homeostasis u1 AoS2

• There are two methods of food digestion, which are both employed by humans:
• • Physical (or mechanical) digestion – to be absorbed, food must first be broken into
• smaller pieces as smaller pieces provide a relatively large surface area for digestion.
• Physical digestion describes the processes whereby the mechanical movement
• of organs and tissues causes this breakdown of food into smaller pieces. These
• movements include chewing, muscle contractions, and the stirring of food and
• digestive juices by muscle movements.
• • Chemical digestion – food pieces undergo chemical digestion by enzymes and stomach
• acid, producing smaller molecules that are capable of being absorbed. Most digestive
• enzymes split food molecules in hydrolysis reactions, by adding water molecules.
• Homeostasis


  Staying stable
• Homeostatic mechanisms


  • Produce a relatively stable internal environment by maintaining key variables within narrow limits
• Major variables subject to homeostasis in humans


  • Temperature
  • Blood glucose
  • Water
  • Ions
  • pH of blood
  • Blood pressure
  • Urea in plasma
• Control mechanisms for homeostasis


  • Nervous system (electrical messages)
  • Endocrine system (chemical messages)
• Nervous system


  Brain, spinal cord and nerves
• Endocrine system


  The hormonal system
• Hormone types


  • Water soluble (polypeptides)
  • Lipid-soluble (steroids)
• Polypeptide hormones


  • Require facilitated diffusion to enter a cell, target proteins are outside the cell
  • Steroids can diffuse naturally through the cell membrane, target proteins can be inside the cell
• Negative feedback


  1. Stimulus

  2. Sensor

  3. Control center

  4. Effector
• Homeostatic physiological processes are not under conscious control but occur automatically
• Decrease in core body temperature


  Triggers physiological responses to increase heat production and reduce heat loss
• Increase in core body temperature


  Triggers physiological responses to increase heat loss
• Ectotherms


  Rely on external sources to maintain warmth
• Endotherms


  Can generate their own body heat internally
• The energy requirement of a reptile is just 10% of a mammal the same body mass
• When negative feedback does not occur, the body is no longer regulating itself properly
• If there is no negative feedback then the body can over-correct and swing too far the other way
• Heat stroke


  Failure of homeostatic mechanisms such as sweating, vasodilation and blood pressure
• Hypothermia


  When core body temperature drops and mechanisms are not in place (or malfunction) to bring up the temperature
• Hyperthyroidism


  Too much thyroid hormone (thyroxine)
• Hypothyroidism


  Too little thyroid hormone (thyroxine)
• Normal thyroid function


  1. Hypothalamus produces TRH

  2. Pituitary gland produces TSH

  3. Thyroid gland produces T3 and T4

  4. Negative feedback from T3 and T4 in blood stops further production
• Hypothyroidism


  1. Hypothalamus produces TRH

  2. Pituitary gland produces TSH

  3. Thyroid gland does not produce enough T3 and T4

  4. No negative feedback, so production continues to be stimulated
• Goiter (enlarged thyroid) continues to grow unless treated with thyroxine hormone injections
• Digestive System


  Physical/chemical breakdown of food, absorption of nutrients
• Digestive System


  • 4 main roles: Ingestion, Digestion, Absorption, Egestion
• Digestion


  1. Mechanical/Physical digestion

  2. Chemical digestion
• Mechanical/Physical digestion


  Large pieces of food are broken down into smaller pieces by mastication (chewing in mouth) or by muscular movement in stomach
• Chemical digestion


  Enzymes break down complex structures into their simplest forms (often using hydrolysis reactions)
• Ingestion


  1. Mastication in mouth

  2. Tongue moves food, increasing contact with teeth

  3. Salivary glands release amylase

  4. Food moves to oesophagus