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Homeostatic mechanisms function to maintain the body in a state of equilibrium and allow a degree of independence from the environment

Principles of Homeostasis

Table 16-11-1: Control of Blood Glucose

Change detected by β-cells in pancreas     Change detected by α-cells in pancreas
Increase in insulin secretion
- Activates enzymes converting
glucose to glycogen
- Increases rate of glucose uptake
    Increase in glucagon secretion
- Activates enzymes converting
glycogen to glucose
Levels return to norm     Levels return to norm

Homeostasis Provides a Constant Internal Environment and Independence from
Fluctuating External Conditions
  • Features that influence internal environment have a set level → norm
  • Any changes from the norm is called deviation
  • Negative feedback / caused by deviation from norm / change results in return to norm
  • External environment is changing → experienced by body
    • Homeostatic system even out variations experienced by body
      • Liver can store or release glucose
    • Blood is kept at a constant, ideal state
      • Glucose conc. of 80mg cm-3
    • Tissue fluid surrounds working cell with constant ideal conditions
      • Optimum glucose for respiration

Negative Feedback Tends to Restore Systems to their Original Level
  • Homeostasis is achieved by a negative feedback and involves
    • Change in level of an internal factor (change from norm level)
    • Detected by receptors / impulse send to hypothalamus
    • Activates effectors / stimulates corrective mechanism
    • Level of factor returns to norm
  • Factors in blood and tissue fluid must be kept constant:
    • Temp and pH
      • Change affects rate of enzyme-controlled/biochemical reactions
      • Extreme changes denatures proteins
      • Humans maintain constant core body temp between 36-37.8°C
      • Body temp refers to core body temp → limbs may be cooler than 37°C
    • Water potential / avoids osmotic problems → cellular disruption
    • Conc of ions (Na, K, Ca)

Temperature Control
Mechanisms Involved in Heat Production, Conversation, and Heat Loss.
The Role of the Hypothalamus and the Autonomic Nervous System in Temperature Control
  • Blood flows through receptors in the hypothalamus
  • Deviation causes the autonomic nervous system to initiate an appropriate response

  1. Receptors in hypothalamus detect increase in core temp/temp of blood
  2. Heat conversation centre stimulated
  3. VASOCONSTRICTION of arterioles
  4. Arterioles leading to capillaries in the skin narrow
  6. Less blood flows to skin surface / less heat is lost by RADIATION
  7. Hair raising / greater insulation / humans have less dense hair \ no effect
  8. Shivering / rapid contraction and relaxation of muscles / heat produced by RESPIRATION
  9. Adrenaline INCREASES METABOLIC RATE of cells //Mammals in cold climates can increase secretion of thyroxine / hormone increases metabolic rate on a more permanent basis
  10. VOLUNTARY CENTRE: put on clothes / seek warmer areas / warm drink

  1. Receptors in hypothalamus detect increase in core temp/temp of blood
  2. Heat loss centre stimulated
  3. VASODILATION of arterioles
  4. Arterioles leading to capillaries in the skin dilate (expand)
  6. More blood flows to skin surface (capillaries) / heat loss by RADIATION
  7. Heat loss by EVAPORATION of sweat / by using energy
    1. High(er) rate of sweating leads to a low(er) skin temp
  8. VOLUNTARY CENTRE: remove clothing / seek cooler area / cold drink

The Role of Temperature Receptors in the Skin
  • Hypothalamus detects temp fluctuation inside the body/internal environment
  • Skin receptors detect temp changes in external environment
  • Information is sent by nerves to voluntary centres of the brain
    • Voluntary activities (jogging, moving into a shade) are initiated
    • Changes behaviour of human

The Structure and Role of the Skin in Temp Regulation
  • Surface area is very large and in direct contact to external environment
  • Skin is divided into two layers: outer epidermis and inner dermis
  • MALPIGHIAN layer is the boundary between these two layers
    • Cells of this layer divide repeatedly by mitosis
    • Older cells are pushed towards the surface/EPIDERMIS
    • Cytoplasm of old cells becomes full of granules / cells die
    • Cells become converted into scales of keratin (waterproof)
  • DERMIS is thicker than epidermis and contains
    • Nerve endings (temp receptors)
    • Blood vessels held together by connective tissue
  • Beneath dermis is a region which contains some subcutaneous fat
    • Adipose tissue (fat storage tissue) provides vital insulations in humans

  • Body temp falls dangerously below normal
    • Heat energy is lost from body more rapidly than it can be produced
  • Brain is affected first → person becomes clumsy and mentally sluggish
  • As body temp falls, metabolic rate falls as well
  • Makes body temp fall even further, causing a POSITIVE FEEDBACK
    • Temp is taken further away from the norm
  • Death when core body temp is below ≈25°C / by ventricular fibrillation / normal beating of the
    heart is replaced by uncoordinated tremors
  • Most at risk are (1) babies and (2) elderly
    • (1) High surface area:volume ratio, undeveloped temp regulation mechanisms
    • (2) Detoriated thermoregulatory mechanisms
  • Deliberate hypothermia is sometimes used in surgical operations on heart
    • Patient is cooled by
      • Circulating blood through a cooling machine
      • Placing ice packs in contact with the body
    • Reduces metabolic rate / O2 demand by brain + other vital tissues is lowered
    • Heart can be stopped without any risks of the patient suffering brain damage through lack of O2
    • Tissues may be permanently damaged if patient is cooled to long

Control of Blood Glucose Concentration
The Factors which Influence Blood Glucose Concentration
  • Digestion of carbohydrates in diet
    • Digestion → polysaccharide → glucose
    • Fluctuation of glucose blood level depend on amount + type of carbohydrate eaten
  • Breakdown of glycogen
    • Excess glucose → glycogen → glucose
    • Storage polysaccharide made from excess glucose by glycogenesis
    • Glycogen is abundant in liver + muscles
  • Conversion of non-carbohydrates to glucose by gluconeogenesis
  • Oxidation of glucose by respiration
    • Glucose → ATP → energy
    • Rate of respiration varies for different activities
    • This affects glucose uptake from blood into cells
  • Brain is unable to store carbohydrates
    • Lack of glucose in blood → no respiratory substrate → insufficient energy for brain
    • Short period of time already causes brain to malfunction
  • Normal glucose level in blood ≈90mg per 100cm³
    • After a meal it rarely exceeds 150mg per 100cm³

Role of Hormones in Activating Enzymes Involved in Interconversion of Glucose and Glycogen
The Role of Insulin and Glucagon in Controlling Blood Glucose
The Pancreas
  • Endocrine role is to produce hormones
  • Contains islets of Langerhans → sensitive to blood glucose conc
  • Islet cells contain
    • α-cells → secrete glucagon and β-cells → secrete insulin
    • capillaries into which hormones are secreted
    • delta cells → produce hormone somatostatin → inhibits secretion of glucagon
  • Insulin mainly affects muscles, liver, adipose tissue
  • Exocrine role is to produce digestive enzymes
  • Active trypsin damages pancreas / digests proteins that make up pancreas / amylase leaks into blood from damaged tissues / amylase conc in blood higher

High Blood Glucose Concentration
  • Detected by β-cells in islet of Langerhans (receptor) → secrete insulin
  • Increase in insulin secretion (corrective mechanism → effectors bring about a return to norm)
    • Speeds up rate of glucose uptake by cells from blood
      • Glucose enters cells by facilitated diffusion via glucose carrier proteins
      • Cells have vesicles with extra carrier molecules present in their cytoplasm
      • Insulin binds to receptor in plasma membrane
      • Chemical signal → vesicles move towards plasma membrane
      • Vesicle fuses with membrane → increases glucose carrier proteins
    • Activates enzymes / Converts glucose to glycogen / Promotes fat synthesis

Low Blood Glucose Concentration
  • Detected by α-cells in islets of Langerhans → secrete glucagon
  • Increase in glucagon secretion
    • Hormone activates enzymes in the liver → convert glycogen to glucose
    • Stimulates formation of glucose form other substances such as amino acids
  • Glucose passes out of cells into blood, raising blood glucose conc until norm is reached

Diabetes and its Control with Insulin and Manipulation of Carbohydrate Intake
  • Diabetes mellitus → inability of control of blood glucose level
  • High levels of blood glucose because
    • Pancreas becomes diseased → fails to secrete insulin
    • Target cells lose responsiveness to insulin
  • Kidney is unable to reabsorb back into blood all the glucose filtered into its tubules
    • Glucose secreted into urine
    • Craving for sweet food and persistent thirst
  • DIAGNOSTIC: glucose tolerance test
    • Patient swallows glucose solution
    • Blood glucose level measured at regular intervals

Two Types of Diabetes Mellitus
  • Type I → insulin dependant/juvenile-onset
    • Occurs in childhood
    • Autoimmune reaction → immune system attacks and destroys own cells
    • Destroys β-cells in islet of Langerhans → unable to produce insulin
    • TREATMENT: insulin given must match glucose intake and expenditure
      • Overdose causes hypoglycaemia (to much glucose withdrawn from blood)
      • Diabetics need to manage their diet and levels of exercise
      • Need to monitor blood glucose conc
  • Type II → insulin independent/late-onset
    • Occurs late in life, more common than type I
    • Causes by gradual loss in responsiveness of cells to insulin
    • TREATMENT: regulated diet
      • Sugar intake must balance with amount of exercises taken
  • Glycogen levels are lower
    • Little insulin / no glucose to glycogen
    • Insulin receptors no longer functional / less glucose taken up by cells
  • Glycogen is an effective storage molecule
    • Insoluble → no osmotic effect
    • Large → cannot diffuse out of cell
    • Branched → easy to break down / hydrolyse to glucose
    • Compact → large amount of glucose stored in small space

Insulin Patches
  • Insulin → peptide chains → digested if swallowed by peptidase → had to be injected
  • Treat skin area with ultrasound → disrupts underlying fat tissues
    • Insulin is not soluble in fat
    • Disrupting tissues allows movement through skin
  • Apply patch containing insulin to that area

References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]

BYA7 SECTION: 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 16.10 16.11 16.12