Higher biology

• Non-specific body defences


  Physical and chemical defences that protect against invading pathogens
• Physical and chemical defences

  • Epithelial cells form a physical barrier
  • Chemical secretions are produced against invading pathogens
• Closely-packed epithelial cells are found in the skin and inner linings of the digestive and respiratory systems
• Chemical secretions


  • Tears
  • Saliva
  • Mucus
  • Stomach acid
• Pathogen


  A bacterium, virus or other organism that can cause disease
• Inflammatory response


  1. Histamine is released by mast cells causing vasodilation and increased capillary permeability

  2. Increased blood flow leads to an accumulation of phagocytes and clotting elements at the site of infection
• Phagocytes


  • Recognise pathogens and destroy them by phagocytosis
  • Release cytokines which attract more phagocytes to the site of infection
• Phagocytosis


  Engulfing of pathogens and their destruction by digestive enzymes contained in lysosomes
• Cytokines


  Protein molecules that act as a signal to specific white blood cells causing them to accumulate at the site of infection
• Lymphocytes


  • The white blood cells involved in the specific immune response
  • Respond to specific antigens on invading pathogens
• Types of lymphocytes


  • B lymphocytes
  • T lymphocytes
• B lymphocytes


  • Produce antibodies against antigens, leading to the destruction of the pathogen
  • Can respond to antigens on substances that are harmless to the body, causing an allergic reaction
• Antibodies


  • Y-shaped proteins with receptor binding sites specific to a particular antigen on a pathogen
  • Become bound to antigens, inactivating the pathogen
  • The resulting antigen-antibody complex can then be destroyed by phagocytosis
• T lymphocytes


  • Destroy infected body cells by recognising antigens of the pathogen on the cell membrane and inducing apoptosis
  • Can normally distinguish between self-antigens on the body's own cells and non-self-antigens on infected cells
  • Failure of the regulation of the immune system leads to T lymphocytes responding to self-antigens, causing autoimmune diseases
• T lymphocyte attack on infected cells

  1. T lymphocytes attach onto infected cells and release proteins

  2. These proteins diffuse into the infected cells causing production of self-destructive enzymes which cause cell death

  3. The remains of the cell are then removed by phagocytosis
• In autoimmunity, the T lymphocytes attack the body's own cells, causing autoimmune diseases such as type 1 diabetes and rheumatoid arthritis
• Atherosclerosis


  1. Accumulation of fatty material (cholesterol, fibrous material, calcium) forming an atheroma or plaque beneath the endothelium

  2. Artery thickens and loses elasticity

  3. Diameter of lumen reduced

  4. Blood flow restricted

  5. Increased blood pressure
• Atherosclerosis is the root cause of various cardiovascular diseases (CVD) — angina, heart attack, stroke and peripheral vascular disease
• Thrombosis


  1. Endothelium damage

  2. Clotting factors activated

  3. Prothrombin converted to thrombin

  4. Thrombin causes fibrinogen to form fibrin threads

  5. Fibrin threads form a meshwork that clots the blood
• Atheromas may rupture damaging the endothelium, releasing clotting factors that activate a cascade of reactions resulting in the conversion of the enzyme prothrombin to its active form thrombin
• A thrombosis in a coronary artery


  May lead to a myocardial infarction (MI), commonly known as a heart attack
• A thrombosis in an artery in the brain


  May lead to a stroke
• Cells are deprived of oxygen leading to death of the tissues
• The formation of a clot (thrombus) is referred to as thrombosis
• In some cases a thrombus may break loose forming an embolus which travels through the bloodstream until it blocks a blood vessel
• Peripheral vascular disease


  1. Narrowing of the arteries due to atherosclerosis of arteries other than those of the heart or brain

  2. Pain experienced in the leg muscles due to a limited supply of oxygen
• A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, most commonly in the leg, which can break off and result in a pulmonary embolism in the lungs
• Cholesterol


  A type of lipid found in the cell membrane, also used to make sex hormones
• Cholesterol is synthesised by all cells, although 25% of total production takes place in the liver
• A diet high in saturated fats or cholesterol causes an increase in cholesterol levels in the blood
• High density lipoproteins (HDL)


  Transports excess cholesterol from the body cells to the liver for elimination, preventing accumulation of cholesterol in the blood
• Low density lipoproteins (LDL)


  Transports cholesterol to body cells
• LDL receptors, negative feedback control and atheroma formation

  1. Most cells have LDL receptors that take LDL into the cell where it releases cholesterol

  2. Once a cell has sufficient cholesterol a negative feedback system inhibits the synthesis of new LDL receptors

  3. LDL circulates in the blood where it may deposit cholesterol in the arteries forming atheromas
• A higher ratio of HDL to LDL will result in lower blood cholesterol and a reduced chance of atherosclerosis
• Regular physical activity tends to raise HDL levels
• Dietary changes aim to reduce the levels of total fat in the diet and to replace saturated with unsaturated fats
• Drugs such as statins reduce blood cholesterol by inhibiting the synthesis of cholesterol by liver cells