Salters-Nuffield advanced biology for Edexcel AS.

by (2008)
ISBN-10 1405896078 ISBN-13 9781405896078
40 Flashcards & Notes
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Summary - Salters-Nuffield advanced biology for Edexcel AS.

  • 1.1 What is Cardiovascular Disease?

  • How Does the Circulation Work?:
    Arteries and veins can easily be distinguished. The walls of both contain collagen for strength, elastic fibres to stretch and smooth muscle cells in the walls allowing to constrict/dilate
    They differences are:
    • narrow lumen (space)
    • thicker walls
    • more collagen, elastic fibres and smooth muscle
    • no valves
    • wide lumen (space)
    • thinner walls
    • less collagen, elastic fibres and smooth muscle
    • valves
    How does blood move through the vessels?:
    • Every time the heart contracts (systole), blood is forces into arteries and their elastic walls stretch to accommodate the blood
    • During diastole (relaxation of the heart), the elasticity of the artery walls causes them to recoil behind the blood, helping to push the blood forward
    • The blood moves along the artery as each section in series stretches and recoils in this way
    • When the blood reaches the smaller arteries and capillaries, there's a steady flow of blood
    • In the capillaries this allows exchange between the blood and the surrounding cells through the one-cell-thick capillary walls
    • The heart has a less direct effect on blood flow through the veins
    • In veins, blood flow is assisted by the contraction of skeletal muscles during the movement of limbs and breathing
    • Low pressure developed in the thorax (chest cavity) when breathing in also helps draw blood back into the heart from the veins
    • The steady flow without pulses of blood means that the blood is under lower pressure in veins
    • The heart needs constant blood supply and this is done via the coronary arteries on the surface of the heart
  • How the Heart Works
    • The chambers of the heart alternately contract (systole) and relax (diastole) in a rhythmic cycle - one complete sequence is a cardiac cycle, or heartbeat
    • During systole, cardiac muscle contracts and the heart pumps blood out through the aorta and pulmonary arteries
    • During diastole, cardiac muscle relaxes and the heart fills with blood
    • The cardiac cycle can be simplified onto three phases: atrial systole, ventricular systole and diastole
    Phase 1: Atrial Systole:
    • Blood returns to the heart due to the action of skeletal and gaseous exchange muscles as you move and breathe
    • Blood under low pressure flows into the left and right atria from the pulmonary viens and vena cava
    • As the atria fill, the pressure of blood against the atrioventricular valves pushes them open and blood begins to leak into the ventricles
    • The atria walls contract, forcing more blood into the ventricles - known as atrial systole
    Phase 2: Ventricular Systole:
    • Immediately follows atrial systole - the ventricles contract from the base of the heart upwards, increasing pressure in the ventricles
    • This pushes blood up and out through the arteries
    • The pressure of blood against the atrioventricular valves closes them and prevents blood flowing back into the atria
    Phase 3: Diastole:
    • The atria and ventricles then relax during diastole - elastic recoil of the relaxing heart walls lowers pressure in the atria and ventricles
    • Blood under high pressure in the arteries is drawn back towards the ventricles, closing the semilunar valves and preventing further backflow
    • The coronary arteries fill during diastole - low pressure in the atria helps draw blood into the heart from the veins
  • Key Biological Principle: Why Have a Heart and Circulation?:
    • Heart and circulation have one primary purpose - to move substances around the body
    • In very small organisms, substances such as oxygen, carbon dioxide etc. move around the organism via diffusion
    • Diffusion is the movement of molecules or ions from a region of high concentration to low concentration
    • More complex organisms are too large for diffusion to move substances quickly enough
    • These animals have blood to carry vital substances around their bodies and a heart to pump it - a circulatory system
    Open Circulatory Systems:
    • In insects and some other animals, blood circulates in large open spaces
    • A simple heart pumps blood out into cavities surrounding the animal's organs
    • When the heart muscle relaxes, blood is drawn from the cavity back into the heart, through small valved openings along its length
    Closed Circulatory Systems:
    • Vertebrates have a circulatory system where the blood is enclosed within tubes
    • This generates higher blood pressures as the blood is forced along narro channels instead of flowing into large cavities
    • The blood leaves the heart under pressure and flows along arteries and arterioles (smaller arteries) to capillaries
    • There are many capillaries - these come into close contact with most body cells, where substance exchange takes place between blood and cells
    • After passing along the capillaries, the blood returns to the heart by means of venules (small veins) and then veins
    • Valves ensure that the blood flows only in one direction
    Single Circulatory Systems:
    This type of circulation is found in (e.g.) fish:
    • The heart pumps deoxygenated blood to the gills
    • Here gaseous exchange takes place; there's diffusion of carbon dioxide from the blood into the water surrounding the gills, and diffusion of oxygen from this water to the blood
    • The blood leaving the gills then flows round the rest of the body before eventually returning to the heart
    • Note that the blood flows through the heart once for each complete circuit of the body
    Double Circulatory Systems:
    Birds and mammals have double circulation:
    • The right ventricle of the heart pumps deoxygenated blood to the lungs where it receives oxygen
    • The oxygenated blood then returns to the heart to be pumped a second time (by left ventricle) out to the rest of the body
    • This means the blood flows through the heart twice for each complete circuit of the body
    • The heart gives the blood returning from the lungs and extra 'boost', which reduces the time it takes for the blood to circulate round the body
    • This allows birds and mammals to have a high metabolic rate, because oxygen and food substances required for metabolic processes can be delivered more rapidly to cells
  • What is Atherosclerosis?:
    • The disease process that leads to CHD and strokes
    • In atherosclerosis, fatty deposits can either block an artery directly, or increase its chance of being blocked by a blood clot - blood supply can be blocked completely
    • In the arteries supplying the heart it leads to heart attack; in the arteries supplying the brain it leads to stroke
    What Happens in Atherosclerosis?:
    It can be triggered by a number of factors, but the following events occur:
    1. The endothelium separating the blood in arteries from the muscular walls become damaged for whatever reason
    2. Once the inner lining of the artery is breached, an inflammatory response occurs. White blood cells accumulate chemicals, e.g. cholesterol. A deposit builds up called an atheroma
    3. Calcium salts and fibrous tissue also build up, resulting in a hard swelling called plaque. This build up causes artery to lose its elasticity, essentially hardening
    4. Plaques cause the artery to become narrower, making it more difficult to pump blood through, raising blood pressure
    Why does the blood clot in arteries?:
    • The direct contact of blood with collagen within the damaged artery triggers a complex series of chemical changes in the blood
    • A cascade of changes result in the soluble plasma protein prothrombin being converted to thrombin
    • Thrombin is an enzyme that catalyses the conversion of another soluble plasma protein, fibrinogen, into long insoluble strands of the protein fibrin
    • The fibrin strands form a tangled mesh that traps blood cells to form a clot
  • 1.2 Who is at Risk of Cardiovascular Disease

  • Probability and Risk
    What do we Mean by Risk?:
    • Risk is defined as 'the probability or occurrence of some unwanted event or outcome'.
    • Probability has a precise mathematical meaning and can be calculated to give a numerical value for the size of the risk
    Working out Probabilities:
    • There are six faces on a die, only one face has six dots, so there's a 1 in 6 chance of throwing that face
    • Expressed as a decimal; 0.1666666 recurring - every time you throw a dice, there's a 17% chance of throwing a six
    Estimating Risks to Health:
    • In 2005, 19,429 people in the UK died due to injuries or poisoning. The total UK population at the times was 60,209,408
    • We can calculate the average risk in a year of someone in the UK dying from injuries or poisoning as:
    • 1 in 60,209,408/19,429
    • =1 in 3099
    • =1/3099 = 0.00032 or 0.032%
    Perception of Risk:
    People will overestimate the risk of something happening if the risk is:
    • involuntary
    • not natural
    • unfamiliar
    • dreaded
    • unfair
    • very small
    There's a tendency to overestimate the risks of sudden imposed dangers where the consequences are sever, and to underestimate a risk if it has an effect in the long-term
    When we lack the data to estimate a risk precisely, we're uncertain about the risk. E.g., we're uncertain about the environmental consequences of many chemicals
  • Different Types of Risk Factor
    In the UK the estimated risk of having a fatal heart disease is 1 in 600, compared to 1 in 1050 for a fatal stroke
    These probabilities are an average figure that don't take risk factors into account, which may increase probability of contracting these conditions
    There are may different factors that contribute to health risks, for example:
    • Heredity
    • Physical environment
    • Social environment
    • Lifestyle and behaviour choices
    Identifying Risk Factors - Correlation and Causation:
    • Two variables are positively correlated when an increase in one sees an increase in the other
    • E.g., the length of a TV programmes and the %age of the class asleep may be positively correlated
    • A negative correlation occurs when one variable decreases while the other increases
    • It's important to realise that a correlation doesn't mean the variables are causally linked
    • Two variables are causally linked when a change in one is responsible for a change in the other
    • E.g., worldwide, speaking English as your first language correlates well with having a higher life expectancy - but they're not causally linked
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