chapter 14

• the endocrine system uses hormones to send information about changes in the environment around the body to bring about a designated response
• endocrine system is made up out of endocrine glands
• endocrine gland

  group of cells which are specialised to secrete chemicals into the bloodstream e.g the pancreas and adrenal glands

  
• pituitary gland

  produces:

  • growth hormone which controls growth of bones and muscles
  • antidiuretic hormone (ADH) that increases absorption of water in the kidneys
  • gonadotrophins which control development of ovaries and testes

  
• thyroid gland

  produces thyroxine which controls rate of metabolism and rate that glucose is used up in respiration, and promotes growth

  
• adrenal gland

  produces adrenaline which increases heart and breathing rate and raises blood sugar levels

  
• testis

  produces testosterone which controls sperm production and secondary sexual characteristics
• pineal gland

  produces melatonin which affects reproductive development and daily cycles

  
• thymus

  produces thymosin which promotes production and maturation of white blood cells

  
• pancreas

  produces insulin which converts excess glucose into glycogen in the liver, and glucagon which converts glycogen back to glucose in the liver

  
• ovary

  produces oestrogen which controls ovulation and secondary sexual characteristics and produce progesterone which prepares the uterus lining for receiving an embryo

  
• exocrine glands

  secrete chemicals through ducts into organs such as in the digestive system, or to the surface of the body
• hormones

  referred to as chemical messengers because they carry information from one part of the body to another, they can be:

  • steroids
  • proteins
  • glycoproteins
  • polypeptides
  • amines
  • tyrosine derivatives
• hormones are secreted directly into the blood when a gland is stimulated, this can occur as a result of a change in concentration of a particular substance such as blood glucose concentration, it can also occur as the result of another hormone or a nerve impulse
• once secreted, the hormones are transported in the blood plasma all over the body, hormones diffuse out of the blood and bind to specific receptors for that hormone which are found on the cell membrane or in the cytoplasm of those cells in the target orans
• once hormones are bound to the target cell receptors, the hormones stimulate the target cells to produce a response, the type of hormone determines the way it causes its effect on a target cell
• steroid hormones

  lipid soluble, pass through the lipid component of the cell membrane and bind to steroid hormone receptors to form a hormone receptor complex, receptors may be present in the cytoplasm or the nucleus depending on the hormon, the complex formed acts as a transcription factor which facilitates or inhibits the transcription of a specific gene e.g oestrogen

  
• non steroid hormones

  hydrophilic so cannot pass directly through the cell membrane, instead they bind to specific receptors on the cell surface membrane of the target cell, this triggers a cascade reaction mediated by chemicals called second messengers e.g adrenaline

  
• as hormones are not released directly onto their target cells, this results in a slower and less specific form of communication than neural communication
• hormones are not broken down as quickly as neurotransmitters so it can result in a much longer lasting and widespread effect e.g insulin and glucagon involve lots of organs in controlling blood glucose concentration
• hormonal system characteristics

  • communication is by chemicals called hormones
  • transmission is by the blood system
  • transmission is relatively slow
  • hormones travel to all parts of the body but only target organs respond
  • response is widespread
  • response is slow
  • response is often long lasting
  • effect may be permanent and irreversible
• nervous systemcharacteristics

  • communication is by nerve impulses
  • transmission is by neurones
  • transmission is rapid
  • nerve impulses travel to specific parts of the body
  • response is localised
  • response is rapid
  • response is short lived
  • effect is temporary and reversible
• adrenal glands

  two small glands located on top of each kidney, made up of 2 parts:

  • adrenal cortex = outer region of the gland, produce hormones that are vital to life such as cortisol and aldosterone
  • adrenal medulla = inner region of the glands, produce non essential hormones such as adrenaline which help body react to stress

  
• adrenal cortex

  production of hormones by adrenal cortex itself is controlled by hormones released from the pituitary gland in the brain there are 3 types of hormones produced by the adrenal cortex:

  • gluocorticoids
  • mineralcorticoids
  • androgens
• glucocorticoids - released by the adrenal cortex

  include cortisol which help regulate metabolism by controlling how the body converts fats, proteins and carbohydrates to energy, also helps regulate blood pressure and cardiovascular function in response to stress, another glucocorticoid is corticosterone which works with cortisol to regulate immune response and suppress inflammatory reactions
• mineralocorticoids

  main one produced is aldosterone which helps control blood pressure by maintaining the balance between salt and water concentrations in the blood and body fluids, its release is mediated by signals triggered by the kidney
• androgens

  small amounts of female and male sex hormones are released, impact is relatively small compared to oestrogen and testosterone, but are vital to woman especially after the menopause
• adrenal medulla

  hormones of the adrenal medulla are released when the sympathetic nervous system is stimulated, occurs when the body is stressed, hormones secreted are:

  • adrenaline
  • noradrenaline
• adrenaline

  increases the heart rate sending blood quickly to the muscles and brain, also rapidly raises blood glucose concentration levels by converting glycogen to glucose in the liver
• noradrenaline

  hormone works with adrenaline in response to stress, producing effects such as increased heart rate, widening of pupils, widening of air passages in the lungs, and the narrowing of blood vessels in non essential organs resulting in high blood pressure
• pancreas has 2 main functions in the body:

  • acts as an exocrine gland, where it can produce enzymes and release them via a duct into the duodenum
  • acts as an endocrine gland, where it can produce hormones and release them into the blood
• pancreas role as an exocrine gland

  most of the pancreas is made up of exocrine glandular tissue which is responsible for producing digestive enzymes and an alkaline fluid known as pancreatic juice, enzymes and juices are secreted into ducts which lead to the pancreatic duct, then they are released into the duodenum, the top part of the small intestine