Revision notes · Homeostasis and response
Hormonal coordination in humans
Human endocrine system4.5.3.1
Definition: The endocrine system is a system of glands that secrete chemical messengers called hormones directly into the bloodstream to target organs.
The endocrine system uses chemical messengers to carry slow, long-lasting signals around the body.
- •Hormones are chemical messengers secreted by endocrine glands and transported in blood plasma.
- •The pituitary gland in the brain is the 'master gland' that secretes several hormones to stimulate other glands.
- •Key endocrine glands include the pituitary, thyroid, pancreas, adrenal glands, ovaries, and testes.
- •Compared to the nervous system, endocrine responses are slower but have longer-lasting effects.
⚠️ Common mistake: Confusing the speed and duration of nervous impulses (fast, short) with hormonal responses (slower, longer-lasting).
🧠 Remember: Hormones travel in blood, nervous impulses travel along neurones.
Control of blood glucose concentration4.5.3.2
Definition: Blood glucose concentration is the amount of glucose dissolved in blood plasma, monitored and controlled by the pancreas.
The body keeps blood glucose levels balanced using the hormones insulin and glucagon.
| Hormone | Released when... | Effect |
|---|---|---|
| Insulin | Blood glucose too high | Liver & muscle cells absorb glucose, convert it to glycogen |
| Glucagon | Blood glucose too low | Liver converts stored glycogen back into glucose |
| Type 1 diabetes | Type 2 diabetes | |
|---|---|---|
| Cause | Pancreas doesn't produce enough insulin | Body cells no longer respond to insulin |
| Managed with | Insulin injections | Carbohydrate-controlled diet & exercise |
⚠️ Common mistake: Confusing glucagon (the hormone) with glycogen (the storage carbohydrate in liver and muscle cells).
🧠 Remember: Glucagon makes glucose go up when it is low.
Maintaining water and nitrogen balance in the body4.5.3.3
Definition: Osmoregulation is the control of water levels and mineral ion concentrations in the blood to prevent cell damage.
The kidneys filter blood to remove waste products like urea and regulate water and ion balances.
- •Excess amino acids are broken down in the liver by deamination into toxic ammonia, which is quickly converted to urea.
- •The kidneys filter blood, reabsorbing all glucose, necessary water, and ions, while excreting all urea and excess water/ions as urine.
- •The hormone ADH (antidiuretic hormone) is released by the pituitary gland when blood is too concentrated, increasing kidney tubule permeability so more water is reabsorbed.
- •Kidney failure can be treated using dialysis machines or a kidney transplant.
⚠️ Common mistake: Thinking that ADH is produced or released by the kidneys rather than the pituitary gland.
🧠 Remember: More ADH = Add Drops (of) H2O back to the blood.
Hormones in human reproduction4.5.3.4
Definition: Reproductive hormones are chemical messengers that cause secondary sexual characteristics to develop and control the menstrual cycle.
Hormones regulate puberty and the monthly female menstrual cycle.
- •Testosterone is the main male reproductive hormone, produced by the testes, stimulating sperm production.
| Hormone | Effect |
|---|---|
| FSH | Causes an egg to mature in the ovary; stimulates oestrogen production |
| Oestrogen | Thickens the uterus lining; inhibits FSH, stimulates LH release |
| LH | Triggers ovulation (egg release) |
| Progesterone | Maintains the uterus lining |
⚠️ Common mistake: Mixing up the order in which hormones act during the menstrual cycle.
🧠 Remember: Menstrual hormone order: FSH, Oestrogen, LH, Progesterone (FOLP).
Contraception4.5.3.5
Definition: Contraception refers to methods used to prevent fertilisation or implantation, avoiding pregnancy.
Pregnancy can be prevented using either hormonal or non-hormonal contraceptive methods.
| Method | How it works |
|---|---|
| Hormonal (pill, injection, patch, implant) | Releases hormones to inhibit FSH, so no eggs mature |
| Barrier (condoms, diaphragms) | Physically prevents sperm reaching an egg |
| IUD | Prevents embryo implantation, or releases hormones to prevent ovulation |
| Surgical (sterilisation) | Cuts or ties sperm ducts/fallopian tubes — permanent |
⚠️ Common mistake: Believing that hormonal contraceptives protect against sexually transmitted infections (STIs)—only condoms protect against STIs.
🧠 Remember: Only condoms protect against both pregnancy and STIs.
The use of hormones to treat infertility4.5.3.6
Definition: Infertility treatments use hormones to assist individuals or couples who experience difficulty conceiving naturally.
Fertility drugs and advanced techniques like IVF help overcome reproductive issues.
- •Women with low FSH levels can take a 'fertility drug' containing FSH and LH to stimulate egg maturation and ovulation.
- •In IVF (In Vitro Fertilisation), FSH and LH are given to stimulate the maturation of several eggs simultaneously.
- •Collected eggs are fertilised by sperm in a laboratory, grown into embryos, and inserted into the woman's uterus.
- •Drawbacks of IVF include high emotional and physical stress, low success rates, an increased risk of multiple births, and high cost, especially if repeated cycles are needed.
⚠️ Common mistake: Thinking fertilisation in IVF takes place inside the female body rather than in a laboratory vessel.
🧠 Remember: IVF sequence: Hormones -> Egg collection -> Lab fertilisation -> Embryo transfer.
Negative feedback4.5.3.7
Definition: Negative feedback is a homeostatic mechanism where a deviation from a normal set point triggers a response that restores conditions to normal.
Negative feedback systems maintain optimal internal conditions by reversing any change in the body.
- •If a factor increases above normal, negative feedback mechanisms work to lower it back to the set point.
- •If a factor decreases below normal, negative feedback mechanisms work to raise it back to the set point.
- •Thyroxine from the thyroid gland regulates basal metabolic rate and is controlled by negative feedback.
- •The pituitary gland releases thyroid stimulating hormone (TSH), which stimulates the thyroid to release thyroxine; rising thyroxine levels then inhibit further TSH release, completing the feedback loop.
- •Adrenaline is released from the adrenal glands during stress to prepare for 'fight or flight', but is not controlled by negative feedback.
⚠️ Common mistake: Assuming adrenaline is controlled by negative feedback; it is released in response to acute stress, not a set point mechanism.
🧠 Remember: Negative feedback negates (reverses) any change from normal.
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