Sense Organs and Homeostatic Mechanisms
Senior 3 · Biology
Sense Organs and Homeostatic Mechanisms
Lesson 5 of 5
25 min
A Senior 3 student has been losing weight for a month, although he eats more than ever. He is always thirsty, he gets up several times each night to pass urine, and he is tired in class. At the health centre a nurse pricks his finger and puts a drop of blood on a test strip: the meter reads far above normal. The student has diabetes. His symptoms make sense once you know how the body normally controls two things in the blood: its water and its glucose.
Definition
Osmoregulation: the control of the water content, and so the concentration, of the blood and tissue fluid.
Antidiuretic hormone (ADH): a hormone released by the pituitary gland that makes the collecting ducts of the kidneys more permeable to water, so more water is reabsorbed into the blood.
Glycogen: a storage carbohydrate made of many glucose units, stored in the liver and muscles.
Diabetes mellitus: a disease in which blood glucose stays too high, because the pancreas makes too little insulin or the body's cells do not respond to it properly. Diabetes insipidus: a rare condition in which the body makes too little ADH, or the kidneys do not respond to it, so large volumes of dilute urine are made.
In Senior 2 you learnt how the kidneys make urine. ADH controls how much water they keep:
| Situation | Blood | Detected by | ADH | Kidneys | Urine |
|---|---|---|---|---|---|
| Sweating, little drinking, salty food | Too concentrated | Receptors in the hypothalamus | More released by the pituitary | Collecting ducts more permeable; more water reabsorbed | Small volume, concentrated, dark |
| Drinking a lot of water | Too dilute | Receptors in the hypothalamus | Less released | Less water reabsorbed | Large volume, dilute, pale |
Both are negative feedback: the response reverses the change. Thirst, also triggered by the hypothalamus, adds a behavioural response.
In diabetes insipidus, too little ADH is released (or the kidneys ignore it). The person passes very large volumes of dilute urine and is always thirsty. It is treated with a synthetic form of ADH. It is a different disease from diabetes mellitus: the urine contains no glucose.
Cells need a steady supply of glucose for respiration; the brain is especially dependent on it. If blood glucose falls too low, a person becomes shaky, confused and may lose consciousness; if it stays too high for years, it damages blood vessels, nerves, the kidneys and the eyes. The pancreas is both the receptor and the control centre (Figure 1).
| Feature | Type 1 diabetes | Type 2 diabetes |
|---|---|---|
| Cause | The body's immune system destroys the insulin-making cells of the pancreas, so little or no insulin is made | The body's cells stop responding well to insulin, and the pancreas may not make enough |
| Usual age at onset | Often in childhood or adolescence | Usually adults, but increasingly young people |
| Risk factors | Not caused by lifestyle | Excess body weight, physical inactivity, unhealthy diet, family history |
| Treatment | Insulin injections every day, with careful diet and testing of blood glucose | Healthy diet, physical activity and weight loss; medicines; sometimes insulin |
Signs of diabetes mellitus include great thirst, passing urine often, tiredness, weight loss without trying, blurred vision and, because the kidneys cannot reabsorb all the glucose, glucose in the urine. Diagnosis is made by measuring blood glucose. The World Health Organization uses a fasting blood glucose of 7.0 mmol per litre or more, or 11.1 mmol per litre or more two hours after a 75 g glucose drink, as signs of diabetes.
Figure 2: illustrative results for two people, A (circles, solid line) and B (squares, dashed line).
Following World Health Organization advice on a healthy diet and physical activity lowers the risk of type 2 diabetes:
Worked example 1: reading Figure 2
Worked example 2: sugar in one bottle
The label on a 500 ml bottle of soft drink says it contains 50 g of sugar (an illustrative label; read the real one). WHO's limit of 10% of daily energy from free sugars is about 50 g a day for an adult eating about 2000 kilocalories. A level teaspoon of sugar is about 4 g.
Warning
Common mistakes. (1) Writing that insulin "breaks down glucose": it makes cells take up glucose and makes the liver and muscles store it as glycogen. (2) Confusing glucagon with glycogen. (3) Mixing up the two diabetes diseases: diabetes mellitus (insulin, glucose in urine) and diabetes insipidus (ADH, very dilute urine).
First aid: low blood glucose. If a person with diabetes becomes shaky, sweaty or confused but is awake and can swallow, give them something sugary, such as a sweet drink or glucose, and stay with them. If they are drowsy or unconscious, give nothing by mouth, place them in the recovery position and get emergency medical help.
Control of Blood Glucose Concentration, Cognito. Watch for how insulin and glucagon act on the liver in a negative feedback loop; compare it with Figure 1.
Tip
Memory hook: "insulin puts glucose IN". Insulin moves glucose into cells and into store; glucagon is released when glucose is gone.
Practice: water and glucose
Key points
Further reading
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2 (Figure 2 shows illustrative results), the tables, the worked examples and the practice questions.
