Glands A gland is an organ which produces and releases substances that perform a specific function in the body. What is a gland? A gland is an organ which produces and releases substances that perform a specific function in the body. There are two types of gland. Endocrine glands are ductless glands and release the substances that they make (hormones) directly into the bloodstream. These glands form part of the endocrine system and information on them is included in this website. There is another type of gland called an exocrine gland (e.g. sweat glands, lymph nodes). These are not considered part of the endocrine system as they do not produce hormones and they release their product through a duct. Information on these glands is not included on this website.
Endocrine glands, such as the pancreas and thyroid gland, use the bloodstream to monitor the bodyâ€™s internal environment and to communicate with each other through substances called hormones, which are released into the bloodstream.
The adrenal glands are small structures attached to the top of each kidney. The human body has two
adrenal glands that release chemicals called hormones into the bloodstream. These hormones affect many parts of the human body. Adipose tissue Adipose tissue (body fat) is crucial for health. Along with fat cells, adipose tissue contains numerous nerve cells and blood vessels, storing and releasing energy to fuel the body and releasing important hormones vital to the body's needs.
Alternative names for adipose tissue Fat; body fat
Where is my adipose tissue? Adipose tissue is commonly known as body fat. It is found all over the body. It can be found under the skin (subcutaneous fat), packed around internal organs (visceral fat), between muscles, within bone marrow and in breast tissue. Men tend to store more visceral fat (fat around their internal organs), leading to obesity around the middle of their abdomen. However, women tend to store more subcutaneous fat within the buttocks and thighs. These differences are due to the sex hormones produced by males and females.
What does adipose tissue do? Adipose tissue is now known to be a very important and active endocrine organ. It is well established that adipocytes (or fat cells) play a vital role in the storage and release of energy throughout the human body. More recently, the endocrine function of adipose tissue has been discovered. In addition to adipocytes, adipose tissue contains numerous other cells that are able to produce certain hormones in response to signals from the rest of the organs throughout the body. Through the actions of these hormones, adipose tissue plays an important role in the regulation of glucose, cholesterol and the metabolism of sex hormones.
What hormones does adipose tissue produce? A number of different hormones are released from adipose tissue and these are responsible for different functions within the body. Examples of these are:
aromatase, which is involved in sex hormone metabolism
TNF alpha, IL-6 and leptin, which are collectively termed â€˜cytokinesâ€™ and are involved in sending messages between cells
plasminogen activator inhibitor-1, which is involved in the clotting of blood
angiotensin, which is involved in blood pressure control
adiponectin, which improves the bodyâ€™s sensitivity to insulin and so helps to protect against developing type 2 diabetes
lipoprotein lipase and apolipoprotein E, which are involved in storage and metabolism of fat to release energy. What could go wrong with adipose tissue? Both too much and too little adipose tissue can have severe health implications. More commonly, too much adipose tissue leads to obesity, mainly from too much visceral fat. Obesity leads to a number of serious health problems. Obesity increases the risk of developing type 2 diabetes as it causes the body to become resistant to insulin. This resistance results in high levels of blood sugar, which is bad for health. Obesity also increases the chance of developing high blood pressure, high cholesterol levels and an
increased tendency for blood to clot. All of these raise the risk of heart attacks and stroke.
A lack of adipose tissue (lipodystrophy) can also cause similar problems and is seen with increasing frequency as a result of medication used to treat HIV/AIDS.
In eating disorders (such as anorexia nervosa), the patient does not eat enough food to maintain their adipose tissues levels. This means that they can lose a dangerous amount of body weight. Adrenal glands The adrenal glands are small structures attached to the top of each kidney. The human body has two adrenal glands that release chemicals called hormones into the bloodstream. These hormones affect many parts of the human body.
Alternative names Suprarenal glands
Where are my adrenal glands?
The human body has two adrenal glands and one sits on top of each kidney. Each adrenal gland weighs 4â€“5 g in an adult. Adrenals are first detected at 6 weeks' gestation.
What do my adrenal glands do? Each adrenal gland is composed of two distinct parts: the outer part called the adrenal cortex and the inner adrenal medulla. The adrenal glands secrete different hormones which act as 'chemical messengers'. These hormones travel in the bloodstream and act on various body tissues to enable them to function correctly. All adrenocortical hormones are steroid compounds made from cholesterol.
What hormones do my adrenal glands produce? The adrenal cortex produces three hormones:
Mineralocorticoids: the most important of which is aldosterone. This hormone helps to maintain the bodyâ€™s salt and water levels which, in turn, regulates blood pressure. Without aldosterone, the kidney loses excessive amounts of salt (sodium) and, consequently, water, leading to severe dehydration and low blood pressure.
Glucocorticoids: predominantly cortisol. This hormone is involved in the response to illness and also helps to regulate body metabolism. Cortisol stimulates glucose production helping the body to free up the necessary ingredients from storage (fat and muscle) to make glucose. Cortisol also has significant anti-inflammatory effects.
Adrenal androgens: male sex hormones mainly dehydroepiandrosterone (DHEA) and testosterone. All have weak effects, but play a role in early development of the male sex organs in childhood, and female body hair during puberty. Adrenocorticotropic hormone (ACTH), secreted by the anterior pituitary gland, primarily affects release of glucocorticoids and adrenal androgens by the adrenal gland and, to a much lesser extent, also stimulates aldosterone release.
the stress response, the so called 'fight or flight' response.
What could go wrong with my adrenal glands? Commonly, overproduction of aldosterone can occur, which causes a condition known as primary hyperaldosteronism. This causes high blood pressure, which is resistant to conventional blood pressure control tablets, and salt disturbances. High blood pressure may cause headaches and visual problems. Some studies have suggested that hyperaldosteronism may account for up to 5% of all people with high blood pressure and an even higher proportion of those who have treatment-resistant hypertension.
In rare cases, the adrenal glands can become either overactive or underactive. The two main glucocorticoid-related disorders resulting from these are Cushing's syndrome and Addison's disease, respectively.
The adrenal medulla produces catecholamines:
Catecholamines include adrenaline, noradrenaline and small amounts of dopamine â€“ these hormones are responsible for all the physiological characteristics of
Cushing's syndrome is due to overactive adrenal glands from excessive production of cortisol. The clinical findings include thinning and bruising of the skin, obesity, diabetes, psychiatric disturbances, high blood pressure, muscle weakness, osteoporosis, excessive facial hair and irregular periods in women. It
can result in growth failure in children. Patients with cortisol excess also have impaired wound healing and an increased susceptibility to infection.
Adrenal cancer is very rare. Adrenal tumours may require surgery if they are large or overproduce hormones.
Addison's disease or adrenal insufficiency is due to underactive adrenal glands associated with lack of hormones. Adrenal insufficiency may be acute or chronic. Symptoms of chronic adrenal insufficiency include low blood pressure, fatigue, weight loss, anorexia, nausea, vomiting, abdominal pain, salt craving and low blood sugar. Skin and mucous membranes may show increased pigmentation. The loss of secondary sex characteristics is seen only in women with the disease. Acute adrenal insufficiency is a medical emergency and must be identified and promptly treated. The hallmarks of acute adrenal insufficiency are circulatory collapse with abdominal pain and low blood sugar.
The treatment of each disorder varies according to the specific cause. Patients with any concerns about these conditions should seek advice from their doctor
Hypothalamus The hypothalamus is a part of the brain that has a vital role in controlling many bodily functions including the release of hormones from the pituitary gland.
Where is my hypothalamus? Overproduction of androgens is also very rare but may result in excessive hair growth and menstrual period disturbances.
Tumours of the adrenal gland are mostly benign and do not result in over or underproduction of adrenal hormones. Most tumours are discovered incidentally when people undergo scans for various other reasons.
Computer artwork of a person's head showing the left side of the brain with the hypothalamus highlighted. Computer artwork of a person's head showing the left side of the brain with the hypothalamus highlighted.
The hypothalamus is located on the undersurface of the brain. It lies just below the thalamus and above the pituitary gland, to which it is attached by a stalk. It is
an extremely complex part of the brain containing many regions with highly specialised functions. In humans, the hypothalamus is approximately the size of a pea and accounts for less than 1% of the weight of the brain.
What does my hypothalamus do? One of the major functions of the hypothalamus is to maintain homeostasis, i.e. to keep the human body in a stable, constant condition.
The hypothalamus responds to a variety of signals from the internal and external environment including body temperature, hunger, feelings of being full up after eating, blood pressure and levels of hormones in the circulation. It also responds to stress and controls our daily bodily rhythms such as the night-time secretion of melatonin from the pineal gland and the changes in cortisol (the stress hormone) and body temperature over a 24-hour period. The hypothalamus collects and combines this information and puts changes in place to correct any imbalances.
they produce down through the pituitary stalk to the posterior lobe of the pituitary gland where these hormones are released directly into the bloodstream. These hormones are anti-diuretic hormone and oxytocin. Anti-diuretic hormone causes water reabsorption at the kidneys and oxytocin stimulates contraction of the uterus in childbirth and is important in breastfeeding.
The other set of nerve cells produces stimulating and inhibiting hormones that reach the anterior lobe of the pituitary gland via a network of blood vessels that run down through the pituitary stalk. These regulate the production of hormones that control the gonads, thyroid gland and adrenal cortex, as well as the production of growth hormone, which regulates growth, and prolactin, which is essential for milk production. The hormones produced in the hypothalamus are corticotrophin-releasing hormone, dopamine, growth hormone-releasing hormone, somatostatin, gonadotrophin-releasing hormone and Thyrotrophin-releasing hormone.
What could go wrong with my hypothalamus? What hormones does my hypothalamus produce? There are two sets of nerve cells in the hypothalamus that produce hormones. One set sends the hormones
Hypothalamic function can be affected by head trauma, brain tumours, infection, surgery, radiation and significant weight loss. It can lead to disorders of energy balance and thermoregulation, disorganised
body rhythms, (insomnia) and symptoms of pituitary deficiency due to loss of hypothalamic control. Pituitary deficiency (hypopituitarism) ultimately causes a deficiency of hormones produced by the gonads, adrenal cortex and thyroid gland, as well as loss of growth hormone.
Lack of anti-diuretic hormone production by the hypothalamus causes diabetes insipidus. In this condition the kidneys are unable to reabsorb water, which leads to excessive production of dilute urine and very large amounts of drinking.
Kidneys The kidneys are specialised organs that ensure that unwanted substances and excess water are removed from the bloodstream.
Where are my kidneys? The kidneys are small bean-shaped organs approximately 6 cm wide and 12 cm long and consist of two main layers â€“ an inner layer called the medulla and an outer layer called the cortex. Most people have two kidneys that are situated at the back of the abdomen on either side of the spine.
Graphic showing a section through the right kidney with the main structures labelled. Graphic showing a section through the right kidney with the main structures labelled.
What do the kidneys do? The kidneys ensure that the make-up and volume of the fluids in the body is correct. They help control the chemical balance of the blood and regulate the body's level of sodium, potassium and calcium. The kidneys remove waste products and excess water from the body and so help to regulate blood pressure. They activate vitamin d, which helps to maintain strong bones, and produce erythropoietin, a hormone that is vital for the production of red blood cells.
Each kidney contains 1.0â€“1.5 million small tubes called nephrons. The kidneys filter blood through a network of small blood vessels called the glomerulus. This produces a solution that then flows through the nephrons. As this fluid passes through the nephron, substances that the body wants to retain (such as sodium, potassium, proteins and most of the water) are re-absorbed back into the blood. The substances that need to be removed from the body, such as waste products including the remains of drugs and alcohol,
are retained in the fluid and removed from the body in the form of urine. The kidneys filter around 200 litres of blood a day and produce between one to two litres of urine.
What hormones do the kidneys produce?
one way in which the production of renin is stimulated. Renin is an enzyme, also produced by the kidneys, that plays an important role in the reninâ€“angiotensinâ€“ aldosterone hormonal system, which helps to control blood pressure. In addition to making hormones, the kidneys also respond to a number of hormones including vitamin D, aldosterone, prostaglandins, cortisol, parathyroid hormone and calcitonin.
The kidneys make two main hormones, vitamin D and erythropoietin. What could go wrong with the kidneys? Vitamin D is essential for a number of different functions in the body. Most of the vitamin D that is in the blood is inactive and it is modified by the kidney and other tissues to activate it. Active vitamin D stimulates the uptake of calcium from food, is important for the maintenance of healthy bones and also helps to regulate the response of the immune system to infection.
Erythropoietin is produced when oxygen levels in the blood are low. It acts in bone marrow to stimulate the production of mature red blood cells, to maintain healthy oxygen levels in our tissues.
The kidneys also produce prostaglandins, hormone-like substances, made from lipid (fat). The substances are
When the kidneys are not working correctly, waste products and excess fluid can build up and the levels of sodium, potassium, phosphate and calcium are not regulated correctly. When these substances gather together, this causes the symptoms of kidney disease, which can include high blood pressure, excessive tiredness, fluid retention and possibly lower back pain.
Kidney damage can occur for a number of reasons â€“ diabetes, high blood pressure, infections and a group of diseases that affect the glomerulus. The kidneys also need an adequate supply of blood, so if there is something wrong with the blood vessels to the kidney, such as a narrowing, this will prevent the kidneys from working efficiently. Ovaries
The ovaries produce and release eggs (oocytes) into the female reproductive tract at the mid-point of each menstrual cycle. They also produce the female hormones oestrogen and progesterone.
Alternative names Ovary (singular)
The ovaries have two main reproductive functions in the body. They produce oocytes (eggs) for fertilisation and they produce the reproductive hormones, oestrogen and progesterone. The function of the ovaries is controlled by gonadotrophin-releasing hormone released from nerve cells in the hypothalamus which send their messages to the pituitary gland to produce luteinising hormone and follicle stimulating hormone. These are carried in the bloodstream to control the menstrual cycle.
Where are the ovaries? Artwork of the female reproductive system, showing the location of the ovaries. Artwork of the female reproductive system, showing the location of the ovaries.
The ovaries form part of the female reproductive system. Each woman has two ovaries. They are oval in shape, about four centimetres long and lie on either side of the womb (uterus) against the wall of the pelvis in a region known as the ovarian fossa. They are held in place by ligaments attached to the womb but are not directly attached to the rest of the female reproductive tract, e.g. the fallopian tubes.
What do the ovaries do?
The ovaries release an egg (oocyte) at the midway point of each menstrual cycle. Usually, only a single oocyte from one ovary is released during each menstrual cycle, with each ovary taking an alternate turn in releasing an egg. A female baby is born with all the eggs that she will ever have.This is estimated to be around two million, but by the time a girl reaches puberty, this number has decreased to about 400,000 eggs stored in her ovaries. From puberty to the menopause, only about 400â€“500 eggs will reach maturity, be released from the ovary (in a process called ovulation) and be capable of being fertilised in the fallopian tubes/uterine tube/oviduct of the female reproductive tract.
The ovarian phases of a 28-day menstrual cycle. Ovulation occurs mid-cycle.
The ovarian phases of a 28-day menstrual cycle. Ovulation occurs mid-cycle.
menstruation. After menstruation, another cycle begins.
In the ovary, all eggs are initially enclosed in a single layer of cells known as a follicle, which supports the egg. Over time, these eggs begin to mature so that one is released from the ovary in each menstrual cycle. As the eggs mature, the cells in the follicle rapidly divide and the follicle becomes progressively larger. Many follicles lose the ability to function during this process, which can take several months, but one dominates in each menstrual cycle and the egg it contains is released at ovulation.
The menopause refers to the ending of a woman's reproductive years following her last menstruation. This is caused by the loss of all the remaining follicles in the ovary that contain eggs. When there are no more follicles or eggs, the ovary no longer secretes the hormones oestrogen and progesterone, which regulate the menstrual cycle. As a result, menstruation ceases.
As the follicles develop, they produce the hormone oestrogen. Once the egg has been released at ovulation, the empty follicle that is left in the ovary is called the corpus luteum. This then releases the hormones progesterone (in a higher amount) and oestrogen (in a lower amount). These hormones prepare the lining of the uterus for potential pregnancy (in the event of the released egg being fertilised). If the released egg is not fertilised and pregnancy does not occur during a menstrual cycle, the corpus luteum breaks down and the secretion of oestrogen and progesterone stops. Because these hormones are no longer present, the lining of the womb starts to fall away and is removed from the body through
What hormones do the ovaries produce? The major hormones secreted by the ovaries are oestrogen and progesterone, both important hormones in the menstrual cycle. Oestrogen production dominates in the first half of the menstrual cycle before ovulation, and progesterone production dominates during the second half of the menstrual cycle when the corpus luteum has formed. Both hormones are important in preparing the lining of the womb for pregnancy and the implantation of a fertilised egg, or embryo.
If conception occurs during any one menstrual cycle, the corpus luteum does not lose its ability to function and continues to secrete oestrogen and progesterone, allowing the embryo to implant in the lining of the
womb and form a placenta. At this point, development of the foetus begins.
What could go wrong with the ovaries? Any medical conditions that stop the ovaries functioning properly can decrease a woman's fertility.
The ovaries naturally stop functioning at the time of menopause. This occurs in most women around the age of 50. If this happens earlier, before the age of 40, it is called premature ovarian failure or premature ovarian insufficiency.
If a woman stops having menstrual periods during her reproductive years, this condition is called amenorrhoea. It can be caused by a number of factors. These include hypothalamic amenorrhea, which may be caused by having a lean/athletic build, high levels of regular exercise, and psychological stress. In these cases, fertility can be restored by reducing the intensity of exercise, weight gain and psychological interventions such as cognitive behavioural therapy. Disorders of the pituitary gland can affect normal ovarian function because a lack of the hormones normally released from the pituitary gland will reduce the stimulation of hormone production and the development of follicles in the ovaries. Thyroid overactivity (thyrotoxicosis) can lead to amenorrhoea, as can any severe illness. Pancreas
The most common disorder of the ovaries is polycystic ovary syndrome, which affects 5â€“10% of women of a reproductive age. In a polycystic ovary, the follicles mature to a certain stage, but then stop growing and fail to release an egg. These follicles appear as cysts in the ovaries on an ultrasound scan. Any abnormality that causes a loss of normal development of the ovaries, such as Turner syndrome, can result in the ovaries not functioning correctly and the loss of a woman's fertility. The ovaries can be damaged by treatments for other conditions, particularly chemotherapy or radiotherapy for cancer treatment.
The pancreas is an organ that serves two vital purposes: to aid food digestion and to produce hormones that mainly serve to control levels of energy in the blood.
Where is the pancreas? The pancreas is a large gland that lies alongside the stomach and the small bowel. It is about six inches (approximately 15 cm) long and is divided into the head, body and tail.
Computer artwork showing the location of the pancreas in the body.
hormones, particularly insulin. Insulin then helps the body to lower blood glucose levels and 'store' the sugar away in fat, muscle, liver and other body tissues where it can be used for energy when required.
click to enlarge
Computer artwork showing the location of the pancreas in the body.
What does the pancreas do? The pancreas carries out two important roles:
It makes digestive juices, which consist of powerful enzymes. These are released into the small bowel after meals to break down and digest food.
It makes hormones that control blood glucose levels. The pancreas produces hormones in its 'endocrine' cells. These cells are gathered in clusters known as islets of langerhans and monitor what is happening in the blood. They then can release hormones directly into the blood when necessary. In particular, they sense when sugar (glucose) levels in the blood rise, and as soon as this happens the cells produce
The pancreas is very close to the stomach. As soon as food is eaten, the pancreas releases digestive enzymes into the bowel to break food down. As the food is digested, and nutrient levels in the blood rise, the pancreas produces insulin to help the body store the glucose (energy) away. Between meals, the pancreas does not produce insulin and this allows the body to gradually release stores of energy back into the blood as they are needed.
Glucose levels remain very stable in the blood at all times to ensure that the body has a steady supply of energy. This energy is needed for metabolism, exercise and, in particular, to fuel the parts of the brain that 'run' on glucose. This makes sure that the body doesn't starve between meals.
What hormones does the pancreas produce? The most important hormone that the pancreas produces is insulin. Insulin is released by the 'beta cells' in the islets of Langerhans in response to food. Its role is to lower glucose levels in the bloodstream
and promote the storage of glucose in fat, muscle, liver and other body tissues.
'Alpha cells' in the islets of Langerhans produce another important hormone, glucagon. This has the opposite effect to insulin, by helping release energy into the bloodstream from where it is stored, thus raising blood sugar levels. Therefore, glucagon and insulin work in tandem to control the balance of glucose in the bloodstream.
Other hormones produced by the pancreas include pancreatic polypeptide and somatostatin. They are believed to play a part in regulating and fine-tuning the insulin and glucagon-producing cells.
What could go wrong with the pancreas? When the cells that make insulin either stop working altogether, or become inefficient and do not make enough insulin, this causes diabetes mellitus. Type 1 diabetes mellitus is caused when the body's immune system attacks its own cells in the islets of Langerhans, meaning that these cells cannot produce insulin. Type 2 diabetes mellitus is a metabolic disorder where the body is no longer able to produce or respond to insulin.
Some women also get diabetes temporarily when they are pregnant. This is called gestational diabetes. There are other rarer forms of diabetes, some of which are inherited. In addition, people will get diabetes if their pancreas is taken away surgically or damaged (for instance by severe pancreatitis).
Very rarely, patients develop growths (tumours) of the cells that make up the islets of Langerhans. These may be benign tumours, where a particular kind of cell multiplies and makes large quantities of its hormone whether it is needed or not. For example, if the tumour is made of insulin-producing cells, it is called an insulinoma. This is where too much insulin is produced when it is not required. This also happens with glucagon-producing cells, or a glucagonoma, which produces too much glucagon. These and other hormone-producing tumours in the pancreas are very rare, but endocrinology specialists have important parts to play in diagnosing patients with these tumours and contributing to their management and treatment.
The digestive cells of the pancreas can be involved in the condition known as pancreatitis. This is a very painful and serious condition caused by digestive enzymes 'leaking' into the pancreas itself and damaging the delicate tissues in and around it.
It is also possible for a tumour to develop in the part of the pancreas that produces the digestive juices that are released into the bowel. This condition is call ed pancreatic cancer.
Computer artwork of a sectioned human brain in side view, showing the pineal gland (highlighted). Front of the brain is at left. The pineal gland is situated deep within the brain, just below the back of the corpus callosum. Computer artwork of a sectioned human brain in side view, showing the pineal gland (highlighted). Front of the brain is at left. The pineal gland is situated deep within the brain, just below the back of the corpus callosum.
Where is the pineal gland?
Pineal gland The pineal gland is situated in the middle of the human brain and is the major site of the body's melatonin production.
Alternative names for the pineal gland Pineal body; epiphysis cerebri; epiphysis
The pineal gland is located deep in the brain in an area called the epithalamus, where the two halves of the brain join. In humans, this is situated in the middle of the brain; it sits in a groove just above the thalamus, which is an area that co-ordinates a variety of functions related to our senses. The pineal gland contains high levels of calcium and can be used by radiographers to mark the middle of the brain in X-ray images.
What does the pineal gland do? The pineal gland is best known for the secretion of the hormone melatonin, which is released into the blood
and possibly also into the brain fluid, known as cerebrospinal fluid. The body's daily (circadian) clock controls the production of pineal melatonin, so melatonin is commonly used in human research to understand the body's biological time. There is a rhythm to the biology of the pineal gland. It varies with changes in day length and this is why the pineal gland is sometimes referred to as both an endocrine clock and an endocrine calendar.
Melatonin secreted by the pineal gland is an important part of the body's circadian timing system and can synchronise daily rhythms (see the articles on jet lag and circadian rhythm sleep disorders). There is considerable research that shows that without the pineal gland and its secretion of melatonin, animals are unable to adapt physiologically to seasonal changes.
not harmful. However, on rare occasions, tumours of the pineal gland are found. There are some extremely rare reports of precocious puberty (early puberty) in individuals with pineal gland cysts or tumours. It is not clear whether these changes in puberty are caused by melatonin or some other hormone, such as human chorionic gonadotrophin, which is reported to be released by some pineal tumours. Otherwise, there are no known diseases associated with over or underactivity of the pineal Placenta The placenta is a temporary endocrine organ formed during pregnancy, which produces hormones important in the maintenance of a healthy pregnancy and in preparation for labour and breastfeeding.
Where is the placenta?
What hormones does the pineal gland produce? The major hormone produced by the pineal gland is melatonin.
What could go wrong with the pineal gland? It is not unusual to see pineal cysts on magnetic resonance imaging (MRI) scans. These are benign and
Blood vessels in the placenta bring oxygen and nutrients to the <a href='/glossary/f#fetus'>fetus</a> and remove waste products. The umbilical vein (red) brings oxygenated blood to the fetus from the placenta while the paired umbilical arteries (red) take deoxygenated fetal blood back towards the placenta.
Blood vessels in the placenta bring oxygen and nutrients to the fetus and remove waste products. The umbilical vein (red) brings oxygenated blood to the fetus from the placenta while the paired umbilical arteries (red) take deoxygenated fetal blood back towards the placenta.
The placenta connects the developing fetus to the wall of the motherâ€™s uterus during pregnancy. It grows in the wall of the uterus and is attached to the fetus within the uterine cavity by the umbilical cord. The placenta is formed by cells that originate from the fetus and is therefore the first of the fetal organs to develop.
What does the placenta do?
The placenta contains a complex network of blood vessels that allow the exchange of nutrients and gases between the mother and the developing fetus. The blood supply of the mother does not actually mix with that of the fetus; this exchange occurs by diffusion of gases and transport of nutrients between the two blood supplies (see figure). The transfer of nutrients and oxygen from the mother to the fetus, and waste products and carbon dioxide back from the fetus to the mother, allows the growth and development of the fetus throughout pregnancy. Antibodies can also pass from the mother to the fetus, providing protection from certain diseases. This benefit can last for several months after birth.
In addition to the transfer of substances, the placenta has two other main functions. It can act as a barrier between the mother and the fetus, preventing some harmful substances in the blood of the mother from damaging the fetus. However, it cannot exclude all harmful substances passing to the fetus. For example, alcohol can cross the placental barrier. The placenta also acts as an endocrine organ, producing several important hormones during pregnancy. These hormones work together to control the growth and development of the placenta and the fetus, and act on the mother to support the pregnancy and prepare for childbirth.
What hormones does the placenta produce?
The placenta produces two steroid hormones â€“ oestrogen and progesterone. Progesterone acts to maintain pregnancy by supporting the lining of the uterus (womb), which provides the environment for the fetus and the placenta to grow. Progesterone prevents the shedding of this lining (similar to that which occurs at the end of a menstrual cycle), since this would result in pregnancy loss. Progesterone also suppresses the ability of the muscular layer of the uterine wall to contract, which is important in preventing labour from occurring before the end of pregnancy. Oestrogen levels rise towards the end of pregnancy. Oestrogen acts to stimulate the growth of the uterus to accommodate the growing fetus and allows the uterus to contract by countering the effect of progesterone. In this way, it prepares the uterus for labour. Oestrogen also stimulates the growth and development of the mammary glands during pregnancy, in preparation for breastfeeding.
The placenta also releases several protein hormones, which include human chorionic gonadotrophin, human placental lactogen, placental growth hormone, relaxin and kisspeptin. Human chorionic gonadotrophin is the first hormone to be released from the developing placenta and is the hormone that is measured in a pregnancy test. It acts as a signal to the motherâ€™s body that pregnancy has occurred by maintaining progesterone production from the corpus luteum, a temporary endocrine gland found in the ovary. The function of human placental lactogen is not completely understood, although, it is thought to promote the growth of the mammary glands in preparation for lactation. It is also believed to help regulate the mother's metabolism by increasing maternal blood levels of nutrients for use by the fetus. A similar role is played by placental growth hormone, which predominates during pregnancy due to suppression of growth hormone produced by the maternal pituitary gland. Relaxin causes the relaxation of pelvic ligaments and softening of the cervix at the end of pregnancy, which aids the process of labour. Kisspeptin is a recently identified hormone, which is important for many aspects of human fertility. In the placenta, kisspeptin appears to regulate placental growth into the lining of motherâ€™s womb (endometrium). A number of other peptide hormones have been recently identified to regulate blood vessel formation within the placenta, which is crucial in allowing the placenta to exchange nutrients from the mother to baby; these peptide hormones include
soluble endoglin (sEng), soluble fms-like tyrosine kinase 1 (sFlt-1) and placental growth factor (PlGF). Testes The testes are two oval-shaped male reproductive glands that produce sperm and the hormone testosterone.
Alternative names Testicles; testis (singular)
Coloured scanning electron micrograph of sperm cells that are developing inside a seminiferous tubule. The developing heads of the sperm cells are embedded in a layer of Sertoli cells (red) that nourish the developing sperm. Magnification: x3750 Coloured scanning electron micrograph of sperm cells that are developing inside a seminiferous tubule. The developing heads of the sperm cells are embedded in a layer of Sertoli cells (red) that nourish the developing sperm. Magnification: x3750
Where are the testes? The testes, also known as testicles or male gonads, lie behind the penis in a pouch of skin called the scrotum.
The testes move freely in the scrotum but each testis is attached to the body wall by a thin cord called the spermatic cord, which passes through a cavity in the pelvis and into the abdomen. The cord contains the nerves and blood vessels for the testes as well as the vas deferens, which carries the sperm from the testes into the urethra; the urethra is the passageway for sperm to the outside of the body at ejaculation.
The testes lie outside of the body and are maintained at a temperature about two degrees Centigrade lower than the body's core temperature. This is because sperm production and quality is optimal at this lower temperature.
What do the testes do? The testes have two functions â€“ to produce sperm and to produce hormones, particularly testosterone.
Sperm are produced in the seminiferous tubules of the testes. There are about 700 of these tubes in each testis, each is the length of an arm and the width of a few hairs; the whole collection of tubules is longer than a football pitch! Once the sperm are produced in the seminiferous tubules they pass into the epididymis, a long coiled tube in which sperm mature as they are
conveyed along it. They are then ready to be released at ejaculation via the vas deferens.
What hormones do the testes produce? The main hormone secreted by the testes is testosterone, an androgenic hormone. Testosterone is secreted by cells that lie between the seminiferous tubules, known as the Leydig cells. The testes also produce inhibin B and anti-mĂźllerian hormone from Sertoli cells, and insulin-like factor 3 and oestradiol from the Leydig cells.
Testosterone is important in the first stages of developing the male reproductive organs in a foetus. It also causes the development of male characteristics such as growth of facial hair, deepening of the voice and the growth spurt that takes place during puberty. Testosterone is important in maintaining these secondary male characteristics throughout a manâ€™s life. From puberty onwards, testosterone provides the main stimulus for sperm production.
What could go wrong with the testes? Many things can wrong with the testes; they can be grouped into physical injury and diseases or conditions that affect the function of the testes:
Physical injury – the testes lie outside of the body and are not protected by muscle and bone so any physical shock (trauma) to the testes can cause severe pain, bruising and swelling. Usually this is not serious, but very rarely a severe trauma can cause blood to leak into the scrotum; this is called testicular rupture. Surgery may be needed to repair the rupture.
Another rare form of injury is a ‘twisted testicle’ or testicular torsion. This is when the spermatic cord becomes twisted from an injury to the testis or after strenuous activity. It happens more often in teenage boys. This torsion cuts off the blood supply to the testis. This is a medical emergency and surgery is needed to untwist the cord, restore the blood supply and save the testis.
Diseases and conditions that affect the function of the testes – there are many reasons for testicular dysfunction caused by diseases and conditions: Male infertility – due to absent or reduced sperm production or the production of sperm that do not function normally. There can be many causes including genetic and lifestyle factors. Men with reduced sperm can undergo assisted reproduction (such as in vitro fertilisation) with their partner. Microsurgical surgical
sperm retrieval is a recently developed technique has the potential to recover sperm from men without any sperm (azoospermia); however, it does not always work, and it is not widely available.
Cryptorchidism – the failure of one or both testes to drop down into the scrotum before birth, so they remain in the abdominal space. This can harm normal development and function of the testes and lead to infertility. Infants are often treated with an operation to bring the undescended testis into the scrotum (orchidopexy).
Epididymitis – an infection of the epididymis caused by a general infection, tuberculosis or sexually transmitted illnesses such as Chlamydia. Epididymitis can be treated with antibiotics.
Testicular cancer – abnormal growth of cells within the testis. The growth can disrupt normal function of one or both testes. It is most common in young men. Urgent medical treatment is required.
Klinefelter's syndrome – this is a genetic condition which stops the testes from developing normally. As a
result, low levels of testosterone are produced and released. Other factors that can affect the function of the testes are radiation and chemotherapy (used in the treatment of cancer), certain drugs, and disorders of the pituitary gland that stop signals from the hormone (endocrine) system that trigger production of testosterone from the testes Thyroid gland The thyroid gland is part of the endocrine system and is responsible for producing and releasing thyroid hormones into the bloodstream.
Where is my thyroid gland? The thyroid gland is located at the front of the neck just below the Adam's apple (larynx). It is butterflyshaped and consists of two lobes located either side of the windpipe (trachea). A normal thyroid gland is not usually outwardly visible or able to be felt if finger pressure is applied to the neck.
Diagram showing the location of the thyroid gland in the neck. It has two lobes and sits in front of the windpipe (trachea). The voice box (larynx) sits just above the thyroid.
Diagram showing the location of the thyroid gland in the neck. It has two lobes and sits in front of the windpipe (trachea). The voice box (larynx) sits just above the thyroid.
What does my thyroid gland do? The thyroid gland produces hormones that regulate the body's metabolic rate as well as heart and digestive function, muscle control, brain development, mood and bone maintenance. Its correct functioning depends on having a good supply of iodine from the diet.
The release of thyroid hormones from the thyroid gland is controlled by Thyrotrophin-releasing hormone from the hypothalamus in the brain and by thyroid stimulating hormone produced by the pituitary gland. This forms part of a feedback loop called the hypothalamicâ€“pituitaryâ€“thyroid axis.
Thyroid stimulating hormone and free thyroid hormones are used to monitor thyroid function in a patient. Typically, modest changes in free thyroid hormones result in large changes in thyroid stimulating hormone.
What hormones does my thyroid gland produce? The thyroid gland produces thyroxine, which is a relatively inactive prohormone and lower amounts of the active hormone, triiodothyronine. Collectively, thyroxine and triiodothyronine are referred to as the thyroid hormones. Twenty percent of the body's triiodothyronine is made by the thyroid gland; the other 80% comes from thyroxine converted by organs such as the liver or kidneys.
The thyroid gland also produces calcitonin from cells called C-cells. Calcitonin is understood to play a role in regulating calcium levels in the body, but its exact function in humans remains unclear.
What could go wrong with my thyroid gland? The thyroid gland can become overactive (hyperthyroidism) or underactive (hypothyroidism). This may, rarely, occur from birth, or develop later on in life. Hypothyroidism is often accompanied by an enlargement of the thyroid gland known as goitre.
Thyrotoxicosis is the term given when there is too much thyroid hormone in the bloodstream. It may be a result of overactivity of the thyroid gland (hyperthyroidism) as in Graves' disease, inflammation
of the thyroid or a benign thyroid tumour. Symptoms of thyrotoxicosis include intolerance to heat, weight loss, increased appetite, increased bowel movements, irregular menstrual cycle, rapid and irregular heartbeat, palpitations, tiredness, irritability, tremor, hair loss and retraction of the eyelids resulting in a 'staring' appearance.
Hypothyroidism is the term given when low levels of thyroid hormones are produced by the thyroid gland. It may result from autoimmune diseases (when the person's immune system starts to attack the body's own organs), poor iodine intake or be brought on by use of certain drugs. Since thyroid hormones are essential for physical and mental development, hypothyroidism during development (that is, before birth and during childhood) can result in learning difficulties and reduced physical growth. Hypothyroidism in adults results in decreased metabolic rate. This causes symptoms that include fatigue, intolerance of cold temperatures, low heart rate, weight gain, reduced appetite, poor memory, depression, stiffness of muscles and infertility.