Topic 6

• Digestive system

  1. Alimentary canal (mouth to anus)

  2. Mouth (mechanical digestion, salivary amylase)

  3. Oesophagus (peristalsis)

  4. Stomach (enzyme mixing, protein digestion)

  5. Small intestine (final digestion, nutrient absorption)

  
• Accessory organs

  • Organs connected by ducts to alimentary canal, play role in digestion but food does not pass through

  
• Liver

  Produces bile to break up lipid droplets
• Gallbladder


  Stores and regulates release of bile
• Pancreas


  Produces and releases enzymes: lipase, pancreatic amylase, endopeptidase
• Humans cannot digest cellulose, which makes up plant cell walls. Cellulose passes undigested out of the body.
• Amylase, lipase and an endopeptidase are secreted by the pancreas into the lumen of the small intestine

  
• Villi


  • Increase surface area of epithelium over which absorption is carried out
• Absorption of digested molecules

  1. Diffusion

  2. Facilitated diffusion

  3. Active transport

  4. Pinocytosis
• Dialysis tubing can be used to model absorption by passive diffusion and osmosis, but not active transport
• Peristalsis

  1. Contraction of circular muscles behind food to prevent backflow

  2. Contraction of longitudinal muscles to move food forward

  3. Controlled by enteric nervous system

  
• Heart chambers and valves

  • Right and left atria, right and left ventricles
  • Atrioventricular valves
  • Semilunar valves

  
• Double circulation


  • Pulmonary circulation (heart-lungs)
  • Systemic circulation (heart-body)
• Cardiac cycle


  1. Atrial systole

  2. Ventricular systole

  3. Diastole

  
• Heart tissue is myogenic, with natural contractions initiated by sinoatrial node pacemaker

  
• Cardiac cycle

  The sequence of events that takes place during one heartbeat

  
• Cardiac cycle


  1. Chambers contract

  2. Blood inside them is forced on its way

  3. Valves in the heart and arteries stop the blood flowing backwards through the heart

  4. Pressure and volume of blood in each of the chambers of the heart change

  
• Heart tissue

  • Made of a special type of muscle that is different from other muscles in our bodies
  • Contracts and relaxes without stimulation from the nervous system
  • Said to be myogenic
• Pacemaker

  A special region of muscle cells in the right atrium that sets the basic pace of the heart
• Heartbeat


  1. SAN produces an impulse

  2. Impulse stimulates both atria to contract

  3. AVN is also stimulated

  4. AVN delays the impulse briefly

  5. AVN transmits the impulse down to the base of the ventricles

  6. Impulses radiate up through the ventricles, which contract simultaneously about 0.1 s after the atria

  
• Nervous system


  Modulates the natural rhythm of the pacemaker so that the heart rate is adjusted to our activity levels
• Heart rate increases

  When we are exercising and need extra oxygen and nutrients
• Heart rate decreases


  As we sleep
• Medulla

  The part of the brain stem that monitors blood pressure and carbon dioxide levels and sends impulses to speed up or slow down the heart rate
• Emotions such as stress, as well as increases in activity level


  Can cause an increase in heart rate
• During periods of excitement, fear or stress

  The adrenal glands release the hormone epinephrine (also called adrenalin), which stimulates the pacemaker to increase the heart rate
• Arteries


  • Blood vessels that carry blood away from the ventricles of the heart
  • Branch and divide many times forming arterioles and eventually the tiny capillaries that reach all our tissues
  • Have thick outer walls of collagen and elastic fibres
  • Have a ring of circular smooth muscle that contracts with each heartbeat to maintain blood pressure and keep blood moving along
  • Have a narrow lumen to keep blood pressure high
  • Have a lining of smooth epithelial cells to reduce friction and keep blood flowing smoothly

  
• Capillaries


  • The smallest vessels
  • Lumen is only about 10 μm in diameter
  • Blood flow is very slow, at less than 1 mm per second
  • Walls are only one cell thick to minimise the distance for diffusion
  • Some have spaces between cells to allow plasma and phagocytes to leak out into the tissues

  
• Veins


  • Carry blood back towards the atria of the heart from body tissues
  • Have much thinner walls with few elastic and muscle fibres
  • Blood inside does not pulse along and the lumen is large to hold the slow-moving flow
  • Can be compressed by adjacent muscles to help squeeze blood along
  • Contain valves to prevent blood flowing backwards

  
• Arteries

  Convey blood at high pressure from the ventricles to the tissues of the body

  
• Capillaries

  Allow an exchange of materials between cells in the tissue and the blood in the capillary
• Veins

  Collect blood at low pressure from the tissues of the body and return it to the atria of the heart

  
• William Harvey discovered the circulation of the blood with the heart acting as the pump

  
• William Harvey

  • Combined earlier discoveries with his own research findings to produce a convincing overall theory for blood flow in the body
  • Overcame widespread opposition by publishing his results and touring Europe to demonstrate experiments that falsified previous theories and provided evidence for his theory
• Harvey demonstrated that blood flow through the larger vessels is unidirectional, with valves to prevent backflow
• Harvey showed that the heart pumps blood out in the arteries and it returns in veins

  
• Harvey predicted the presence of numerous fine vessels too small to be seen with contemporary equipment that linked arteries to veins in the tissues of the body

  
• Pathogen


  An organism or virus that can cause a disease
• Skin and mucous membranes

  • Form the first line of defence against pathogens
  • Unbroken skin is a tough barrier to potential invaders
  • Openings in the skin are protected by mucous membranes which contain lysozyme to attack bacteria
• Blood clot formation


  1. Platelets, erythrocytes and leucocytes are involved

  2. Clotting factors cause platelets to stick to the damaged area

  3. Prothrombin is activated to thrombin

  4. Thrombin activates fibrinogen to insoluble fibrin which forms a mesh to trap blood cells and plug the gap