7.1.1 Endocrine glands and hormone transport

Endocrine glands release hormones

1. An endocrine gland is a gland that secretes hormones directly into the blood.
2. A hormone is a chemical messenger produced by an endocrine gland and carried in the blood to a target organ, where it causes a response.
3. A target organ is an organ containing cells that respond to a particular hormone.
4. Endocrine glands have no ducts, so their hormones enter nearby capillaries and join the circulation.

Hormones reach their target organs

1. The bloodstream carries a hormone away from the gland and around the body.
2. Hormones travel dissolved in the blood plasma, rather than inside red blood cells.

Target organs respond to hormones

1. Blood delivers each hormone to its target organ, where it produces its specific effect.
2. A hormone can circulate around the whole body, but only its target organs respond to it.

Nervous system vs Endocrine system

1. The nervous system is formed by the brain, spinal cord and nerves, and it coordinates rapid responses using electrical impulses.
2. Nervous control sends electrical impulses along neurones, whereas hormonal control sends chemical hormones through the blood.
3. Nervous responses are usually very fast because impulses travel directly along neurones to an effector.
4. Hormonal responses are usually slower because hormones must be secreted and carried through the circulation.
5. Nervous effects are usually short-lived because the impulses stop once the response is no longer required.
6. Hormonal effects can be long-lasting because hormones remain in the blood and may continue to affect target organs.
7. A nervous impulse travels to a specific effector, while a hormone circulates widely but affects only organs that respond to that hormone.
8. Nervous control is suited to immediate actions such as muscle contraction, while hormonal control is suited to longer processes such as growth, puberty and metabolic control.

Choose control by speed and duration

1. A nerve impulse is an electrical signal that travels along a neurone.
2. Touching a hot surface needs nervous control because the response must reach the arm muscles quickly and stop after the hand moves away.
3. Growth during puberty needs hormonal control because it develops slowly and continues over a long period.

Main glands and their hormones

1. The pituitary gland lies at the base of the brain and releases hormones that regulate several other endocrine glands.
2. The thyroid gland lies in the neck and secretes thyroxine.
3. The pancreas lies in the upper abdomen and secretes insulin and glucagon.
4. The adrenal glands sit above the kidneys and secrete adrenaline.
5. The ovaries lie in the pelvis and secrete oestrogen and progesterone.
6. The testes lie in the scrotum and secrete testosterone.

7.1.2 Adrenalin and fight or flight

Adrenaline starts fight or flight

1. The adrenal glands secrete adrenaline when the brain detects fear, stress or danger.
2. The fight or flight response is a rapid set of body changes that prepares a person to confront danger or escape from it.
3. Adrenaline travels in the blood and changes the activity of several target organs at the same time.

Four effects supply active muscles

1. Adrenaline increases heart rate, so the heart pumps blood to the muscles more quickly.
2. Adrenaline increases blood pressure, which helps blood reach the muscles rapidly.
3. Adrenaline increases blood flow to the muscles, so muscle cells receive more oxygen and glucose.
4. Adrenaline stimulates the liver to convert $\mathrm{glycogen}\to \mathrm{glucose}$, which raises the blood glucose concentration.
5. The extra oxygen and glucose increase the rate of aerobic respiration in muscle cells, releasing energy more quickly for contraction.

7.1.3 Thyroxine and negative feedback

Thyroxine sets metabolic rate

1. The thyroid gland is an endocrine gland in the neck that produces and secretes thyroxine.
2. Thyroxine enters the blood and is carried to cells throughout the body.
3. Thyroxine changes how quickly cells carry out chemical reactions, so it controls the body's metabolic rate.
4. The body needs thyroxine to remain near its normal concentration because reactions that run too slowly or too quickly disrupt normal cell function.

Metabolic rate changes energy use

1. Metabolic rate is the overall rate of the chemical reactions in the body.
2. A higher thyroxine concentration increases the rate of many cellular reactions, including reactions involved in respiration and energy transfer.
3. A lower thyroxine concentration slows these reactions, so the body releases and uses less energy in a given time.
4. The effect is not instant because thyroxine must be secreted, transported in the blood and act on its target cells.

The hypothalamus detects low thyroxine

1. The hypothalamus is a region of the brain that monitors internal conditions and controls hormone release from the pituitary gland.
2. When the blood thyroxine concentration falls below normal, the hypothalamus responds by producing and releasing more $\mathrm{TRH}$.
3. A low thyroxine concentration therefore begins a response that will increase thyroxine secretion and oppose the original fall.

TRH activates the pituitary

1. $\mathrm{TRH}$ is thyrotropin-releasing hormone produced by the hypothalamus.
2. $\mathrm{TRH}$ travels to the pituitary gland and stimulates it to produce and release $\mathrm{TSH}$.
3. More $\mathrm{TRH}$ therefore causes more $\mathrm{TSH}$ to enter the blood.

The pituitary releases TSH

1. The pituitary gland is an endocrine gland at the base of the brain that releases hormones which control other endocrine glands.
2. In this pathway, the pituitary is the source of $\mathrm{TSH}$ and the thyroid gland is its target.
3. The pituitary does not release thyroxine, because thyroxine is produced by the thyroid gland.

TSH stimulates the thyroid

1. $\mathrm{TSH}$ is thyroid-stimulating hormone released by the pituitary gland.
2. $\mathrm{TSH}$ travels in the blood to the thyroid gland and stimulates it to produce and secrete more thyroxine.
3. As the thyroid releases more thyroxine, the blood thyroxine concentration rises towards its normal level.
4. The higher thyroxine concentration raises metabolic rate towards its normal value.

Negative feedback reduces stimulation

1. Negative feedback is a control mechanism in which a change away from the normal level causes responses that reverse the change and restore the normal level.
2. As thyroxine returns to its normal concentration, thyroxine inhibits the hypothalamus from producing and releasing more $\mathrm{TRH}$.
3. Thyroxine also inhibits the pituitary gland from producing and releasing more $\mathrm{TSH}$.
4. Less $\mathrm{TSH}$ reaches the thyroid gland, so the thyroid receives less stimulation and secretes less thyroxine.
5. This response prevents the thyroxine concentration and metabolic rate from continuing to rise above the normal level.
6. If thyroxine later falls below normal, inhibition of the hypothalamus and pituitary weakens, so $\mathrm{TRH}$ and $\mathrm{TSH}$ secretion increase again.
7. If thyroxine rises above normal, stronger inhibition reduces $\mathrm{TRH}$ and $\mathrm{TSH}$ secretion until thyroxine falls towards normal.

7.2.1 Stages of the menstrual cycle

The cycle prepares the uterus for pregnancy

1. The menstrual cycle is a repeating sequence of changes in the ovaries and uterus that releases an egg and prepares the uterus lining for possible implantation.
2. Day $1$ is the first day of menstruation, when the uterus lining breaks down and leaves the body through the vagina.
3. A cycle is often modelled as about $28$ days, but the length varies between individuals and between cycles.
4. During the first half of the cycle, an egg matures in a follicle in an ovary while the uterus lining repairs and becomes thicker.
5. Around the middle of the cycle, a mature egg is released from an ovary.
6. During the second half, the uterus lining is maintained so it can support an embryo if fertilisation and implantation occur.

Ovulation separates the two halves

1. Ovulation is the release of a mature egg from an ovary.
2. In a model $28$-day cycle, ovulation occurs at about day $14$, after the egg has matured inside a follicle.
3. The released egg enters an oviduct, where fertilisation may occur if a sperm cell reaches it.
4. If fertilisation does not occur, the egg breaks down and the hormone changes later in the cycle lead to menstruation.

Oestrogen rebuilds the uterus lining

1. Oestrogen is a hormone made mainly by the ovaries that repairs and thickens the uterus lining after menstruation.
2. Its concentration rises during the first half of the cycle as the follicle develops.
3. A thicker, well-supplied lining creates suitable conditions for an embryo to implant if the egg is fertilised.

Progesterone maintains the uterus lining

1. Progesterone is a hormone made by the ovary after ovulation that maintains the thick uterus lining.
2. If pregnancy does not occur, progesterone and oestrogen concentrations fall near the end of the cycle.
3. The fall in these hormones means the lining is no longer maintained, so it breaks down and a new menstruation begins.
4. If pregnancy begins, progesterone remains high and helps keep the uterus lining in place.

7.2.2 Hormonal control of the menstrual cycle

FSH matures an egg

1. FSH is released by the pituitary gland and travels in the blood to the ovaries.
2. FSH stimulates a follicle in an ovary to develop, so the egg inside it matures.
3. FSH also stimulates the developing follicle to release oestrogen.

Oestrogen rebuilds the lining

1. Oestrogen released by the developing follicle repairs and thickens the uterus lining after menstruation.
2. As oestrogen rises, it inhibits further FSH release so several follicles do not usually continue maturing.
3. A high oestrogen concentration also stimulates the pituitary gland to release a surge of LH.

LH triggers ovulation

1. LH is released by the pituitary gland and travels to the ovaries in the blood.
2. The sharp LH surge causes the mature follicle to release its egg, which is ovulation.
3. After ovulation, LH supports the change of the empty follicle into a structure that releases progesterone.

Progesterone maintains the lining

1. Progesterone is released by the ovary after ovulation and keeps the uterus lining thick.
2. Progesterone inhibits FSH and LH, preventing another follicle from maturing and another ovulation during the same cycle.
3. If the egg is not fertilised, the progesterone-producing structure breaks down and progesterone falls.

Falling hormones start menstruation

1. When progesterone and oestrogen fall, the uterus lining is no longer maintained and it breaks down.
2. The fall removes inhibition of FSH, allowing FSH to rise and begin the next cycle.
3. On a graph, the LH peak marks ovulation, the progesterone peak follows ovulation, and the fall in progesterone matches the start of menstruation.

7.2.3 Hormonal contraception

Hormonal contraception prevents ovulation

1. Contraception is the deliberate prevention of pregnancy.
2. The combined pill contains artificial versions of oestrogen and progesterone.
3. These hormones maintain feedback inhibition of the pituitary gland, so less FSH and LH are released.
4. Without enough FSH, a follicle and its egg do not mature fully.
5. Without an LH surge, ovulation does not occur, so there is no egg available for fertilisation.

Methods deliver hormones in different ways

1. Hormonal contraception can be delivered as a daily pill, skin patch, injection, implant or hormone-releasing intrauterine system.
2. Long-acting methods release hormone over weeks, months or years, so their effect does not depend on remembering a tablet every day.
3. A progesterone-only method mainly uses an artificial version of progesterone, while the combined pill supplies artificial oestrogen and progesterone.
4. The exact pattern of bleeding can change because these methods alter the normal menstrual cycle.

Other effects also prevent pregnancy

1. Artificial progesterone can thicken the mucus at the cervix, making it harder for sperm to enter the uterus.
2. Some hormonal methods also keep the uterus lining thin, reducing the chance that an embryo implants.
3. These effects support the main mechanism of preventing ovulation.

Hormonal methods do not prevent infections

1. Hormonal contraception reduces the chance of pregnancy but does not form a physical barrier between partners.
2. It therefore does not protect against sexually transmitted infections.
3. A condom may be used as well when protection from sexually transmitted infections is needed.

7.2.4 Evaluating contraception methods

Evaluation needs a supported judgement

1. Effectiveness is the proportion of users for whom a method prevents pregnancy over the stated time and under the stated conditions.
2. A comparison should also consider side effects, ease of correct use, reversibility, medical suitability and protection from sexually transmitted infections.
3. The best method depends on the needs and circumstances in the question, so a conclusion must be supported by relevant evidence.
4. When data are given, compare methods over the same time period and check whether the figures describe perfect use or typical use.

Hormonal methods have trade-offs

1. Hormonal methods can be highly effective because they prevent ovulation and may also thicken cervical mucus.
2. Implants and injections are less affected by forgetting than a daily pill because they release hormone for a longer period.
3. Possible disadvantages include headaches, nausea, mood changes, irregular bleeding and, for some combined methods, a small increased risk of blood clots.
4. Hormonal methods do not protect against sexually transmitted infections and may be unsuitable for some people after medical assessment.
5. Most hormonal methods are reversible, but normal fertility may take time to return after some injections.

Barrier methods block sperm

1. Barrier methods, including condoms and diaphragms, physically prevent sperm from reaching an egg.
2. They do not alter hormone concentrations, so they avoid hormone-related side effects.
3. Condoms reduce the transmission of sexually transmitted infections as well as reducing the chance of pregnancy.
4. Their effectiveness depends on correct use every time, and a condom can tear, slip or be put on incorrectly.
5. A diaphragm must be positioned correctly over the cervix and is usually used with spermicide.

Data must match the person

1. A person who cannot use oestrogen may need a progesterone-only or barrier method even if another method has a higher quoted effectiveness.
2. A person who wants long-term contraception may prefer an implant because it does not require action at every episode of intercourse.
3. A person who needs protection from sexually transmitted infections should use condoms because hormonal methods do not provide that protection.
4. A strong evaluation states the choice, supports it with data, acknowledges a relevant drawback and explains why the benefit outweighs that drawback.

7.2.5 Hormones in Assisted Reproductive Technology

ART uses treatment to support pregnancy

1. Assisted Reproductive Technology uses fertility drugs or procedures involving eggs, sperm or embryos to increase the chance of pregnancy.
2. The choice of treatment depends on the cause of infertility, such as failure to ovulate, blocked oviducts or difficulty with fertilisation.
3. Hormones used in treatment act on the same ovaries and follicles as the hormones of the natural menstrual cycle.

Clomifene increases FSH and LH

1. Clomifene is taken as a fertility drug when ovulation is absent or irregular.
2. It stimulates the pituitary gland to release more FSH and LH.
3. The higher FSH concentration increases follicle development and egg maturation.
4. The higher LH concentration increases the chance that a mature egg is released by ovulation.
5. If intercourse occurs around ovulation, the egg may then be fertilised inside the body.

IVF fertilises eggs outside the body

1. First, FSH and LH are given so several follicles develop and several eggs mature.
2. The mature eggs are collected from the ovaries, and a sperm sample is collected.
3. Eggs and sperm are mixed in a laboratory dish, or one sperm is injected into an egg when needed.
4. Fertilised eggs divide by mitosis and develop into embryos.
5. One or more selected embryos are transferred into the uterus.
6. A pregnancy begins only if an embryo implants successfully in the uterus lining.

Clomifene and IVF solve different problems

1. Clomifene mainly treats infertility caused by failure to ovulate because it increases FSH and LH release.
2. IVF can help when fertilisation inside the oviduct is unlikely, including when an oviduct is blocked.
3. IVF also allows several eggs to be collected, increasing the chance that at least one embryo develops for transfer.

Embryo transfer does not guarantee pregnancy

1. An embryo must implant in the uterus lining and continue developing for IVF to result in pregnancy.
2. IVF may need more than one attempt because fertilisation, embryo development, transfer or implantation can fail.
3. Transferring more than one embryo can increase the chance of a multiple pregnancy, which carries greater risks for the pregnant person and babies.