homeostasis and response

• 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.
• what does homeostasis control?

  • blood glucose concentration
  • body temperature
  • water levels.
• 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.
• The human nervous system

  Structure and function

  The nervous system enables humans to react to their surroundings and to coordinate their behaviour.

  Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS). The CNS is the brain and spinal cord. The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones.

  stimulus →receptor →coordinator →effector →response 
• Reflex actions are automatic and rapid; they do not involve the conscious part of the brain
• The brain (biology only)

  The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions.
• Neuroscientists have been able to map the regions of the brain to particular functions by studying patients with brain damage, electrically stimulating different parts of the brain and using MRI scanning techniques. The complexity and delicacy of the brain makes investigating and treating brain disorders very difficult.
• The eye (biology only)

  The eye is a sense organ containing receptors sensitive to light intensity and colour.
• Accommodation is the process of changing the shape of the lens to focus on near or distant objects.

  To focus on a near object:

  • the ciliary muscles contract
  • the suspensory ligaments loosen
  • the lens is then thicker and refracts light rays strongly.

  To focus on a distant object:

  • the ciliary muscles relax
  • the suspensory ligaments are pulled tight
  • the lens is then pulled thin and only slightly refracts light rays.
• Two common defects of the eyes are myopia (short sightedness) and hyperopia (long sightedness) in which rays of light do not focus on the retina.

  • Generally these defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina.
  • New technologies now include hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens in the eye.
• Control of body temperature (biology only)

  Body temperature is monitored and controlled by the thermoregulatory centre in the brain. The thermoregulatory centre contains receptors sensitive to the temperature of the blood. The skin contains temperature receptors and sends nervous impulses to the thermoregulatory centre.
• Control of body temperature (biology only)

  If the body temperature is too high, blood vessels dilate (vasodilation) and sweat is produced from the sweat glands. Both these mechanisms cause a transfer of energy from the skin to the environment.

  If the body temperature is too low, blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver).
• Human endocrine system

  The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ where it produces an effect. Compared to the nervous system the effects are slower but act for longer.
• Human endocrine system

  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.
• Control of blood glucose concentration

  Blood glucose concentration is monitored and controlled by the pancreas.
• Control of blood glucose concentration

  If the blood glucose concentration is too high, the pancreas produces the hormone insulin that causes glucose to move from the blood into the cells. In liver and muscle cells excess glucose is converted to glycogen for storage
• Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin. It 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. A carbohydrate controlled diet and an exercise regime are common treatments. Obesity is a risk factor for Type 2 diabetes
• (HT only) 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.
• Maintaining water and nitrogen balance in the body (biology only)

  Water leaves the body via the lungs during exhalation.

  Water, ions and urea are lost from the skin in sweat.

  There is no control over water, ion or urea loss by the lungs or skin.

  Excess water, ions and urea are removed via the kidneys in the urine.

  If body cells lose or gain too much water by osmosis they do not function efficiently.
• Maintaining water and nitrogen balance in the body (biology only)

  The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. In the liver these amino acids are deaminated to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion.
• The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
•  The water level in the body is controlled by the hormone ADH which acts on the kidney tubules. ADH is released by the pituitary gland when the blood is too concentrated and it causes more water to be reabsorbed back into the blood from the kidney tubules. This is controlled by negative feedback.
• People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis.
• During puberty reproductive hormones cause secondary sex characteristics to develop.
• Oestrogen is the main female reproductive hormone produced in the ovary. At puberty eggs begin to mature and one is released approximately every 28 days. This is called ovulation.
• Testosterone is the main male reproductive hormone produced by the testes and it stimulates sperm production.
• Several hormones are involved in the menstrual cycle of a woman.

  • Follicle stimulating hormone (FSH) causes maturation of an egg in the ovary.
  • Luteinising hormone (LH) stimulates the release of the egg.
  • Oestrogen and progesterone are involved in maintaining the uterus lining.
• Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception.

  These include:

  • oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature
  • injection, implant or skin patch of slow release progesterone to inhibit the maturation and release of eggs for a number of months or years
  • barrier methods such as condoms and diaphragms which prevent the sperm reaching an egg
  • intrauterine devices which prevent the implantation of an embryo or release a hormone
• Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception. (2)

  • spermicidal agents which kill or disable sperm
  • abstaining from intercourse when an egg may be in the oviduct
  • surgical methods of male and female sterilisation.
• In Vitro Fertilisation (IVF) treatment.

  • IVF 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).
• Although fertility treatment gives a woman the chance to have a baby of her own:

  • it is very emotionally and physically stressful
  • the success rates are not high
  • it can lead to multiple births which are a risk to both the babies and the mother.
• Feedback systems (HT only)

  Adrenaline is produced by the adrenal glands in times of fear or stress. It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’.
•  Feedback systems (HT only)

  Thyroxine from the thyroid gland stimulates the basal metabolic rate. It plays an important role in growth and development.

   

  Thyroxine levels are controlled by negative feedback.
• Plant hormones (biology only)

  Plants produce hormones to coordinate and control growth and responses to light (phototropism) and gravity (gravitropism or geotropism). Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
• Plant hormones (biology only)

  (HT only) Gibberellins are important in initiating seed germination.

  (HT only) Ethene controls cell division and ripening of fruits.
• Use of plant hormones (HT only)

  Plant growth hormones are used in agriculture and horticulture.

  Auxins are used:

  • as weed killers
  • as rooting powders
  • for promoting growth in tissue culture.
• Use of plant hormones (HT only)

  Ethene is used in the food industry to control ripening of fruit during storage and transport.
•  Use of plant hormones (HT only)

  Gibberellins can be used to:

  • end seed dormancy
  • promote flowering
  • increase fruit size.