Capillary shift mechanisms aid in the maintenance of the blood pressure. Due to the permeability of the capillaries, when there is an increase in blood pressure, the high pressure forces fluid to move out of the capillaries into the interstitial space thus reducing the volume thus reducing the pressure. The opposite takes place when there is a decrease in blood pressure. Due to the low pressure, fluid diffuses into the capillaries increasing the volume thus the pressure. Vascular stress relaxation is another mechanism in which blood pressure is regulated. Blood vessels are able to stretch when there is an increase in volume. This will then cause a drop in blood pressure to return it to normal limits. The opposite is also true, blood vessels narrows due to the decrease in volume in order to increase the pressure and returning it to normal levels.
The Renin Angiotensin Aldestrone System (RAAS) is a very important mechanism in maintaining blood pressure and water balance. The mechanism is activated when the blood volume is low. The liver secretes Angiotensinogen which is converted to Angiotensin I using renin (secreted by the kidneys). Angiotensin is converted to Angiotensin II using the Angiotensin converting enzyme (ACE). Angiotensin II has a number of effects which include 1) increasing sympathetic activity – this will increase the blood pressure by increasing the heart rate which in turn increases the cardiac output. Increasing the cardiac output will increase the volume of blood ejected from the heart thus increasing the blood pressure.
2) Increases the reabsorption of Na+ and Cl- and excretion of K+. This will therefore increase water retention thus increasing the blood pressure due to the increase in the volume of fluid in the blood vessels. 3) Stimulates the adrenal gland cortex to secrete Aldestrone which also increases the reabsorption of Na+ and Cl- and excretion of K+ thus increasing water retention. 4) Angiotensin II causes arteriolar vasoconstriction thus increasing the blood pressure. 5) Finally Angiotensin II stimulates the pituitary gland to secrete ADH (anti diuretic hormone). ADH promotes water reabsorption in the collecting duct and the distal convoluted tubule thus increasing the volume of fluid in the vessels therefore increasing the blood pressure.
Hypertension is a condition where blood circulates through the vessels at a high pressure (higher pressure than normal). One of the big problems with hypertension is that sometimes there are no symptoms associated. This means that it can cause more problems if it is unnoticed. High risk hypertension can cause a number of symptoms including headache, sleepiness, confusion and coma. Hypertension is caused by a number of reasons which are classified as primary (90% of cases) and secondary (10% of cases). A primary cause is where the cause is unknown, whereas secondary causes include 1) Chronic kidney diseases 2) Chronic alcohol abuse 3) Hormonal disturbances and 4) Endocrine tumours.
A number of factors can contribute to the increase in the risk of hypertension. These factors include 1) Family history 2) Obesity 3) High salt intake 4) Diabetes 5) High alcohol intake 6) kidney diseases 7) lack of exercise and 8) certain medications such as steroids.
The chance of getting a cardiovascular disease is increased if an individual has hypertension as well as diabetes; therefore the targets for blood pressure control in diabetes are tighter. An individual suffering from diabetes as well as hypertension will need to have a blood pressure of less than 130mmHg (systolic) and less than 80mmHg (diastolic),whereas an individual only suffering from hypertension will need to have a blood pressure of less than 140 mmHg (systolic) and less than 85 mmHg (diastolic).
Hypotension is a condition where blood circulates through the vessels at a low pressure (lower pressure than normal). When the blood pressure is low, the body organs will not have sufficient blood supply which prevents them from functioning or sometimes permanently damaging them. A common type of hypotension is orthostatic hypotension in which standing suddenly causes dizziness or even fainting. An example would be the brain not getting sufficient blood supply. This means that the brain cells will not receive enough oxygen and nutrients to function. This will make the individual feel lightheaded, dizzy or even faint. The common symptoms associated with low blood pressure are 1) Dizziness 2) Confusion 3) Blurred vision 4) Tiredness 5) Feeling weak and 6) Fainting. A number of factors can be associated with the causes of hypotension. Certain drugs such as diuretics, nitrates and some drugs used for the treatment of hypertension can cause hypotension. Dehydration, diarrhoea, vomiting and loss of blood can cause hypotension due to the decrease in the volume of fluid circulating in the blood vessels. Dehydration occurs due to excessive sweating and low water intake. Dehydration can therefore be treated by drinking appropriate amount water. Loss of blood can either be internal or external which can be treated using blood transfusions.
There are a number of drugs available for the treatment of hypertension. These drugs are called antihypertensive drugs. The Pharmacodynamics for each drug is different as each drug act on different receptors (different mechanisms) to restore the normal blood pressure. There are a number of classes of antihypertensive drugs which include, 1) Diuretics 2) Angiotensin converting enzyme (ACE) inhibitors 3) α-Blockers 4) Angiotensin receptor blockers 5) Beta blockers and 6) calcium channel blockers.
Diuretics are also known as water tablets (as they increase diuresis). These drugs help in blood pressure regulation by increasing the urinating rate of the person being treated. Increasing the urination rate will therefore decrease the volume of fluid in the vessels thus reducing the blood pressure. There are a number of diuretics available which include loop diuretics, potassium sparing diuretics and thiazide diuretics. Loop diuretics function by inhibiting the body’s ability to reabsorb sodium at the ascending limb of the kidney. This means that water will follow the sodium thus reducing the volume of fluid. Examples of loop diuretic drugs are: Bumetanide, Ethacrynic acid, Furosemide and Torsemide. Some of the side effects associated with loop diuretics include: Reduced levels of potassium and magnesium, urinating more frequently, short term increase in blood sugar levels and cholesterol levels, an increased level of uric acid, sexual dysfunction in men and digestive upset. Potassium sparing diuretics function by either blocking sodium channels directly (Amiloride) or by acting as antagonists with Aldestrone for intracellular cytoplasmic receptors. The advantage of using the potassium sparing diuretics is that it reduces potassium excretion. They are usually used in combination with other diuretics such as loop diuretics so that the potassium level is maintained. The side effects of using a potassium sparing diuretics are as follows: breast enlargement in men and menstrual cycle irregularities in women. Examples of potassium sparing diuretics include: Amiloride, Eplerenone, Spironolactone and Triamtrene. Finally thiazide diuretics function in the distal convoluted tubule by blocking Na+ and Cl- transfer. They are also associated with widening the blood vessels to lower the blood pressure. The side effects associated with thiazide diuretics include the following, 1) Decreased levels of potassium and magnesium 2) increased levels of calcium and uric acid 3) sexual dysfunction in men and 4) digestive upset. Thiazide diuretics include: Chlorothiazide, Chlorthalidone, Hydrochlorothiazide, Indapamide and Metolazone.
The other group of drugs used to for the treatment of hypertension are classified as angiotensin converting enzyme (ACE) inhibitors. These include Benazepril, Captopril, Enalapril, Fosinopril, Lisinopril, Moexipril, Perindopril, Quinapril, Ramipril and Trandolapril. Angiotensin converting enzyme inhibitors function by blocking the action of the angiotensin converting enzyme. The enzyme’s function is to convert angiotensin I to angiotensin II. Angiotensin II is responsible for a number of mechanisms that cause an increase in the blood pressure, for example it increases sympathetic activity, constriction of blood vessels and promotes the secretion of Aldestrone. When the enzyme is inhibited, Angiotensin II will not be produced from Angiotensin I thus all the mechanisms causing an increase in blood pressure will be inhibited. The major drawback in using the ACE inhibitors is that it has a major side effect, dry cough. Other side effects are also present which include an increased in potassium levels, rash, angioedema (allergic swelling that affects the face, lips, and windpipe and may interfere with breathing), and, in pregnant women, serious injury to the fetus.
As discussed above, ACE inhibitors have a major side effect, dry cough. A new class of antihypertensives were developed to eliminate this major drawback. Angiotensin receptor blockers (or sometimes known as angiotensin II receptor antagonists) has the same effect as the ACE inhibitor but without the major drawback, dry cough. The way that this drug functions is different than the ACE inhibitor because the drug will not inhibit the ACE enzyme. The drug will compete with the substrate binding to the Angiotensin converting enzyme thus blocking its activity. Angiotensin receptor blockers come with a number of side effects which includes: Dizziness, an increased potassium level, angioedema (highly likely), and, in pregnant women, serious injury to the fetus. As can be seen the side effects are quite similar to the ACE inhibitor drugs but without the major side effect, dry cough (which occurs in 20% of people). Angiotensin receptor blockers include Candesartan, Eprosartan, Irbesartan, Losartan, Olmesartan, Telmisartan and Valsartan.
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