, the cortex 皮质,
• an inner region, the medulla 髓质,
• a central space, the renal pelvis 肾盂, which collects urine,
• the ureter 输尿管, which carries urine to the bladder,
• branches of the renal artery (bringing blood in) and renal vein (taking blood out).

The nephron

Each kidney holds about a million tiny tubes called nephrons 肾单位. Along a nephron are: the glomerulus 肾小球 (a knot of capillaries), the Bowman's capsule 鲍曼囊 around it, the proximal convoluted tubule 近曲小管, the loop of Henle 亨利环, the distal convoluted tubule 远曲小管, and the collecting duct 集合管.

Making urine

1. Ultrafiltration 超滤 happens in the Bowman's capsule. The blood in the glomerulus is under high pressure, so water and small molecules (glucose 葡萄糖, ions 离子, urea) are pushed out into the capsule, forming the filtrate 滤液. Blood cells and large proteins are too big to pass, so they stay in the blood.
2. Selective reabsorption 选择性重吸收 happens in the proximal convoluted tubule. Useful substances are taken back into the blood. All the glucose and much of the water and ions are reabsorbed here. The tubule wall is well suited to this: its cells have microvilli 微绒毛 to give a large surface area, and many mitochondria 线粒体 to power active transport 主动运输.

How glucose is reabsorbed. Glucose is taken back even though the cell may already contain more glucose than the filtrate. The cell does this by co-transport 协同运输 (follow the numbers):

1. sodium–potassium pumps on the side of the cell facing the blood use ATP to pump sodium ions out of the cell. This keeps the sodium ion concentration inside the cell low.
2. sodium ions diffuse into the cell from the filtrate, down their concentration gradient, through a co-transporter protein 协同转运蛋白. Each one carries a glucose molecule in with it, even against the glucose concentration gradient.
3. glucose diffuses out of the other side of the cell into the blood, by facilitated diffusion.
4. as solutes leave the filtrate, its water potential rises, so water follows by osmosis 渗透.

So glucose reabsorption is indirectly active: the energy from ATP is used by the pump, not by the co-transporter. This is why the cells need many mitochondria. If the blood glucose concentration is very high (as in untreated diabetes), the co-transporters are all in use and some glucose stays in the urine.

Common mistakes.

• Saying glucose is "actively transported" by the co-transporter. The co-transporter uses the sodium gradient; ATP is used by the sodium–potassium pump.
• Saying ultrafiltration is selective. It sorts only by size; selection happens afterwards, in the proximal convoluted tubule.
• Saying ADH "makes the kidney produce more water". ADH makes the collecting duct more permeable to water, so more water is reabsorbed into the blood.

Osmoregulation 渗透调节 controls the water content of the blood. It is run by the brain:

• the hypothalamus 下丘脑 detects the water potential 水势 of the blood.
• when the blood is too concentrated, the pituitary gland 垂体 releases antidiuretic hormone 抗利尿激素 (ADH).
• ADH makes the collecting ducts more permeable to water by adding water channels called aquaporins 水通道蛋白.
• more water is then reabsorbed back into the blood, so less, more concentrated urine is made. This is negative feedback.

When blood glucose falls, the hormone glucagon 胰高血糖素 is released. It shows how a hormone passes its message into a cell — cell signalling 细胞信号传递:

1. glucagon binds to a receptor on the liver cell surface, causing a conformational change 构象变化 (a change in the receptor's shape).
2. this activates a G-protein G蛋白, which switches on the enzyme adenylyl cyclase 腺苷酸环化酶.
3. adenylyl cyclase makes a second messenger 第二信使 inside the cell, called cyclic AMP 环腺苷酸 (cAMP).
4. cAMP activates protein kinase A 蛋白激酶A, which starts an enzyme cascade 酶级联反应 — one enzyme 酶 switches on the next, by phosphorylation 磷酸化.
5. because each enzyme switches on many of the next, the signal is greatly amplified 放大.
6. the final enzyme breaks down glycogen 糖原 into glucose, which raises the blood glucose level.

Common mistakes.

• Saying glucagon enters the liver cell. Glucagon is a protein and cannot cross the membrane; only the message gets in, through the receptor and the second messenger.
• Saying glucagon "turns into" glucose, or that cAMP breaks down glycogen. cAMP only activates protein kinase A; the enzymes at the end of the cascade break down glycogen.
• Mixing up glucagon and glycogen. Glucagon is the hormone; glycogen is the stored carbohydrate it causes to be broken down.

Negative feedback and blood glucose

Blood glucose is held steady by two hormones working against each other:

• when glucose is high, insulin 胰岛素 makes muscle and liver cells take in glucose and store it as glycogen, lowering the level.
• when glucose is low, glucagon makes liver cells break glycogen back into glucose, raising the level.

Measuring glucose

Test strips 试纸 and biosensors 生物传感器 measure glucose in blood or urine. They use the enzymes glucose oxidase 葡萄糖氧化酶 and peroxidase 过氧化物酶, which react with glucose to give a colour change (or an electric signal in a biosensor) that shows how much glucose is present.

Worked example. A person drinks a large volume of water. Trace the homeostatic response. The blood's water potential rises (becomes less negative). Osmoreceptors in the hypothalamus detect this, so the posterior pituitary releases less ADH. With less ADH, fewer aquaporins are inserted into the collecting duct's membranes, so the duct becomes less permeable to water. Less water is reabsorbed, so a large volume of dilute urine is produced and the blood's water potential falls back towards normal. That return to the set point is what makes it negative feedback - the response reverses the original change. Name the receptor, the hormone, the effector and the correction: an answer that jumps from "drinks water" straight to "more urine" skips every marking point in between.

Stomata 气孔 are pores in a leaf. The plant opens and closes them to balance two needs: letting in carbon dioxide 二氧化碳 for photosynthesis 光合作用, and limiting water loss by transpiration 蒸腾作用. Stomata usually open by day and close at night, following a daily rhythm.

Each stoma is opened and closed by two guard cells 保卫细胞 around it:

• to open: the guard cells pump in ions, so their water potential falls and water enters by osmosis. They swell and bend apart, opening the pore.
• to close: ions leave, water follows out, the guard cells go floppy, and the pore closes.

When the plant is short of water (water stress 水分胁迫), the hormone abscisic acid 脱落酸 is released. It makes the guard cells lose ions and water so the stomata close, saving water. In this signalling, calcium ions 钙离子 act as a second messenger inside the guard cells.

• Frame every answer as negative feedback: receptor → coordinator → effector → correction; name each part.
• Blood glucose: insulin lowers it (uptake, glycogenesis), glucagon raises it (glycogenolysis) — explain through cell signalling.
• Kidney: know ultrafiltration (Bowman's capsule), selective reabsorption (proximal tubule) and the role of ADH in osmoregulation.
• Stomata open when guard cells become turgid (K+ enters, water follows).