Welcome to Animal Coordination, Control, and Homeostasis!
In this chapter, we are going to explore how your body sends "messages" to different parts to keep everything running smoothly. We will look at hormones (the body's chemical messengers), how we control our internal environment (homeostasis), and how our bodies get rid of waste. Don't worry if some of the long names look scary—we will break them down step-by-step!
Note: This content is specifically designed for Paper 2 of your Edexcel GCSE Biology course.
1. Hormones and the Endocrine System
Your body has two main ways of sending messages: the nervous system (fast electrical impulses) and the endocrine system (slower chemical messages). These chemical messages are called hormones.
Where do hormones come from?
Hormones are produced in endocrine glands and released directly into the blood. The blood carries them to target organs, which are the specific places where the hormone needs to work.
Key Glands you need to know:
1. Pituitary Gland: The "master gland" in the brain. It makes many hormones that control other glands.
2. Thyroid Gland: In the neck. Controls your metabolic rate.
3. Pancreas: Controls blood sugar levels.
4. Adrenal Glands: Sit on top of the kidneys. Produce adrenalin for "fight or flight."
5. Ovaries (female) and Testes (male): Produce reproductive hormones.
Adrenalin: The "Fight or Flight" Hormone
When you are scared or stressed, your adrenal glands release adrenalin. This prepares your body for action by:
- Increasing heart rate and blood pressure: To pump more oxygen and glucose to your cells.
- Increasing blood flow to muscles: So they have the energy to run or fight.
- Raising blood sugar levels: By stimulating the liver to change glycogen back into glucose.
Thyroxine and Negative Feedback
Thyroxine controls your metabolic rate (how fast your body uses energy). It works through a negative feedback loop. Think of this like a thermostat in a house: when it gets too cold, the heating turns on; when it’s warm enough, it turns off.
How it works:
1. Low thyroxine levels trigger the hypothalamus to release TRH.
2. TRH tells the pituitary gland to release TSH.
3. TSH tells the thyroid to make thyroxine.
4. When thyroxine levels are back to normal, they "switch off" the production of TRH and TSH.
Quick Review: Hormones are chemicals in the blood. Adrenalin prepares you for action. Thyroxine controls energy use via negative feedback.
2. The Menstrual Cycle
The menstrual cycle is a monthly process (usually 28 days) that prepares a woman's body for potential pregnancy.
The Four Hormones Involved
It can be hard to remember all four, but they work in a specific order:
1. FSH (Follicle Stimulating Hormone): Produced by the pituitary gland. It causes an egg to mature in the ovary.
2. Oestrogen: Produced by the ovaries. It builds up the lining of the uterus (womb).
3. LH (Luteinising Hormone): Produced by the pituitary gland. It triggers ovulation (the release of the egg at Day 14).
4. Progesterone: Produced by the empty follicle in the ovary. It maintains the uterus lining.
The "Key Takeaway": If no pregnancy occurs, progesterone levels fall, the lining breaks down, and the period begins. If a woman is pregnant, progesterone stays high to keep the uterus lining thick.
3. Controlling Fertility
We can use our knowledge of hormones to either prevent pregnancy (contraception) or help people have children (ART).
Contraception
There are two main types:
- Hormonal methods: Like the "pill" or implants. These use hormones (like oestrogen or progesterone) to stop the body from releasing eggs.
- Barrier methods: Like condoms or diaphragms. These physically stop the sperm from meeting the egg and also protect against STIs.
Assisted Reproductive Technology (ART)
1. Clomifene therapy: A drug given to women who don't ovulate regularly. It triggers the release of FSH and LH.
2. IVF (In Vitro Fertilisation): Eggs are taken from the mother and fertilised by sperm in a lab. The resulting embryos are then placed back into the mother's uterus.
Memory Aid: In Vitro means "in glass" – because the fertilisation happens in a lab dish!
4. Homeostasis: Keeping the Balance
Homeostasis is the maintenance of a constant internal environment. Your body needs to keep things like temperature and water levels steady so your cells can work properly.
Why is it important?
- Thermoregulation (Temperature): If you get too hot, your enzymes will denature (stop working).
- Osmoregulation (Water): If you have too much or too little water, your cells could swell up and burst or shrivel up and die.
How we control temperature (Thermoregulation)
The hypothalamus in your brain acts as your body's "brain center" for temperature. It receives signals from the epidermis and dermis (layers of your skin).
If you are too HOT:
- Vasodilation: Blood vessels near the skin get wider so more blood flows near the surface, losing heat to the air.
- Sweating: Water evaporates from your skin, taking heat away with it.
If you are too COLD:
- Vasoconstriction: Blood vessels near the skin get narrower to keep blood (and heat) deep inside your body.
- Shivering: Your muscles contract rapidly, which releases energy as heat from respiration.
Quick Review: Homeostasis is about "staying the same." Vasodilation = Cooling down. Vasoconstriction = Keeping warm.
5. Control of Blood Glucose
Your body needs glucose for energy, but too much in the blood is dangerous. The pancreas monitors this.
Insulin and Glucagon
These two hormones are like a see-saw:
- If blood glucose is TOO HIGH: The pancreas releases insulin. This tells the liver to take glucose out of the blood and store it as glycogen.
- If blood glucose is TOO LOW: The pancreas releases glucagon. This tells the liver to turn glycogen back into glucose and release it into the blood.
Mnemonic: Gluca-gon is used when the glucose is gone!
Diabetes
Type 1 Diabetes: The pancreas produces little or no insulin. It is usually controlled by insulin injections.
Type 2 Diabetes: The body's cells stop responding to insulin. It is often linked to obesity and can be controlled by diet and exercise.
Evaluating Health: BMI
Doctors use the Body Mass Index (BMI) to see if someone is at risk of Type 2 diabetes. The formula is:
\( BMI = \frac{mass (kg)}{(height (m))^2} \)
6. The Urinary System: Waste and Water
Your body produces a waste product called urea in the liver when it breaks down excess amino acids. This urea must be removed from the blood by the kidneys and turned into urine.
The Nephron: The Kidney's Filter
Each kidney contains millions of tiny tubes called nephrons. They work in three stages:
1. Filtration: Blood is filtered at high pressure in the glomerulus and Bowman’s capsule. Small things (water, urea, glucose) are pushed out of the blood.
2. Selective Reabsorption: The body takes back all the glucose and some water/ions that it still needs.
3. Excretion: Whatever is left over (urea, excess water) forms urine and goes to the bladder.
ADH and Water Balance
A hormone called ADH (Anti-Diuretic Hormone) controls how much water your kidneys reabsorb. It changes the permeability of the collecting duct in the nephron.
- If you are dehydrated: The brain releases more ADH. This makes the collecting duct more permeable, so more water goes back into the blood. You produce a small amount of dark, concentrated urine.
- If you have plenty of water: The brain releases less ADH. Less water is reabsorbed. You produce a lot of pale, dilute urine.
Treating Kidney Failure
If kidneys stop working, a person can be treated with:
- Dialysis: A machine filters the blood artificially.
- Organ Donation: A healthy kidney from a donor is transplanted into the patient.
Common Mistake to Avoid: Don't confuse glucagon (a hormone) with glycogen (stored sugar) or glycerol (part of a lipid). They look similar but are very different!
Key Takeaway: The kidneys filter the blood, take back the "good stuff" like glucose, and use ADH to decide exactly how much water to keep.