AQA Biology AS Unit 1: The Lungs/ The Heart/ Diseases

Covers chapters 4 and 5

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4 Lungs and Lung Disease

4.1 Structure of the Human Gas-Exchange System

  • Lungs act as an interface for gas exchange
  • All aerobic organisms require constant oxygen to release ATP
  • The volume of oxygen absorbed and the CO2 removed is large in mammals:
    • Have large volume of living cells
    • Maintain a high body temperature so have high metabolic and respiratory rates

Mammalian Lungs

  • Protected by the rib cage which can move allowing a tidal stream to flow through lungs
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Structure of the Human Gas-Exchange System Continu

  • Main parts of Lungs:
    • Lungs: lobed structure made up of bronchioles with alveoli
    • Trachea: flexible airway supported by cartilage rings which stop it collapsing under air pressure . Tracheal walls made of muscles lined with epithelium and goblet cells. Goblet cells produce mucus that traps dirt and bacteria, cilia move mucus up the throat into the stomach
    • Bronchi: Produce mucus to stop bacteria and dirt, larger bronchi supported by cartilage
    • Bronchioles: series of branching subdivisions of bronchi, walls are made of muscle lined with epithelial cells, muscles constrict allowing the control of air in and out
    • Alveoli: minute air sacs 100-300 micrometers in diameter, contain some collagen and elastic fibres, lined with epithelium. Elastic fibres allow alveoli to stretch as fill with air. The alveoli membrane is the gas exchange surface
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4.2 The Mechanism of Breathing

  • The process of moving air in and out of the lungs is called breathing/ventilation
  • When air pressure of atmosphere is greater than inside the lungs air is forced into the alveoli: inspiration/inhalation
  • When air pressure of atmosphere is lower than in the lungs air is forced out of the lungs: expiration/ exhalation
  • Pressure change is brought by two sets of muscles:
    • The diaphragm: sheet of muscles separating the thorax from abdomen
    • Intercostal muscles: lie between the ribs there are two sets: internal and external intercostal muscles.
      • Internal contractions lead to expiration
      • External contractions lead to inspiration

Inspiration (Requires energy as is an active process)

  • External intercostal muscles contract, internal relax
  • Ribs are pulled up and out increasing thorax volume
  • Diaphragm muscles contract so it flattens increasing thorax volume
  • Increased thorax volume reduces lung pressure forcing air into the lungs as lower than atmospheric
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The Mechanism of Breathing Continued

Expiration (Passive process)

  • Internal intercostal muscles contract, external relax
  • Ribs move down and in decreasing thorax volume
  • Diaphragm muscles relax so it domes decreaing thorax volume
  • Decreased thorax volume increases pressure in the lungs
  • Pulmonary pressure is greater than atmospheric so air is forced out of lungs

Pulmonary Ventilation

  • The total volume of air that is moved into lungs during one minute
  • Pulmonary ventilation=Tidal volume X Ventilation rate
  • Tidal volume is volume of air taken in at each breath
  • Ventilation rate is the number of breaths taken in one minute
  • PV is measured in dm3min-1
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4.3 Exchange of Gases in the Lungs

Essential feature of exchange surfaces

  • Large Surface area to volume ratio: speeds up exchange
  • Thin: keep diffusion pathway short
  • Partially Permeable: allow selected material to diffuse
  • Movement of environmental medium: e.g. air to maintain diffusion gradient
  • Movement of internal medium: e.g. blood to maintain diffusion gradient
  • Ficks Law: Diffusion is proportional to: Surface Area X Difference in Concentration
  •                                                                            Length of Diffusion Path

Role of the Alveoli in Gas Exchange (Improve diffusion rate)

  • Red blood cells slowed as pass pulmonary capillaries
  • Distance between alveolar air and red blood cells reduced as RBC flattened against capillary wall
  • Walls are very thin reducing diffusion distance
  • Alveoli and pulmonary capillaries have large surface area
  • Steep concentration gradient due to blood circulation and breathing movement of lungs
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4.4 Lung Disease - Pulmonary Tuberculosis

Causes and Symptoms

  • TB is caused by mycobacterium tuburculosis or mycobacterium bovis
  • Symptoms include persistant cough, tiredness and loss of appetite
  • As the disease develops fever and coughing up blood occur

Transmission

  •  Through air by droplets released in the air by coughs, sneezes or laughing
  • M. tuberculosis is a resistant bacterium so can survive seven weeks after droplets dry
  • It develops over time with close contact
  • Can be spread from cattle to humans
  • Some groups are at greater risk: people with close contact to TB e.g. due to overcrowding, working or living with TB carriers, certain nationalities, those with reduced imunity (children, people with AIDS, the elderly)
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Lung Disease - Pulmonary Tuberculosis Continued

Course of Infection

  • Bacteria grows and divides within upper region of the lungs where plentiful oxygen
  • Immune System Responds and white blood cells accumulate at site of infection
  • Leads to inflammation of the lymph nodes that drain the area: Primary Infection
  • The infection, in a healthy person, is controlled within weeks
  • Some bacteria usually remains and the infection re-emerges: Post-Primary Infection
  • It isn't so easily controlled, the bacteria destroys the tissue of the lungs causing cavities and scar tissue
  • The sufferer coughs up damaged lung tissue containing bacteria
  • Without treatment TB spreads and can be fatal
  • Preventing TB depends on scientific understanding and public, political and economic circumstances
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4.5 Lung Disease - Fibrosis

Pulmonary Fibrosis

  • Arises when scars form on lung epithelium causing them to thicken
  • This causes oxygen to not be able to diffuse efficiently
  • It also reduces the elasticity of the lungs making it difficult to breath out
  • Effects on lung function:
    • Shortness of breath: due to reduced volume of air space because of fibrous tissue, less oxygen enters the lungs. Thickened epithelium increases length of diffusion pathway so diffusion is slower. Loss of elasticity makes ventilating difficult reducing concentration gradient
    • Chronic, dry cough: fibrous tissue obstructs airways causing the cough
    • Pain in chest: due to pressure from fibrous tissue causing coughing thus scarring
    • Weakness: reduced intake of oxygen into blood, causing cellular respiration to be reduced so less energy
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Lung Disease - Asthma

Asthma

  • It is a localised allergic reaction
  • Allergens that stimulate it are: pollen, animal fur and dust mites faeces
  • It can be triggered/made worse by air pollutants, exercise, cold air, infection and stress
  • Allergens cause white blood cells on bronchi and bronchioles lining to release histamine:
    • Causes the linging of airways to be inflamed
    • Cells of epithelial lining secrete larger quantities of mucus
    • Fluid leaves capillaries and enters airways
    • Muscle surrounding bronchioles contract and constrict airways
  • Symptoms of asthma:
    • Difficulty breathing: constricted bronchi/bronchioles as inflammed and mucus filled
    • Wheezing: caused by air passing through constricted bronchi/bronchioles
    • Tight chest: not able to ventilate lungs easily due to constriction
    • Coughing: response due to blocked bronchi/bronchioles
  • Genetics can play a role in asthma, so does air pollution and chemicals in food products
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Lung Disease - Emphysema

Emphysema

  • Develops over 20 years and is impossible to detect
  • Healthy lungs contain large quantities of elastic tissue mostly elastin
  • Tissue stretches when we breath in and springs back when we breath out
  • In emphysema lungs, due to smoking, elastin is permanently stretched
  • This stops lungs being able to force out air from alveoli
  • Surface area of alveoli is also reduced, alveoli sometimes burst
  • Little gas exchange occurs causing symptoms:
    • Shortness of breath: Difficulty exhaling air due to loss of elastin, difficult to inhale fresh oxygen causing breathlessness. Smaller alveolar surface area reduces levels of oxygen in the blood so breathing more rapidly
    • Chronic Cough: Due to lung damage and body's effort to remove damaged tissue and mucus but can't as cilia were destroyed by smoking
    • Bluish skin colour: Low levels of oxygen in blood due to poor diffusion
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The Heart and Heart Disease

5.1 The Structure of the Heart

  • Lies in the thoracic cavity behind sternum
  • Separated into two separate pumps side by side
  • Left pump deals with oxygenated blood from lungs
  • Right pump deals with deoxygenated blood from the body
  • Both contain two chambers:
  • Atrium: thin walled and elastic, it stretches as it collects blood, thin walled as only has to pump short distance to ventricle
  • Ventricle: thicker muscular wall as has to pump to lungs or the rest of the body
  • There are two separate pumps in order to maintain pressure, blood is pumped back to the heart to increase pressure to distribute blood to the rest of the body
  • Deoxygenated blood has to be kept separate from oxygenated blood
  • The right ventricle is thinner than the left as it only has to pump to the lungs
  • The left ventricle has to create enough pressure to send the blood around the body
  • Both sides of the heart pump in time  with both atria contracting together as well as ventricles
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The Structure of the Heart Continued

  • Between atrium and ventricle are valves that prevent blood backflow into atria when ventricles contract:
    • Left Atrioventricular valve: formed of two cup-shaped flaps on left side of the heart
    • Right Atrioventricular valve: formed of three cup-shaped flaps on right side
  • Ventricles pump blood away from the heart into the arteries
  • Atria receive blood from the veins
  • Vessels connecting lungs to heart are the pulmonary vessels:
    • Aorta: connected to left ventricle and carries oxygenated blood to body
    • Vena Cava: connected to right atrium, brings deoxygenated blood back from tissues
    • Pulmonary Artery: connected to right ventricle, carries deoxygenated blood to lungs
    • Pulmonary Vein: connected to left atrium, brings oxygenated blood back from lungs

Supplying the Heart Muscle with Oxygen

  • The heart muscle is supplied by its' own blood vessels: coronary arteries, (branch off the aorta)
  • Blockage of arteries could lead to myocardial infarction- muscle deprived of oxygen dies
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5.2 The Cardiac Cycle

  • Two phases: contraction (systole) and relaxation (diastole)
  • Contraction occurs separately in the ventricles and atria in two stages

Relaxation of the Heart (Diastole)

  • Blood returns to atria through pulmonary vein and the vena cava
  • As atria fill pressure rises pushing open atrioventricular valves allowing blood into ventricles
  • Muscular walls of atria and ventricles relax
  • Ventricle relaxation reduces pressure within them so it is lower than in aorta and pulmonary artery
  • Causes semi lunar valves in aorta and pulmonary artery close causing 'dub' sound

Contraction of Atria (Atrial Systole)

  • When contracts forces blood into ventricles
  • Pushed a short distance so atria walls thin
  • Muscles of ventricle walls remain relaxed
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The Cardiac Cycle Continued

Contraction of  Ventricles (Ventricular Systole)

  • Short delay to allow ventricles to fill then walls contract
  • Increases blood pressure within them forcing shut atrioventricular valves preventing backflow into atria
  • 'Lub' sound occurs as valves close
  • With the valves closed pressure rises forcing semi lunar valves open making blood flow into aorta and pulmonary artery

Valves in Control of Blood Flow

  • Blood flowing in the right direction is done by pressure created by the heart
  • Valves are used to prevent unwanted backflow
  • Blood usually travels from high pressure to low pressure
  • Valves open when difference in blood pressure favours movement of blood in required direction
  • Designed to close if blood flow tends to flow in opposite direction to favoured
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The Cardiac Cycle Continued

  • Atrioventricular valves: between atrium and ventricle , prevent backflow of blood when contraction of ventricles causes ventricular pressure to exceed atrial pressure. Closure ensures blood within ventricles moves to aorta and pulmonary artery not atria
  • Semi-Lunar valves: in aorta and pulmonary artery, prevent backflow of blood into ventricles when recoil action of elastic walls of vessels created greater pressure in vessels than ventricles
  • Pocket valves: in veins, ensure when veins squeezed blood flows back to heart
  • All valves are made up of tough fibrous tissue
  • When pressure is greater on the convex side they pull apart letting blood pass
  • When pressure is greater on concave side they push together to form a tight grip
  • Due to the great pressure the ventricles could become inverted, this is prevented by the valves having string like tendons attached to pillars of muscle in ventricle wall

Cardiac Output

  • Cardiac Output = Heart Rate X Stroke Volume
  • Heart rate is rate heart beats, Stroke volume is volume of blood pumped out
  • Measured in dm3min-1
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The Cardiac Cycle Continued

How the Cardiac Cycle is Controlled

  • Cardiac muscle is myogenic so controlls its own contractions
  • Within right atrium is the sinoatrial node (SAN) - known as the pacemaker
  • The cardiac cycle follows:
  • Wave of electrical activity spreads from SAN across atria causing them to contract
  • The atrioventricular septum (layer of non-conducting tissue) prevents wave crossing to ventricles
  • Wave passes through atrioventricular node (AVN) - between atria
  • After short delay AVN conveys electrical wave along muscle fibres - bundle of His
  • Bundle of His conducts wave through atrioventricular septum to base of ventricles
  • Wave of electrical activity released through small fibres causing ventricles to contract from the apex to the heart
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The Cardiac Cycle Continued

Pressure and Volume changes of the Heart

  • Mammals have closed circulatory systems so blood confined to vessels allowing pressure to be maintained
    • Aortic Pressure: rises when ventricles contract as blood forces through, gradually falls never bellow 12kPa due to elasticity of the wall as has recoil action.Recoil causing temporary rise in pressure at start of relaxation
    • Atrial Pressure: relatively low due to thin walls, highest when contracting but drops when left atrioventricular valve closes and walls relax. Atria then fill with blood, leads to build up of pressure so left atrioventricular valve opens moving blood to ventricles
    • Ventricular Pressure: low at first but increases as ventricles fill with blood as atria contract. The left atrioventricular valves close and pressure rises as thick muscular walls of ventricle contract. Pressure rises above aorta causing blood to flow into aorta past semi-lunar valves. Pressure falls as ventricles empty and walls relax
    • Ventricular Volume: rises as atria contract and ventricles fill with blood, then drops suddenly as blood forced out into aorta when semi-lunar valve opens. Volume increases again as ventricles fill with blood
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5.3 Heart Disease

  • Coronary Heart Disease (CHD) affects the coronary arteries which supply the heart muscles with glucose and oxygen required for respiration
  • Build up of atheroma stops blood flow to the heart causing mycardial infarction

Atheroma

  • It is a fatty deposit that forms within walls of artery
  • Begins as fatty streaks that are accumulations of white blood cells that have taken up low density lipoproteins (LDLs)
  • The streaks enlarge to form an irregular patch/atheromatous plaque
  • Most commonly occurs in larger arteries made up of deposits of cholesterol, fibres and dead muscle cells
  • They bulge into the lumen causing it to narrow so blood flow is reduced
  • Atheroma increases the risk of thrombosis and aneurysm
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Heart Disease Continued

Thrombosis

  • If atheroma breaks through endothelium of the blood vessel it forms a rough surface
  • It can result in formation of a blood clot/thrombus
  • The thrombus may block the blood vessel reducing blood flow to tissues
  • The blood deprived tissues often die as have lack of glucose, oxygen and nutrients
  • Sometimes the thrombus moves and lodges itself in other arteries

Aneurysm

  • Atheroma that leads to formation of thrombus weakens artery walls
  • Weakened points swell to form balloon like, blood filled structures: aneurysm
  • Aneurysms frequently burst leading to a haemorrhage which causes lose of blood to area
  • A brain aneurysm is a cerebrovascular accident or stroke
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Heart Disease Continued

Myocardial Infarction/ Heart Attack

  • It is the reduced supply of oxygen to the muscle of the heart
  • Results from blockage in coronary artery, if it occurs near junction with aorta the heart stops beating as block supply is completely cut off
  • Symptoms will be milder if further away since smaller area of muscle has oxygen deffiecentcy

Factors associated with Coronary Heart Disease

  • Smoking:
    • Carbon Monoxide: combines irreversibly with haemoglobi to form carboxyhaemoglobin, reduces oxygen carrying capacity so the heart must work harder to gain enough oxygen, this leads to raised blood pressure increasing risk of CHD and strokes. Less oxygen leads to chest pain during exercise
    • Nicotine: stimulates adrenaline production which increases heart rate and raises blood pressure. Also makes red blood cells more sticky increasing risk of thrombosis hence strokes and heart attacks
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Heart Disease Continued

Factors associated with Coronary Heart Disease

  • High Blood Pressure: factors like prolonged stress, certain diets and lack of exercise increase blood pressure, factors like genetics cannot be changed, CHD is increased:
    • Already high blood pressure in arteries so the heart has to pump harder
    • More likely to develop an aneurysm and burst causing haemorrhage
    • To resist high pressure the arteries walls become thickened and may harder restricting blood flow
  • Blood Cholesterol:essential component of membranes carried in plasma as tiny spheres of lipoproteins:
    • High Density Lipoproteins (HDL) remove cholesterol from tissues transporting it to the liver for excretion, help protect against heart disease
    • Low Density Lipoproteins (LDL) transport cholesterol from liver to tissues including artery walls, which thety infiltrate leading to atheroma and heart disease
  • Diet: high salt levels increase blood pressure, high saturated fats increase LDLs hence blood cholesterol concentration
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