Topic B5 - Homeostasis and Response

Reducing fertility 

Oestrogen can be used to prevent the release of an egg - so it can be used as a method of contraception (preventing fertility of an egg). If oestrogen is taken everyday to keep the level of it permanently high, it inhibits the the production of FSH (the hormone that helps the production of an egg), and after a while egg development and production stop and stay stopped. 

Progesterone also reduces fertility, eg. by stimulating the production of thick mucus which prevents any sperm gettting through and reaching the egg. The pill is an oral contraceptive containing oestrogen and progesterone (known as the combined oral contraceptive pill). It's over 99% effective at preventing pregnancy, but it can cause side effects like headaches and nausea and doesn't protect against sexually transmitted disease (STDs). 

There's also a progesterone-only pill - it has fewer side effects than the pill, and is just as effective. 

Stopping the egg and sperm meeting with barriers 

Non-hormonal forms of contraception are designed to stop the sperm from gettign to the egg. 

Condoms are worn over the penis during intercourse to prevent the sperm entering the vagina. There are also female condoms that are worn inside the vagina. Condoms are the only form of contraception that will protect against sexually transmitted disease (STDs). 

A diaphragm is a shallow plastic cup that fits over the cervix (the enterance to the uterus) to form a barrier. It has to be used with spermicide (a substance that disables or kills the sperm). Spermicide can be used alone as a form of contraception, but it's not as effective (only about 70-80%).

More drastic ways of stopping pregnancy 

Steralisation - Steralisation involves cutting or tying the fallopian tubes (which connects the ovaries to the uterus) in a female, or a sperm duct (the tube between the testes and penis) in a male. This is a permanent procedure. However, there is a very small chance that the tubes can rejoin. 

More drastic ways - continued

'Natural' methods - pregnancy may be avoided by finding out when the menstrual cycle in thewoman is most fertile and avoiding sexual intercourse on those days. It's popular with people who think that hormonal and barrier methods are unnatural, but it's not very effective. 

Abstinence - the only way to be completely sure that sperm and egg don't meet is to not have intercourse. 

More ways of reducing fertility 

The contraceptive patch contains oestrogen and progesterone (the same as the combined pill on card #1). It's a small (5cm x 5cm) patch that's stuck to the skin. Each patch lasts up to one week. 

The contraceptive implant is inserted under the skin of the arm. It releases a continuous amount of progesteron, which stops the ovaries from releasing eggs, makes it harder for the sperm to swim to the egg, and stops any fertilised egg implanting in the uterus. An implant can last for three years.

This is the menstrual cycle. It shows the build up of blood lining the uterus in 28 to 30 days and when the egg is released. It also shows the amount of oestrogen and progesterone over the time period. 

Maintaining a stable environment

The conditions inside your body need to be kept steady, even when the external environment changes. This is really important because because your cells need the right conditions in order to function properly, including the right conditions for enzyme reaction. 

Homeostasis is all about the regulation of the conditions inside your body (and cells) to maintain a stable internal environment, in response to changes in both external and internal conditions. 

You may have loads of automatic control systems in your body that regulate your internal environment - these include both nervous and hormonal communication systems. For example there are control systems that maintain your body temprature, blood glucose level and your water content. 

All your automatic control systems are made up of three main components which work together to maintain a steady condition - cells called receptors, coordination centres (including the brain, spinal cord and pancreas) and effectors