Blood Clotting
In my previous post about blood, I mentioned that platelets are responsible for blood clotting but I didn't go into too much detail. I'm still not going to go into too much detail, but I will go into more detail than I did last time ;)
For most small tears to blood vessels, not too much has to be done. First of all, the muscles in the walls of the blood vessels constrict to reduce blood loss, and then platelets stick to the rough surface caused by damage (normally the internal walls of blood vessels are smooth, but not when damaged). As more and more platelets are attracted to the injury, the platelets begin to stick to each other and form a plug which reduces blood loss further. Next the platelets release substances that prolong the constriction of the damaged vessels until they heal themselves.
If the damage is much worse, then coagulation (a.k.a. blood clotting) is necessary. Many reactions take place involving a large number of chemical substances in the plasma called clotting factors. These result in the formation of threads of insoluble protein, which trap the cells in the blood, forming a clot. The threads hold the clot in position, before a slower process called clot retraction takes place, in which the threads contract, becoming denser and stronger and pulling the edges of the blood vessels together. While this happens, a fluid called serum is squeezed out. After this, the clot dries and forms a scab which protects the wound from infections.
There are several factors that stop blood from forming clots in undamaged vessels. First of all, as mentioned before, undamaged blood vessels have smooth walls that platelets can't stick to. Secondly, the plasma also contains anticlotting factors.
Blood Groups
I'm sure you've all heard of the ABO and Rhesus blood group systems, right? Well, here I'm going to explain how they work.
Basically your red blood cells (erythrocytes) may or may not contain specific antigens which determine your blood group. Antigens are substances that can stimulate the formation of antibodies, and the antigens on your red blood cells can consist of either antigen A only, antigen B only, both antigens A and B, or neither antigen. This corresponds to blood groups A, B, AB and O, respectively.
Meanwhile, your plasma contains the antibodies that react against the antigens that you don't have- for example, if you have group A blood (i.e. your red blood cells only have antigen A), you will have anti-B antibodies. Similarly, group B blood has anti-A antibodies. Type AB blood doesn't have any antibodies, while type O blood has both anti-A and anti-B antibodies.
You can't receive blood that contains antigens that your antibodies are going to attack, otherwise the donated blood will clump and disintegrate. For example, if someone has group B blood, with anti-A antibodies in the plasma, receiving A or AB blood will just result in a lot of cells clumping together before disintegrating. Type AB is probably the best off in this regard- their blood has no anti-A or anti-B antibodies, so they can receive blood from any other blood group! Type O, on the other hand, can only receive type O blood (since type O has both anti-A and anti-B antibodies), but thankfully type O is the most common type of blood (followed by type A, then type B, then type AB). Generally attempts are made to give the patient blood of the same type, but if that fails then, unless the patient is O negative (more on the Rhesus +/- system in a bit), another compatible type is given.
One thing that I don't really understand is why it isn't a problem the other way. As in, if someone has type AB blood and they receive type O, why don't the anti-A and anti-B antibodies in the donated blood react with the A and B antigens in the patient's blood? Could someone please answer this for me?
Now we're onto the Rhesus system! As well as the antigens listed above, there are also Rh antigens which might also appear on our red blood cells! If you have the antigens, you're Rh positive; if not, you're Rh negative. If you're Rh negative, your body will most likely produce anti-Rh antibodies when confronted with Rh positive blood. This is fine the first time, but subsequent exposures will cause your newly-created antibodies to spring into action.
This can also be a bit of a problem during pregnancy, if an Rh-negative woman gives birth to multiple Rh-positive children. The first child will cause the mother's body to produce anti-Rh antibodies; a second Rh-positive child might have its blood cells attacked by these antibodies. This can be prevented or managed through several medical interventions, including Rh immune-globulin injections. (See http://kidshealth.org/parent/pregnancy_center/your_pregnancy/rh.html for more info.)
Transfusions
Determining the patient's blood type is necessary for transfusions. As stated above, attempts are made to get the best match possible, or at the very least a compatible blood type. Another kind of transfusion is an autologous transfusion where the patient's own blood is used, which carries fewer risks, but obviously has a lot of drawbacks (for example, they are pretty much only used for non-emergency surgeries that are planned in advance, so they know when to collect your blood).
Aside from whole blood transfusions, there are several other kinds of transfusions, including red cell concentrates (the red cells are separated out, but the concentrate may also include platelets and leucocytes), plasma, platelet concentrates, cryoprecipitate (it's what you get if you freeze the plasma and let the other bits thaw out. Contains substances necessary for blood clotting) and immunoglobulins (contain antibodies against particular diseases).
Cardiovascular Diseases
Cardiovascular diseases, or heart diseases, come in many different varieties. Many of them are related to arteriosclerosis, which is a "hardening of the arteries." One type of arteriosclerosis, atherosclerosis (which looks too similar to "arteriosclerosis" IMO, which might be why I keep mixing the two up), is caused by the deposition of fatty substances onto the artery walls which creates fatty deposits known as plaques, which then cause fibrous tissue to develop, which results in the deposition of salts, which results in hard, calcified areas. These hard, calcified areas then cause obstructions and result in the artery walls losing their elasticity.
One treatment for atherosclerosis is balloon angioplasty. First the patient is given drugs that help to dissolve clots, and then a tube with a tiny balloon at the tip is inserted. When the balloon is inflated, the artery expands, loosening the plaque from the wall.
Bypass surgery can be used to treat more severe cases where the blockage is leaving the patient at risk of a heart attack. Veins are taken and placed on the outside of the heart to allow the circulatory system to bypass those arteries affected by atherosclerosis.
Arteriosclerosis can lead to several other heart cardiovascular diseases:
- Coronary heart disease is a result of atherosclerosis in the coronary arteries (arteries that supply blood to the heart- see my post on the circulatory system). The obstructing plaques restrict the supply of blood to the heart muscle, so there might not be enough oxygen supply to the heart, particularly during exercise.
- Angina is pain in the chest that normally occurs during physical activity. It is normally called by atherosclerosis of the coronary arteries. Normally, the pain subsides during rest and there is no damage to the heart muscle, so people with angina can still lead relatively normal lives.
- A heart attack, or myocardial infraction, is a result of a complete stop in blood flow to a part of the heart muscle. A severe obstruction in the coronary arteries is usually the cause- sometimes this is due to plaque, or it might be due to a blood clot (clots can form on the rough surface caused by plaque). As oxygen supply is halted, the muscle in that area dies, resulting in sudden and severe chest pain. If only a small area is affected, it can heal- firstly, scar tissue forms, and then arterial branches develop to supply blood. Heart attacks, can, however, have very adverse effects: if it is severe enough to disturb the normal beat of the heart, cardiac arrest, where the heart pumps little to no blood, may occur. This can result in death within minutes.
If someone is displaying the signs of a heart attack, you should get them to hospital immediately. Symptoms include chest pain, pain radiating to the left arm, shoulder, neck or jaw, sweating, breathlessness, faintness and palpitations. - A stroke occurs when atherosclerosis affects the cerebral arteries (arteries supplying the brain with blood). This causes part of the brain tissue to die. Aside from blocked arteries, another cause of a stroke is cerebral haemorrhage, where bleeding occurs at a weak point in one of the arteries. Strokes range from mild to severe, some allowing people to continue leading normal lives, others resulting in death.
- A transient ischaemic attack (TIA) is like a stroke, but shorter, with less severe symptoms. They do not cause any permanent damage, but people who have a TIA should seek treatment as a TIA shares the same causes as other cardiovascular diseases (i.e. if you have a TIA, you're at more risk of stroke etc.)
- Peripheral vascular diseases are cardiovascular diseases affecting the limbs. One type of peripheral vascular disease is arteriosclerosis in the limb arteries, slowing blood supply to a limb. Other types of peripheral vascular disease include phlebitis (inflammation of a vein) which may cause blood clots, or varicose veins (enlarged and lengthened veins which cannot carry blood back to the heart efficiently, potentially resulting in the accumulation of blood in lower limbs).
Aside from these diseases, there are many other kinds of heart diseases. One other is congestive heart failure, or simply "heart failure," which is where the heart is too weak to pump sufficient amounts of blood. This could be due to excessive alcohol consumption, a heart attack, or an infection.
Now that we know what types of heart disease there are, let's look at factors that influence your risk at getting a heart disease so that you can avoid some of them! (Granted, there are some that you can't avoid, for example those related to gender and age, but lifestyle changes can go a long way.)
- Age: Incidence of atherosclerosis increases with age.
- Gender: Men are more likely to suffer from cardiovascular diseases at a younger age than women are (women rarely suffer from cardiovascular diseases until around 45 years old).
- Blood cholesterol: Blood cholesterol, a measure of the fat content of the blood, can be controlled via saturated fat intake and exercise levels. Higher blood cholesterol puts you more at risk of heart disease.
- Blood pressure: Smoking, high alcohol consumption and high salt consumption are all factors that can lead to hypertension, or high blood pressure. High blood pressure is another risk factor for heart disease.
- Weight: Being overweight is another risk factor for heart disease, as there is more strain on the heart and lungs.
- Smoking: Smoking constricts blood vessels, causing hypertension. Also, the CO in smoke combines with haemoglobin, reducing the capacity of the blood to carry oxygen. Not a good combination.
- Alcohol: Consuming too much can lead to hypertension or heart muscle weakness.
- Heredity: If you have a family member with cardiovascular disease, you might be at greater risk.
So, basically, if you want the best chance at avoiding cardiovascular diseases, reduce your saturated fat and salt intake, get in a decent amount of exercise, don't smoke and don't drink excessively. Oh, and choose your parents wisely. (Not that you can. Oops.)
Next up: the respiratory system! When I can be bothered to get off the computer and do some study, that is.
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