Human body, the physical substance of the human organism, composed of living cells and extracellular materials and organized into tissues, organs, and systems.
Human anatomy and physiology are treated in many different articles. For detailed discussions of specific tissues, organs, and systems, see human blood; cardiovascular system; human digestive system; human endocrine system; renal system; skin; human muscle system; nervous system; human reproductive system; human respiration; human sensory reception; and human skeletal system. For a description of how the body develops, from conception through old age, see aging; growth; prenatal development; and human development.
For detailed coverage of the body's biochemical constituents, see protein; carbohydrate; lipid; nucleic acid; vitamin; and hormone. For information on the structure and function of the cells that constitute the body, see cell.
Many entries describe the body's major structures. For example, see abdominal cavity; adrenal gland; aorta; bone; brain; ear; eye; heart; kidney; large intestine; lung; nose; ovary; pancreas; pituitary gland; small intestine; spinal cord; spleen; stomach; testis; thymus; thyroid gland; tooth; uterus; and vertebral column.
The gland and the tooth 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 body'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.
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 are more prone to storing visceral fat (fat around their internal organs), leading to 'central 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. In addition to adipocytes, adipose tissue contains several other cells that can produce hormones in response to signals from the rest of the organs in 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- an enzyme involved in converting androgens to oestrogens.
TNF alpha, IL-6 and leptin- these are collectively termed 'cytokines' and are involved in sending messages between cells. Leptin is involved in satiety regulation.
Plasminogen activator inhibitor-1, which is involved in the clotting of blood.
Angiotensin, which is involved in blood pressure control.
Adiponectin, which improves the body's sensitivity to insulinand 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. Obesity can lead to several health problems, especially if there is too much visceral fat. Obesity increases the risk of developing type 2 diabetes as it causes the body to become resistant to the normal glucose-lowering effect of insulin. This resistance to the action of insulin results in high levels of blood sugar, which is harmful for health. Obesity also increases the chance of developing high blood pressure, high cholesterol levels, and an increased tendency for blood to clot. These factors increase the risk of heart attacks, stroke, and abnormal blood clotting in the legs or lungs (venous thrombo-embolism).
Abnormal storage of adipose tissue (lipodystrophy) can also cause similar problems. This can be seen due to rare inherited conditions (inherited lipodystrophy) and can also be due to use of medications for the treatment of HIV (acquired lipodystrophy).
In eating disorders (such as anorexia nervosa), the patient does not eat enough food to maintain their adipose tissues levels. This can also cause low levels of important reproductive hormones such as oestrogen resulting in thinning of bones (osteoporosis), and stopping of menstrual periods (amenorrhea).
Where are my adrenal glands?
The human body has two adrenal glands and one sits on top of each kidney. Each adrenal gland weighs 4–5 g in an adult.
Each adrenal gland is composed of two distinct parts: the outer part called the 'adrenal cortex' and the inner part called the 'adrenal medulla'. The adrenal glands secrete different hormones which act as 'chemical messengers'. These hormones travel via the bloodstream and act on various other body tissues. All adreno-cortical hormones (ie hormones produced by the adrenal cortex) are steroid compounds made from cholesterol.
What hormones do my adrenal glands produce?
The adrenal cortex produces three hormones, from three different layers:
1. Mineralocorticoids: the most important of which is aldosterone. This hormone helps to maintain the body's salt and water balance, which is important for maintaining blood pressure. Aldosterone helps the kidney conserve salt when it is needed. It acts in the kidney to cause sodium and water to be retained, and potassium to be lost. This action is important if the body has low salt and water levels, that could risk a low blood pressure. However, too much aldosterone can cause high blood pressure (hypertension) and low potassium. Without aldosterone (for example if the adrenal glands are not working properly), the kidney loses excessive amounts of salt (sodium) and, consequently, water, leading to severe dehydration and low blood pressure. In summary, if we do not have enough salt in the body, aldosterone production is 'switched on' and the kidney retains salt, but if we have too much salt, the amount of aldosterone being made is reduced and the kidney can excrete the excess salt.
2. Glucocorticoids: predominantly cortisol, which is also known as the body's 'natural steroid'. This hormone is involved in the response to illness and also helps to regulate body metabolism. Cortisol is released during the 'stress response' to illness. Cortisol stimulates glucose production to help maintain blood glucose levels. Cortisol also has anti-inflammatory effects especially at higher amounts.
3. Adrenal androgens: these male sex hormones, mainly dehydroepiandrosterone (DHEA) and testosterone, are present in both men and women. All have weak effects, but play a role in early development of the male sex organs in childhood, and are important for the normal onset of female body hair following puberty.
Adrenocorticotropic hormone (ACTH), secreted by the anterior pituitary gland, primarily affects the release of glucocorticoids and adrenal androgens by the adrenal gland and, to a much lesser extent, also stimulates aldosterone release.
The adrenal medulla produces catecholamines:
Catecholamines include adrenaline, noradrenaline and small amounts of dopamine – these hormones are responsible for all the physiological characteristics of the stress response, the so called 'fight or flight' response, which can include increased heart rate, blood pressure, breathing rate, dilated pupils in