5.3 Hormonal coordination in humans Flashcards
Human endocrine system
The human endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream.
The blood carries the hormone around the body, and when it reaches a target cell/organ it produces an effect.
Compared to the nervous system the effects of hormones are slower but they act for longer.
Pituitary gland
The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions.
These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.
Pancreas
The pancreas produces insulin and glucagon, both of which are involved in regulating blood glucose levels.
Thyroid
This gland is in our neck and releases thyroxine.
Thyroxine regulates our metabolism, heart rate and temperature.
Adrenal gland
The two adrenal glands are situated above each kidney.
They produce adrenaline, which triggers the ‘fight or flight’ response when an organism is presented with a dangerous situation.
Ovaries
The ovaries produce oestrogen, which controls puberty and is one of the main hormones in the regulation of the menstrual cycle.
Testes
The testes produce testosterone, which controls puberty and sperm production.
Thyroxine
Thyroxine is the main hormone secreted (released) by the thyroid gland. It increases the basal (resting) metabolic rate.
Thyroxine is responsible for regulating the growth and brain development of young animals.
Production is regulated by a negative feedback loop. This allows the body to maintain a fairly constant concentration of thyroxine in the blood.
When the concentration of thyroid hormones, such as thyroxine, in the blood is low, the production of thyroid-stimulating hormone (TSH) is increased. This stimulates the secretion of thyroxine by the thyroid gland.
When the concentration of thyroid hormones in the blood is high, TSH production is decreased.
Hyperthyroidism, caused by an overactive thyroid gland secreting too much thyroxine into the bloodstream which causes an increase in BMR and protein synthesis.
Hypothyroidism caused by an underactive thyroid gland secreting too little thyroxine into the bloodstream which can lead to heart and nerve problems, and death.
Adrenaline
Adrenaline is a hormone secreted (discharged) by the adrenal glands. It has different functions:
Adrenaline increases heart rate and blood pressure, and dilates the pupils in our eyes.
Adrenaline enlarges the air passages of the lungs and alters metabolism in order to boost the delivery of oxygen and glucose to the brain and the muscles.
In times of fear or stress, adrenaline is part of the so-called ‘fight or flight’ response, preparing the body to move and think quickly in response to danger.
Control of blood glucose
Blood glucose concentration is monitored and controlled by the pancreas.
The pancreas is an endocrine gland (making and secreting hormones into the bloodstream) and it also plays a vital (but separate) role in digestion (making and secreting enzymes into the digestive system).
Blood glucose concentration must be kept within a narrow range, so it’s another example of homeostasis (like temperature control).
Eating foods containing carbohydrate results in an increase of glucose into the bloodstream.
If the blood glucose concentration is too high, the pancreas produces the hormone insulin to bring it back down.
Too high a level of glucose in the blood can lead to cells of the body losing water by osmosis, which can be dangerous.
Insulin stimulates cells to take in glucose from the bloodstream (particularly liver and muscle cells)
In liver and muscle cells excess glucose is converted into glycogen (a polymer of glucose) for storage.
Type 1 diabetes
Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin to control blood glucose levels.
Scientists think this is a result of a person’s own immune system destroying the cells of the pancreas that make insulin during development.
Type 1 diabetes is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.
Type 2 diabetes
In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas - the person still makes insulin but their cells are resistant to it and don’t respond as well as they should.
This can also lead to uncontrolled high blood glucose levels.
A carbohydrate-controlled diet and an exercise regime are common treatments for Type 2 diabetes.
Obesity is a big risk factor for Type 2 diabetes; probably because a person who is obese may consume a diet high in carbohydrates, and over-production of insulin results in resistance to it developing.
Negative feedback of blood glucose
If the blood glucose concentration is too low, the pancreas produces the hormone glucagon that causes glycogen to be converted into glucose and released into the blood.
Glucagon and insulin interact as part of a negative feedback cycle to control blood glucose (sugar) levels in the body.
Insulin
Insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored – this reduces the blood glucose level.
Glucagon
Glucagon is produced when blood glucose falls too low and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the bloodstream – this increases the blood glucose level.
Maintaining water levels
Maintaining water levels in the body is vital to prevent harmful changes occurring to cells of the body as a result of osmosis.
If body cells lose or gain too much water by osmosis they do not function efficiently:
Too much water in the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting).
Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death.
Water loss in the body
There are two sources of water in the body: water produced as a result of aerobic respiration and water in the diet.
The cytoplasm of all cells is largely composed of water, as is the blood plasma.
Water is lost from the body in the following ways:
Water leaves the body via the lungs during exhalation (breathing out).
Water, ions and urea are lost from the skin in sweat.
However, the lungs and skin have no control over how much water, ion or urea is lost via exhalation or sweating.
Controlled loss of excess water, ions and urea is controlled by the kidneys when they filter the blood to produce urine.
Deamination
The digestion of proteins from the diet results in excess amino acids which need to be excreted safely, as they cannot be stored by the body in the same way that excess glucose can.
Deamination is the process of breaking down excess protein and it predominantly occurs in the liver.
Enzymes in the liver split up amino acid molecules, with the part containing carbon turned into glycogen and the other part containing nitrogen (the amino part) turned into ammonia (this is why we say the amino acid has been deaminated).
Ammonia is toxic to cells and so it is immediately converted to urea which can be transported around the body via the blood safely for excretion by the kidneys.
Structure and function of the kidney
The kidneys regulate water and ion levels by filtering blood through branched capillary networks with 3-nanometer pores. Under high pressure, small molecules like glucose, urea, water, and ions pass into the filtrate while larger molecules remain in the bloodstream. The kidneys then actively reabsorb essential substances—recovering all glucose and some ions—back into the blood. The remaining filtrate, enriched with urea (a liver deamination byproduct), becomes urine. Reabsorption of water concentrates the urea, resulting in urine with urea levels far exceeding those in blood plasma. Thus, the kidneys effectively maintain body fluid balance.
Control of water levels
Water lost through the lungs or skin is uncontrolled, but the kidneys regulate water loss in urine. In kidney tubules, water is reabsorbed based on blood water content: if blood water is high, less is reabsorbed, and if low, more water is reabsorbed. The pituitary gland continuously releases ADH (antidiueretic hormone), which determines tubule permeability. Less ADH is released when blood water is abundant, reducing reabsorption, and more ADH is released when blood water is low, increasing reabsorption. This regulation by the tubules is a negative feedback mechanism that maintains the body’s water balance.
Kidney failure
The kidneys might not work properly for several reasons, including accidents or disease.
Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body which will be fatal if not removed.
Kidney dialysis
Dialysis is the usual treatment for someone with kidney failure.
Patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and salt balance of the blood.
Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then returned to a vein in the arm.
Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane – the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists.
Dialysis fluid contains:
A glucose concentration similar to a normal level in blood.
A concentration of salts similar to a normal level in blood.
No urea.
Kidney transplant
Kidney transplants are a better long term solution to kidney failure than dialysis; however, there are several disadvantages to kidney transplants, including:
Donors won’t have the same antigens on cell surfaces so there will be some immune response to the new kidney (risk of rejection is reduced - but not removed – by ‘tissue typing’ the donor and the recipient first).
This has to be suppressed by taking immunosuppressant drugs for the rest of their lives – these can have long term side effects and leave the patient vulnerable to infections
There are not enough donors to cope with demand
However, if a healthy, close matched kidney is available, then the benefits of a transplant over dialysis include:
The patient has much more freedom as they are not tied to having dialysis several times a week in one place.
Their diets can be much less restrictive than they are when on dialysis.
Use of dialysis machines is very expensive and so this cost is removed.
A kidney transplant is a long term solution whereas dialysis will only work for a limited time.
Puberty
During puberty, reproductive hormones cause secondary sex characteristics to develop.
The main male reproductive hormone is testosterone which is produced by the testes; testosterone stimulates sperm production.
The main female reproductive hormone is oestrogen which is produced by the ovaries.
Stages of the menstrual cycle
During the menstrual cycle, the lining of the uterus builds up and ovulation occurs.
The average menstrual cycle is 28 days long and there are four overall stages:
Menstruation – loss of lining from the uterus, occurs at the start of the cycle if no fertilisation has occurred
The lining starts to thicken.
Ovulation occurs around the middle of the cycle (about day 14), the egg travels down the oviduct towards the uterus.
The lining is maintained ready to accept a fertilized egg.
Hormonal control of menstruation cycle
Four hormones control the events that occur during the menstrual cycle.
Two of these hormones are produced by the pituitary gland in the brain:
Follicle-stimulating hormone (FSH) causes maturation of an egg in the ovary.
Luteinising hormone (LH) stimulates the release of the egg.
The other two hormones, oestrogen and progesterone are involved in maintaining the uterus lining with oestrogen being made by the ovaries and progesterone specifically by an empty egg follicle called the corpus luteum.
Interaction of the hormones during the menstrual cycle
The pituitary gland produces FSH which stimulates the development of a follicle in the ovary.
An egg matures inside the follicle and the follicle produces the hormone oestrogen – so FSH stimulates the production of oestrogen.
Oestrogen causes growth and repair of the lining of the uterus wall and inhibits the production of FSH.
When oestrogen rises to a high enough level it stimulates the release of LH from the pituitary gland which causes ovulation (usually around day 14 of the cycle).
The follicle becomes a corpus luteum and starts producing progesterone.
Progesterone maintains the uterus lining (the thickness of the uterus wall).
If the egg is not fertilised, the corpus luteum breaks down and progesterone levels drop
This causes menstruation – commonly known as having a period.
If fertilisation does occur the corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down (breakdown of the lining would prevent a pregnancy).
Once the placenta has developed, it starts secreting progesterone and continues to do so throughout the pregnancy to maintain the lining.
Changes in the levels of the pituitary hormones FSH and LH in the blood during the menstrual cycle
FSH (follicle-stimulating hormone) is released by the pituitary gland and causes an egg to start maturing in the ovary.
It also stimulates the ovaries to start releasing oestrogen
The pituitary gland is stimulated to release luteinising hormone (LH) when oestrogen levels have reached their peak.
LH causes ovulation to occur and also stimulates the ovary to produce progesterone.
Changes in the levels of oestrogen and progesterone in the blood during the menstrual cycle
Oestrogen levels rise from day 1 to peak just before day 14.
This causes the uterine wall to start thickening and the egg to mature.
The peak in oestrogen occurs just before the egg is released.
Progesterone stays low from day 1 - 14 and starts to rise after ovulation.
The increasing levels cause the uterine lining to thicken further; a fall in progesterone levels causes the uterine lining to break down (menstruation).
Control of fertility
Fertility can be controlled by a variety of hormonal and non-hormonal methods.
Contraceptive methods aim to prevent fertilisation and pregnancy, and include the use of hormones (oestrogen and progesterone) as well as non-hormonal methods (such as the use of barriers or surgery).
Hormones can also be used to increase the chance of pregnancy occurring when it previously might not have done.
Surgical methods of sterilisation
In a female, the oviducts which connect the ovaries to the uterus (also called the fallopian tubes) can be cut and tied
In a male, the sperm ducts (the tube connecting the testes to the penis) can also be cut and tied in a procedure called a vasectomy.
Both methods are highly effective but there have been a small number of cases where tubes have rejoined.
Barrier method of sterilisation
Barrier methods such as condoms and diaphragms prevent the sperm from reaching an egg.
Condoms are the only barrier method that can prevent the spread of sexually transmitted infections.
Spermicidal agents which kill or disable sperm – these are only 70 - 80% effective.
Progesterone only pill
Stimulates the production of thick, sticky mucus, which is very difficult for any sperm to penetrate.
Inhibits the release of FSH, so that eggs don’t mature.
Just as effective as the combined pill but with fewer side effects.
Combined pill
Contains both oestrogen and progesterone.
By taking it every day, enough oestrogen builds up in the female body to inhibit the production of FSH so that no eggs mature.
Infertility
Infertility occurs when a couple find it difficult or are unable to conceive naturally.
This can be a result of insufficient or too low levels of reproductive hormones affecting the development of egg and sperm cells, or as a result of issues with the reproductive system of the female.
Use of hormones to treat infertility
Artificial hormones are used as part of modern reproductive technologies to treat infertility, particularly when the female is not producing enough eggs, usually as a result of the pituitary gland not producing sufficient FSH to cause egg maturation.
The hormones FSH and LH are given as a ‘fertility drug’ to stimulate egg production.
An important social issue to consider with this is that several eggs can be released at once so this increases the chance of multiple births (twins or triplets etc).
It also doesn’t have a particularly high success rate and can be expensive.
IVF treatment
An alternative treatment is for eggs to be fertilised by sperm outside of the body (‘in vitro’ means ‘in glass’) – this is used particularly when there are issues with both male and female fertility.
The process involves:
Giving a mother FSH and LH to stimulate the maturation of several eggs.
The eggs are collected from the mother and fertilised by sperm from the father in the laboratory.
The fertilised eggs develop into embryos.
At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb)
The success rate of IVF is low (~30%) but there have been many improvements and advancements in medical technologies which are helping to increase the success rate.
These advancements include improvements in microscope techniques and micro-tools that enable single cells to be removed from an embryo for genetic testing to identify if the embryo is healthy or has genetic defaults the couple might want to consider.
Issues with fertility treatment
Fertility treatments can give a couple the chance to have a baby of their own, which is a big positive.
Potential issues to consider include:
As several embryos are implanted, the risk of multiple births is quite high (which increases the risk of miscarriage or stillbirths).
The success rate is not very high (although it is increasing); IVF treatment failures can be very emotionally upsetting and physically stressful for couples.
Some women use IVF to get pregnant at a later age than they would be able to conceive naturally.
Some people are against IVF as more embryos can be produced than are used; the issue of who owns these embryos and whether they are used in research before eventually being destroyed is contentious (as embryos are a potential life).
The use of genetic testing is controversial as there is potential it could be misused in choosing characteristics of offspring (this is not allowed).
