Introduction to the Menstrual Cycle
In this chapter, we explore how the human body coordinates reproduction. This is part of the Animal coordination, control and homeostasis topic because it involves hormones (chemical messengers) traveling through the blood to control specific organs. We will look at how the menstrual cycle works, how we can prevent pregnancy using contraception, and how Assisted Reproductive Technology (ART) helps people who find it difficult to conceive.
The Menstrual Cycle
The menstrual cycle is a recurring process (usually lasting about \( 28 \) days) in which the lining of the uterus (womb) is prepared for pregnancy. If pregnancy does not occur, the lining is shed, and the cycle begins again.
The Four Main Stages:
Stage 1 (Days \( 1 \) to \( 4 \)): The lining of the uterus breaks down and is lost from the body. This is known as a period (menstruation).
Stage 2 (Days \( 4 \) to \( 14 \)): The uterus lining starts to build up again. It becomes a thick, spongy layer full of blood vessels, ready to receive a fertilized egg.
Stage 3 (Day \( 14 \)): Ovulation occurs. This is when a mature egg is released from one of the ovaries.
Stage 4 (Days \( 14 \) to \( 28 \)): The lining is maintained. If no fertilized egg lands on the uterus wall by day \( 28 \), the lining starts to break down again, and the cycle repeats.
Hormonal Control (Higher Tier Only)
Four main hormones interact to control the menstrual cycle. They are produced in different glands and work together using negative feedback and stimulation.
1. FSH (Follicle Stimulating Hormone)
Produced by the pituitary gland. It causes an egg to mature in one of the ovaries and stimulates the ovaries to produce oestrogen.
2. Oestrogen
Produced in the ovaries. It causes the lining of the uterus to thicken. High levels of oestrogen stimulate a surge in LH and inhibit (stop) the release of FSH so only one egg matures.
3. LH (Luteinising Hormone)
Produced by the pituitary gland. A sudden surge in LH levels triggers ovulation (the release of the egg).
4. Progesterone
Produced in the ovaries (specifically by the remains of the follicle after the egg is released). It maintains the lining of the uterus. When progesterone levels fall, the lining breaks down (the period starts). Progesterone also inhibits the release of both LH and FSH.
Memory Tip: Remember F-O-L-P (FSH, Oestrogen, LH, Progesterone) for the order in which they peak during the cycle!
Contraception
Contraception is used to prevent pregnancy. There are two main types: hormonal and barrier methods.
Hormonal Methods
These use synthetic hormones to disrupt the menstrual cycle. Examples include the oral contraceptive pill, implants, or patches.
How they work: They often use progesterone (or a mix of progesterone and oestrogen) to inhibit FSH so that no eggs mature, and thicken cervical mucus to stop sperm from reaching the egg.
Barrier Methods
These create a physical block so the sperm cannot reach the egg. Examples include condoms and diaphragms.
How they work: They simply stop the sperm and egg from meeting.
Evaluating Contraceptive Methods
When comparing methods, we look at effectiveness and safety.
Hormonal methods: These are usually more effective at preventing pregnancy if used correctly (\( >99\% \)). However, they can have side effects like mood swings or high blood pressure, and they do not protect against STIs (Sexually Transmitted Infections) like HIV or chlamydia.
Barrier methods: Condoms are slightly less effective at preventing pregnancy due to the risk of breaking or slipping, but they are the only method that also helps protect against STIs.
Assisted Reproductive Technology (ART) (Higher Tier Only)
If a couple has difficulty conceiving naturally (infertility), hormones can be used as medical treatments.
1. Clomifene Therapy
Some women do not ovulate regularly. Clomifene is a drug that works by stimulating the release of more FSH and LH. This triggers egg maturation and ovulation, increasing the chances that the woman will become pregnant through normal intercourse.
2. IVF (In Vitro Fertilisation)
IVF involves fertilizing the egg outside of the body ("In Vitro" means "in glass").
Step 1: The woman is given FSH and LH to stimulate the maturation of several eggs at once.
Step 2: The eggs are collected from the ovaries.
Step 3: The eggs are fertilized in a laboratory using the father's sperm.
Step 4: The fertilized eggs grow into embryos in an incubator.
Step 5: One or two embryos are transferred into the woman’s uterus to improve the chance of a successful pregnancy.
Key Takeaways
The Menstrual Cycle: Controlled by hormones to prepare the body for pregnancy every \( 28 \) days.
Hormones: FSH matures the egg; Oestrogen thickens the lining; LH triggers ovulation; Progesterone maintains the lining.
Contraception: Hormonal methods are highly effective but don't stop STIs; Barrier methods (condoms) prevent STIs.
ART: Clomifene triggers ovulation; IVF involves fertilizing eggs in a lab and placing embryos back into the uterus.