Hormones: Maintaining Blood Homeostasis

• What is homeostasis in biology?

  • Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes
  • Homeostasis maintains optimal conditions for enzyme action and all cell functions
  • In the human body, these include control of:
  • Blood glucose concentration
  • Body temperature
  • Water levels
• Control of Homeostasis

  • Maintaining controlled conditions within the body is under involuntary (automatic) control
  • This means that the brain stem (or non-conscious part of the brain) and the spinal cord are involved in maintaining homeostasis – you don’t consciously maintain your body temperature or blood glucose level
  • These automatic control systems may involve nervous responses or chemical responses
• • All control systems include:
  • Cells called receptors, which detect stimuli (changes in the environment)
  • Coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors
  • Effectors (muscles or glands) which bring about responses which restore optimum levels
• Endocrine System Structure & Function

  • Students should be able to describe the principles of hormonal coordination and control by the 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
• • Important structures in the endocrine system are:
  • Pituitary gland: a ‘master gland’ making hormones such as FSH and LH
  • Pancreas: produces insulin which regulates the blood glucose level
  • Thyroid: produces thyroxine which controls metabolic rate and affects growth
  • Adrenal glands: produces adrenaline
  • Ovaries (females): produce oestrogen
  • Testes (males): produce testosterone
• 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
  • For example, in certain conditions, the pituitary gland makes and secretes thyroid-stimulating hormone (TSH) which stimulates the thyroid to release thyroxine
• • 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 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
• • 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 Control 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 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 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
• The negative feedback cycle - Control of blood glucose

  
• Exam Tip

  The terms glucagon and glycogen are very often mixed up by students as they sound similar. Remember:

  • Glucagon is the hormone
  • Glycogen is the polysaccharide glucose is stored as