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Homeostasis Basics

The maintenance of a constant internal environment- kept in a state of dynamic equilibrium (fluctuating aroung a normal level)

The importance of homeostasis

  • Homeostasis is mainly involved in the maintenance of three factors: temperature, pH and blood glucose levels in order to control the rate of metabolic reactions and the water potential of cells
  • Temperature- If the temperatue is too low, enzyme activity is reduced, slowing down the rate of metabolic reactions. As the temperature increases so does the kinetic energy meaning there will be more successful collisions betweent the substrate and enzyme, increasing the rate of metabolic reactions. If the temperature is too high, hydrogen bonds in the enzyme will break and the 3D structure will change shape meaning enzyme substrate complexes cannot form.
  • pH- If the pH is too high or too low the enzyme will be denatured as the hydrogen bonds are affected which will change the 3D shape and therefore the active site
  • Blood glucose- If it is too high, the water potential will reduce to a point that water molecules will diffuse out of the cells= crenation. If it is too low the water potential will increase to a point that water molecules diffuse into the cells= cell lysis.
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Negative Feedback

Negative Feedback

  • When the level of something that needs to be controlled (temperature, pH, blood glucose ect.) deviates to far from the set point, a response is triggered to return the level back to the set point.
  • Normal level-->level changes from normal-->receptors detect change-->nervous/hormonal system-->effectors respond-->normal level (repeats)
  • Only works within certain limits, is the change is too big the effectors may not be able to counteract it e.g. a massive drop/rise in body temperature
  • Homeostasis involves multiple feedback mechanisms, this gives you more control over changes in the internal environment compared to having only one mechanism. It allows you actively increase or decrease the level.

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Positive Feedback

Positive Feedback

  • The effectors responf t keep the level deviating from the set point
  • The mechanism aplifies the change away from the normal level
  • Normal level-->normal level changes-->receptors detect change-->nervous/hormal system-->effectors respond-->change aplified-->normal level changes
  • Not incolved in homeostasis becaus eit dosn't keep your internal envionment constant
  • Useful in rapidly active processes in the body
  • Can happen when a homeostaic system breaks down e.g. hypo/hyperthermia
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Control of Body Temperature


  • 32 degrees and below
  • Symptoms- excessive shivering up to a point, blue lips, pale, incoherent, paradoxial ********** (severe)


  • 38 degrees and above
  • Symptoms- hyperactive, sweating (up to a point)

Death: below 25 degrees or above 43 degrees

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Ectotherms and Endotherms


  • Can't control their body temperature internally, instead they control it by changing their behaviour
  • The internal temperature of ectotherms depends on the external termperature
  • Have variable metabolic rates
  • Generate very little heat themselves
  • Their activity level depends on the external temperature- more active at higher temperatures


  • Control their body internally by homeostasis as well as by altering their behaviour
  • Internal temperature is less affected by the external temperature
  • Constantly high matbolic rate- generate lots of heat
  • Activity level is relatively independant from the external temperature
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Mechanisms for Body Temperature Control

Heat Loss

  • Sweating- Sweat is secreated from sweat glands, the water in the sweat evaporates from the surface of the skin and takes heat from the body cooling the skin
  • Hairs lie flat- The eractor pili muscles relax so that hairs on the skin lie flat, less air is trapped meaning the skin is less insulated and heat can be lost more easily
  • Vasodilation- Arterioles near the surface of the skin dilate allowing more blood to flow through the capillaries in the surface layers of the dermis, more heat is lost from the skin by radiation causing the temperature to be lowered

Heat Production

  • Shivering- Muscles contract in spasms making the body shiver, more heat is therefore produced as a result of increased respiration
  • Hormones- The body releases adrenaline and thyroxine which increase metabolism and so moreheat is produced
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Mechanisms for Body Temperature Control

Heat Conservation

  • Much less sweat- Less sweat is secreted from the sweat glannds, reducing heat loss via exaporation
  • Hairs stand up/ pilo erection- Erector pili muscles contract making the hairs stand up, this traps more air against the skin, reducing heat loss
  • Vasoconstriction- Arterioles near the surface of the skin constrict, reducing the blood flow throught the capillaries in the surface layers of the dermis which decreases heat loss via radiation
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The Hypothalamus

  • Involved in the maintenance of a constant internal body temperature
  • Thermoreceptors send informtation about the internal (thermoreceptors in the aorta and carotid artery) and external (thermoreceptors in the skin) tempertatures to the hypothalamus
  • Thermoreceptors send impulses along sensory neurones to the hypothalamus, which sends impulses along motor neurones to effectors (muscles and glands) which respond to restore the body temperature back to normal
  • The neurones are part of the autonomic nervous system- an unconscious process
  • Rise in body temperature-
    rise in body temperature-->thermoreceptors detect change--.activates heat loss centre in hypothalamus-->impulses sent to effectors-->effectors respond-->normal body temperature
  • Fall in body temperature-
    fall in bosy temperature-->theromorecteptors detect change-->activates heat gain centre in hypothalamus-->impuses sent to effectors-->effectors respond-->normal body temperature
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Control of Blood Glucose Concentration

Concentration of glucose in the blood is normally around 90mg per 100cm^3 and is monitered by cells in the pancreas. Blood glucose rises after eating carbohydrates and falls after exercise

Islets of Langerhans

  • A cluster of cells in the pancreas which secrete the two hormones: insulin and glucagon
  • Beta (β) cells- Secrete insulin into the blood
  • Alpha (α) cells- Secrete glucagon into the blood


  • Lowers the blood glucose concentration when it's too high
  • Binds to specific receptors on the cell membranes of liver (hepatocytes) and muscle cells
  • Increases the permability of cell mebranes to glucose, so more glucose is taken up and therefore removed from the blood
  • Activates enzymes that convert glucos into glycogen which is store in the cytoplasm of liver and muscle cell as an energy source- GLYCOGENESIS
  • Increases the respiration of glucose, especially in muscle cells
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Control of Blood Glucose Concentration


  • Raises blood glucose concentration when it's too low
  • Binds to specific receptors on the cell membranes of liver cells (hepatocytes)
  • Activates enzymes that break down glycogen into glucose- GLYCOGENOLYSIS
  • Promotes the formation of glucose form glycerol and amino acids- GLUCONEOGENESIS
  • Decreases the rate of respiration in cells
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Negative feedback mechanisms in glucose concentrat

Rise in glucose concentration

Rise in blood glucose concentration-->Pancreas detects change-->Beta cells in the islets of langerhans secrete insulin-->Liver and muscle cells respond-->Normal blood glucose concentration

Fall in blood glucose concentration

Fall in blood glucose concentration-->Pancreas detects change-->Alpha cells in the islets of langerhans secrete glucagon-->Liver cells respond-->Normal blood glucose concentration

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  • A hormone secreted from the adrenal glands when there is a low concentration of glucose in the blood, when you're stressed or exercising
  • Binds to receptors on the cell membrane of liver cells
  • Activates the breakdown of glycogen to glucose- GLYCOGENOLYSIS
  • Inhibits the synthesis of glycogen from glucose- GLYCOGENESIS
  • Activates glucagon secretion and inhibits insulin secretion
  • Makes glucose more readily available for respiration

Secind messengers

  • Both adrenaline and glucagon can activate glycogenolysis inside a cell even though they bind to receptors on the outside of the cell- second messenger model
  • The binding of the hormone to cell receptors activates an enzyme on the inside of the cell membrane. which then produces a chemical known as a second messenger- activates other enzymes in the cell to bring about a response
  • Activating glycogenolysis- adrenaline and glucagon bindo to specific receptors which cause the enzyme adenylate cyclase to be activated inside the cell. Adenylate cyclase converts ATP into a chemical called cyclic AMP (cAMP)- a second messenger. cAMP activates a cascade that break down glycogen into glucose
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Type 1

  • β cells don't produce any insulin- blood glucose levels remain high after eating= hyperglycaemia
  • The kidneys can't reabsorb all the glucose- some is secreted into the urine
  • Treated with regular injections of insulin

Type 2

  • Acquired later in life and linked with obesity
  • β cells don't produce enough insulin or the body's cells don't respond properly to insulin because the insulin receptors don't work properly
  • Blood glucose level is higher than normal
  • Treated through a controlled diet an weight loss
  • Glucose-lowering tablets can also be taken
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The human menstrual cycle

  • Also known as the Oestrous cycle and lasts around 28 days
  • A cycle where the female body prepares for reproductyion
  • If fertilisation does not occur, mentruation takes place, marking the end of the cycle and the beginning of a new one

Menstrual hormones

  • Follicle stimulating hormone (FSH)- stimulates the ova to develop which produces a follicle
  • Leutinising hormone (LH)- stimulates ovulation and the corpus leutium (empty follicle) to develop
  • Oestrogen- stimulates the uterus lining to thicken
  • Progestreone- maintains the thick uterus lining, ready for implantation of an embryo
  • Gondanotrophin hormone (gnrtl)- pregnancy hormone that is released by the embryo to stop the cycle

FSH and LH are secreted from the pituatry glan in the brain.
Oestrogen and Progesterone are secreted from the ovaries.

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1. Hormone changes in the menstual cycle

Stage 1

  • A high FSH concentration in the blood stimulates follicle development, the follicle contains a single ovum cell surrounded by other cells
  • The follicle releases oestrogen which causes the uterus lining to thicken
  • Oestrogen inhibits FSH being released from the anterior pituitary gland- Negative feedback

Stage 2

  • Oestrogen concentration increases to a high level and no longer inhibits FSH release but instead stimulated the anterior pituitary gland to release LH and FSH
  • LH stimulates the ovaries to release more oestrogen which further stimulates the release of LH from the anterior pituitary gland- Positive feedback
  • This causes a surge in LH concentration which stimulates ovulation- the follicle ruptures and the egg is released (day 14)
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2. Hormone changes in the menstrual cycle

Stage 3

  • The corpus luteum which is the result of ovulation, releases progesterone
  • Progesterone levels rise, inhibiting the release of FSH and LH from the anterior pituitary gland, so the concentration of these fall- Negative feedback
  • The uterus lining is maintained by progesterone

No fertilisation

  • The corpus luteum breaks down over the next 10 days and stops releasing progesterone
  • FSH levels increase again
  • The uterus lining is no longer being maintained so it breaks down- menstruation


  • The corpus luteum doesn't degenerate and so progesterone continues to be produced, preventing mentruation
  • Progesterone stops the release of FSH and so no more ova are matured
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