m

the heart pumps deoxygenated blood to the gills

here gaseous exchange takes place; there is diffusion of carbon dioxide from the blood into the ater that surrounds the gills and diffusion of oxygen from this water into the blood

the blood leaving the gills then flows round the rest of the body beofre eventuallt returing to the heart.

animals such as fish have a single cirulaotry system.

stokes are caused by an interruption to the normal blood supply in an area of the brain this may be bleeding from damaged capillarys.or a blockage  cutting of the blood supply to the brain. usally by a blood clot. a blockage in ine of the main arteries leading to the brain can cause a serious strokes or even death. in one of the smaller arterioles the damage may be less serious.

the symptoms of strokes

dizzieness, confusion, slurred speech, blurred vision, or loss of part pf vision. and numbness. 

plaque can build up slowly in the cornory arteries, reduucing blood flow to the parts of the hearrt muscule beyond the plaque. often symptoms are first noticed during exercise. when the cardicac muscle is working harder and needs more oxygen. the narrowed cononary arteries cannot supply enough oxygenated blood and the heart micle has to resort to anaerobic respiration. ehis causes a gripping pain in the chest. 

angina can be helped by eating a low fat diet, taking regular exercise adn losing weight, also not smoking. the symptoms can be treated by drugs, that cause rapid dialations of the conory blood vessles so that they supply the cardiac muscle with the oxygen it needs.

many heartattacks are cause by blood clots leading to atheroscerosis. the wall of the arterienis affected by a plaque is stiffened. making it much more likley to suffer cracks or damage. platelers come into contact with the damaged surface of the damaged surface and a clotting mechanism is triggered.

means hardening of the arteries and is a build of fatty deposites on the inside of the arteries.

once the damage has occoured the body inflammatory response begins, white blood cells accumulate chemical from the blood, in particular cholesterol. this leads to fatty deposits known as atheroma forming on the endthelial lining of the arterie.

fibrous tissire and calcium salts also build up around the atheroma, turning into hardened plaque

this hardened area means that part of the artery wall, so it is less elastic 

the lumen of the artery becomes much smaller as a rsult of plaque 

this increases the blood pressure.

The oxygen dissociation curve is a graph that shows the percent saturation of hemoglobin at various partial pressures of oxygen. Commonly a curve may be expressed with the P₅₀ value.  This is a value which tells the pressure at which the erythrocytes are fifty percent saturated with oxygen

At high partial pressures of oxygen, usually in the lungs, hemoglobin binds to oxygen to form oxyhemoglobin.  When the blood is fully saturated all the erythrocytes are in the form of oxyhem

The sigmoid shape of the oxygen dissociation curve is a result of the cooperative binding of oxygen to the four polypeptide chains.  Cooperative binding is the characteristic of a hemoglobin to have a greater ability to bind oxygen after a subunit has bound oxygen ).  Thus, hemoglobin is most attracted to oxygen when three of the four polypeptide chains are bound to oxygen.  oglobin.  As the erythrocytes travel to tissues deprived of oxygen the partial pressure of oxygen will decrease.  Consequently, the oxyhemoglobin releases the oxygen to form hemoglobin