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Define homeostasis.
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All Flashcards in Topic 4.9
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4.9.19 cards
Define homeostasis.
Keeping the body's **internal environment** near a constant **set point**, despite changes outside.
What is a 'set point'?
The **normal value** a regulated variable is held close to (e.g. ~37 °C core temperature, ~pH 7.4 blood).
Name four blood/body variables kept constant by homeostasis.
**Blood glucose**, **temperature**, **blood pH**, and **water/solute balance**.
Define negative feedback.
A control loop where the **response opposes the change**, returning the variable toward its set point.
List the stages of a homeostatic control loop, in order.
**Stimulus → receptor → control centre → effector → response** (the response opposes the change).
Why is it called NEGATIVE feedback?
Because the response acts in the **opposite direction** to the change — a rise triggers a fall, a fall triggers a rise.
How does positive feedback differ from negative feedback?
Positive feedback **amplifies** the change (drives it further from normal); it is **not** homeostatic.
Give one example of positive feedback in the body.
The **LH surge** before ovulation (or **oxytocin** in childbirth) — the change is amplified to drive a process to completion.
Which brain structure is the control centre for several homeostatic loops?
The **hypothalamus** — it compares the variable with the set point and signals the effectors.
4.9.210 cards
What is thermoregulation?
Keeping the **core body temperature** close to a set point (about **37 °C**) despite changes in the environment.
Which type of feedback controls body temperature?
**Negative feedback** — the response opposes the change and returns temperature to the set point.
Which part of the brain is the temperature control centre?
The **hypothalamus** — it acts as the body's thermostat.
What two responses cool the body when it is too HOT?
**Vasodilation** (skin arterioles widen → more heat radiated) and **sweating** (evaporation removes heat).
What responses warm the body when it is too COLD?
**Vasoconstriction** (less heat lost), **shivering** (muscle respiration makes heat) and **non-shivering thermogenesis** in brown fat.
What is the difference between vasodilation and vasoconstriction?
**Vasodilation** = skin arterioles widen to **lose** heat (too hot); **vasoconstriction** = they narrow to **conserve** heat (too cold).
How does brown adipose tissue raise body temperature?
By **non-shivering thermogenesis** — it **oxidises lipids** and releases the energy **directly as heat**.
How does shivering raise body temperature?
Rapid **skeletal-muscle contractions** increase **respiration**, releasing **heat**.
Why is vasodilation a cooling response?
Widening the skin arterioles brings **more blood near the surface**, so **more heat is radiated/lost**.
Is thermoregulation an example of positive or negative feedback, and why?
**Negative feedback** — the response (e.g. sweating, shivering) **opposes** the temperature change.
4.9.39 cards
Which organ monitors and controls blood glucose?
The **pancreas** — it secretes insulin and glucagon.
Name the two hormones that regulate blood glucose.
**Insulin** (lowers high glucose) and **glucagon** (raises low glucose).
What does insulin do, and when is it released?
Released when glucose is **too high**: it makes **liver and muscle cells take up glucose** and store it as **glycogen**, lowering blood glucose.
What does glucagon do, and when is it released?
Released when glucose is **too low**: it makes the **liver break glycogen down** into glucose, raising blood glucose.
What is glycogen?
The **storage form of glucose** (many glucose units), kept mainly in the **liver and muscles**.
What does 'antagonistic hormones' mean here?
Insulin and glucagon have **opposite effects** — one lowers glucose, the other raises it.
Why is blood glucose control negative feedback?
Each response **opposes the change** and returns glucose to its **set point**, holding it within narrow limits.
On a graph, what explains blood glucose FALLING after a meal?
Glucose rose → **insulin** released → cells **take up and store** glucose as glycogen → glucose falls.
Glucagon vs glycogen — what's the difference?
**Glucagon** is the **hormone** that raises glucose; **glycogen** is the **storage molecule**.
4.9.48 cards
What is the normal set point for blood pH?
About **pH 7.4** (slightly alkaline).
Why does a rise in blood CO2 lower the blood pH?
CO2 dissolves to form **carbonic acid**, which releases **H+**, making the blood **more acidic** (lower pH).
What detects a change in blood pH / CO2?
**Chemoreceptors** — in the **medulla** of the brain and in the walls of the **aorta** and **carotid arteries**.
Which part of the brain controls breathing rate?
The **medulla** (in the brainstem).
What does the body do when blood pH falls below 7.4?
It **increases ventilation rate and depth** (breathes faster and deeper) to **remove more CO2** and raise pH back to the set point.
What does the body do when blood pH rises above 7.4?
It **decreases ventilation** so **CO2 is retained**, which lowers pH back to the set point.
Why is blood-pH control an example of negative feedback?
Because the response **reverses the change** (pH down → breathe faster → CO2 removed → pH up) and then switches off at the set point.
Which gas, not oxygen, mainly drives the urge to breathe?
**CO2** — rising CO2 (and falling pH) is what the chemoreceptors mainly sense.
4.9.57 cards
Which part of the brain is the appetite control centre?
The **hypothalamus**.
What is the source, target and function of leptin?
Source: **adipose (fat) tissue**; target: the **hypothalamus**; function: **suppresses appetite**.
Where is ghrelin made and what does it do?
Made by the **(empty) stomach**; it **stimulates appetite** (the 'hunger hormone').
Name two hormones that suppress appetite and where they come from.
**Leptin** (from adipose/fat tissue) and **insulin** (from the pancreas).
Why does more body fat lead to a stronger 'stop eating' signal?
More fat tissue secretes **more leptin**, which acts on the hypothalamus to **suppress appetite**.
What does thyroxin do, and where is it made?
Made by the **thyroid gland**; it sets the **basal metabolic rate** (how fast cells use energy).
A patient is tired, gaining weight and feels cold. Which hormone is likely low?
**Thyroxin** — too little lowers the metabolic rate, causing these symptoms.
Topic 4.9 study notes
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