Excretion and Homeostasis
Senior 5 · Biology
Excretion and Homeostasis
Lesson 2 of 4
25 min
After a plate of rice and beans at lunch, glucose floods into your blood from the gut. During a football match that afternoon, your muscles pull glucose out of it fast. Yet a blood test at almost any moment would show nearly the same glucose concentration. Your body temperature, blood pH, water content and carbon dioxide level are held steady in the same way. Keeping the internal environment constant is called homeostasis. In Unit 8 you saw one example, the control of breathing. This lesson sets out the general principles and applies them to blood glucose.
Definition
Homeostasis: the maintenance of a relatively constant internal environment (the blood and tissue fluid around the cells) despite changes inside and outside the body.
Set point (norm): the value around which a factor is kept, such as about 37 °C for core body temperature.
Negative feedback: a change from the set point is detected and triggers a response that reverses the change, bringing the factor back to the set point.
Positive feedback: a change triggers a response that increases the change further.
Glycogenesis: making glycogen from glucose. Glycogenolysis: breaking glycogen down to glucose. Gluconeogenesis: making new glucose from non-carbohydrates such as amino acids and glycerol.
Every homeostatic mechanism has the same parts: a receptor that detects a change (the stimulus), a coordinator that compares it with the set point, and effectors that bring about a response. The message travels by nerves, by hormones, or both.
Cells, and above all brain cells, need a steady supply of glucose for respiration. If blood glucose falls too low (hypoglycaemia), a person becomes shaky, confused and may lose consciousness. If it stays too high (hyperglycaemia), the blood's water potential falls, water leaves cells by osmosis, glucose appears in the urine, and over years blood vessels, nerves, eyes and kidneys are damaged.
The pancreas is mostly exocrine tissue, making digestive enzymes, but scattered through it are small groups of endocrine cells, the islets of Langerhans. They act both as receptors and as coordinators: their β (beta) cells detect a rise in blood glucose and secrete insulin; their α (alpha) cells detect a fall and secrete glucagon.
| Insulin | Glucagon | |
|---|---|---|
| Secreted by | β cells, when glucose rises | α cells, when glucose falls |
| Main target cells | Liver, muscle and fat cells | Liver cells |
| Effects | More glucose transporter proteins (GLUT4) inserted into muscle and fat cell membranes, so more glucose enters; glycogenesis; more glucose respired; glucose turned into fat | Glycogenolysis; gluconeogenesis; glucose released into the blood |
| Result | Blood glucose falls | Blood glucose rises |
How a β cell knows. Glucose enters the β cell and is respired, so the ATP concentration rises. ATP closes potassium ion channels in the membrane, so the cell depolarises; this opens calcium ion channels, and calcium ions entering the cell trigger the exocytosis of vesicles of insulin.
How glucagon works inside a liver cell: the second messenger model. Glucagon cannot enter the cell. It binds to a receptor on the cell surface, which activates the enzyme adenylyl cyclase; this makes many molecules of cyclic AMP (cAMP), the "second messenger". cAMP activates a protein kinase, which starts a chain of enzymes that ends with the enzyme that breaks down glycogen. Each step multiplies the signal, so a few hormone molecules release a great deal of glucose. Adrenaline, released in stress and exercise, works on liver cells in the same way.
Worked example 1: reading a blood glucose result
Some meters give glucose in mg dL⁻¹. Glucose has , so . After an overnight fast, a man's plasma glucose is . The WHO criterion for diabetes is a fasting plasma glucose of or more. Convert his result and interpret it.
The World Health Organization's fact sheet on diabetes (14 November 2024) reports that the number of people living with diabetes rose from 200 million in 1990 to 830 million in 2022, with prevalence rising faster in low- and middle-income countries, and that more than half of people with diabetes were not taking medication for it in 2022.
| Type 1 | Type 2 | |
|---|---|---|
| Cause | The β cells are destroyed, usually by the body's own immune system, so little or no insulin is made | Cells respond poorly to insulin (insulin resistance), and the β cells may later make too little |
| Usual onset | Often in childhood or youth, quickly | Usually in adults, slowly; linked to excess body weight and physical inactivity |
| Treatment | Daily insulin injections, matched to food and exercise | Healthy diet, physical activity, weight control; medicines; sometimes insulin |
| Prevention | Its cause and prevention are not known (WHO) | Can often be prevented or delayed by a healthy diet, regular activity, a healthy weight and avoiding tobacco (WHO) |
Insulin must be injected, not swallowed, because it is a protein and would be digested in the gut. In a glucose tolerance test, a person fasts overnight, drinks 75 g of glucose, and has their blood glucose measured over the next two hours (Figure 3). WHO criteria for diabetes include a fasting plasma glucose of 7.0 mmol dm⁻³ or more, or a value of 11.1 mmol dm⁻³ or more two hours after the glucose drink.
Worked example 2: interpreting a glucose tolerance test
Use Figure 3. (a) Compare the two curves. (b) Which person's results meet the WHO criteria for diabetes, and why? (c) Explain the shape of person A's curve.
Warning
Common mistakes and safety
Miss Estruch Biology: "Control of blood glucose: the role of insulin, glucagon and adrenaline. The second messenger model". Watch for how insulin and glucagon act on their target cells, and for how cAMP amplifies the signal inside a liver cell.
Tip
Memory hook: glucaGON is released when the glucose is GONE. And α cells Add glucose to the blood (glucagon), while β cells Bring it down (insulin).
Practice: homeostasis and blood glucose
Key points
Sources and credits
Figures 2 and 3, the tables, the worked examples and the practice questions are by Ganzaa. The results in Figure 3 and Worked example 1 are illustrative.
