As the term indicates blood pressure is the pressure by which the blood is circulated in blood vessels. Heart is the muscular pump that supplies the pressure to move the blood along the blood vessels. Blood vessels have elastic walls and provide some resistance to the flow. Hence there is pressure in the system even between heartbeats. 
It is necessary to keep the blood pressure at a certain level is to ensure the adequate blood supply (nutrients and oxygen) to various organs. 

Prevalence in India: 
Two to four percentage of rural population and ten to fifteen percentage of urban population are hypertensive. The prevalence increases with age. Above the age of 60 years, 19 % were found to be hypertensive.
“ Rule of halves ” 
Hypertension is an “ice berg” disease. It became evident that only about half of the hypertensive subjects in the general population of most developed countries were aware of the condition, only about half of those aware of the problem were being treated and about half of those treated were considered adequately treated.

DEFINITION:
Systemic hypertension is the persistent rise of basal blood pressure above the arbitrary level of 140 / 90 mm of Hg recorded on three or more successive occasions.
  Or
Chronic elevation of systolic blood pressure above 140 mm of Hg and / or diastolic blood pressure above 90 mm of hg is defined as arterial hypertension                 
  
Normal    Below 130 mm of hg systolic
Below 85 mm of hg diastolic 
High normal 130 -- 139 mm of hg systolic
                             85 -- 89 mm of hg diastolic 
Elevation of systolic blood pressure without elevation of diastolic blood pressure is called systolic hypertension.
Physiological variation: Throughout the day.
Exercise
Mental stress
Sexual intercourse. 
At rest / during sleep.    

How is BP kept normal? 
BP is controlled mainly by the brain, autonomous nervous system, kidney, some of the endocrine gland and to certain extent by the heart and arteries. 
Brain is the controlling center. 
 Autonomous nervous system brings signals from all parts of the body to inform the brain of the status of BP, the volume of blood and specialized need of all organs. Here, blood pressure is controlled either by narrowing of blood vessels or by opening of blood vessels. 
Kidney – regulator of fluid in the body. Kidney regulates the blood pressure through the rennin- angiotensin- aldosteron system. In response to anything that lowers the BP and there by threatens the normal function of the kidney, release of hormone rennin take place. Rennin converts angiotensinogen to angiotensin -1, which further converted in to angiotensin- 2. Angiotensin -2 stimulates the production of aldosteron leads to salt and water retention. Angiotensin- 2 has a vasoconstrictive effect and thereby BP is elevated. 
Endocrine gland – Mainly by the adrenal and thyroid gland. Cortisone, adrenaline and aldosteron have a hypertensive action. Thyroxin has important role in BP control. Estrogen – increase BP.  
Heart -- Heart secrete natriuretic hormone- a substance that can rid the body of excess salt and help keep the blood vessels properly dilated. Natruretic hormone inhibits the secretion of renal rennin and aldosteron and opposing the vasoconstrictor effect of angiotensin – 2. 
Artery -- Can dilate to increase the blood supply to an organ. (When we exercise, the blood is shifted to the exercising muscles.) 

Classification:
Based on the diastolic blood pressure, hypertension can be classified in to  
1. Mild               --- 90 to 110 (diastolic blood pressure) 
2. Moderate --- 110 to 130 
3. Severe --- above 130 mm of hg.  
Based on etiology hypertension can be classified into 
1. Primary or essential hypertension and 
2. Secondary hypertension.
 
Essential hypertension:
When the blood pressure is elevated with out an evident organic cause. (When the causes are generally unknown) 
v Most prevalent form of hypertension.
v Accounting for 90-- 95 % of all cases of hypertension.
 
Secondary hypertension:               
When some other disease process or abnormalities are involved in its causation then it is said to be secondary hypertension. 
Eg: - Kidney diseases,
Tumors of adrenal gland,
Congenital narrowing of aorta,
Toxemias of pregnancy etc. 

Stages: 
Classified in to four stages (based on the average of two or more readings taken at each of two or more visit after initial screening) 

Stage     Systolic blood Pressure              Diastolic blood Pressure
 
Stage 1    140 -- 159                                     90 --99
 
Stage 2    160 --179                                     100 –109
 
Stage 3    180 -- 209                                    110 -- 119
 
Stage 4    Above 210                                   Above 120

Causes of hypertension: (in decreasing order of occurrence)
1. Essential (95 % of patients) – unknown cause.
2. Diseases of kidney
3. Decreased blood supply to the kidney. (Reno vascular)
4. Primary aldosteronism.
5. Coarctation of aorta.
6. Cushing syndrome.
7. Oral contraceptives
8. Pheochromocytoma. 
[Causes of hypertension can be classified in the following way        
1. Vascular causes: Coarctation of aorta
Poly arthritis nodosa 
2. Renal causes:  Renal artery stenosis
Glomerulonephritis
Pyelonephritis
Radiation nephritis
Nephrocalcinosis
Tuberculosis of the kidney
Polycystic kidney
Renal tumor     
3. Endocrine causes: Disease of the adrenal cortex
Hyper aldosteronism
Cushing syndrome 
4. Adverse reaction to drugs: Oral contraceptives
Corticosteroids 
5. Toxemia of pregnancy
6. Alcohol  
 
                                            Risk factors for hypertension:

Two types 1. Non modifiable risk factors and 2. Modifiable risk factors 
Non-modifiable risk factors:
Age – Blood pressure rises with age in both sexes and the rise is greater in those with higher initial blood pressure.
Genetic factors -- Genetic and familial predisposition. Family study have shown that the children of two normotensive parents have 3 % possibility of developing hypertension, where as the possibility is 45 % in children of two hypertensive parents.
Person with diabetes mellitus and impaired glucose tolerance has a higher prevalence of hypertension – up to 50 %. 
Modifiable risk factors: 
Obesity – The greater the weight gain, the greater the risk of blood pressure. 
Salt intake -- High salt intake increase blood pressure. Increased incidence in Japan is most probably due to the high intake of salt. 
It has been postulated that essential hypertensives have a genetic abnormality of the kidney, which makes salt excretion difficult except at raised level of arterial pressure.                                                                                                                                                                
Potassium antagonizes the biological effect of sodium and there by decrease blood pressure. Potassium supplement lower the blood pressure. 
Saturated fat -- Increases blood pressure as well as serum cholesterol. 
Alcohol -- High alcohol intake is associated with an increased risk of developing blood pressure, especially systolic. Alcohol induced elevation may not be fixed. Blood pressure returns to normal with abstinence. 
Physical activity – Help in reducing body weight and there by reducing blood pressure. 
Environmental factors -- Tense personality. Psychosocial factors operate through mental process consciously or unconsciously to produce hypertension. 
Other factors -- Oral contraceptives
Noise
Vibration
Temperature
Humidity 

Pathology:
Main pathological changes seen in heart and parts of the arterial tree.
Cardiac changes
Left ventricular hypertrophy in 20 – 50 % of mild to moderate hypertension. Because of these left ventricular hypertrophy, diastolic dysfunction and arrhythmias results. Diastolic dysfunction accelerates coronary atheroma and left ventricular failure. Cardiac failure due to degeneration and lysis of myofibrils. 
Arterial changes 
Arterial wall shows thickening – more marked in kidney. Renal vessels show medial hypertrophy and intimal fibrosis. Heart vessels show coronary atherosclerosis and atheromatous changes. Small arteries of the brain show micro aneurysm (Charcot Bouchard aneurysm). This aneurysm may rupture and may leads to cerebral hemorrhage.
Thrombotic and embolic occlusion of arteries may result in infarction in heart, brain, kidney etc 

Clinical features: 
Essential hypertension-- Usually asymptomatic. 50% of the patients are unaware of the condition. Many of them develop symptoms after knowing that they are hypertensive. Patient may felt fatigue, dizziness, palpitation, anxiety and head ache. Throbbing headache felt in sub occipital region waking after sleep suggestive of hypertension. 
On physical examination, there may heaving apex beat; loud aortic second sound and increase tension pulse.

Secondary hypertension -- Evidence of primary disorder present.
Eg: - Weak or delayed femoral pulse,
        Hypertension in the upper limb,
Hypotension in lower limb is seen in secondary hypertension due to coarctation of aorta.

Complication:
If untreated for a long period of time can cause damage to the arteries of the body and to the organs they supply. Eg: - heart, brain and kidney.   [Smoking, high cholesterol and diabetes mellitus can cause similar damage.] 
Heart: forced to do extra work, it become hypertrophied and dilated when it has to pump against higher pressure. 
Because the heart has the difficulty in pumping blood to the muscle, when they need extra oxygen for exercise or work, a person will feel short of breath. At first only during active exercise. Later difficulty even during rest—congestive cardiac failure. 
Oxygen supply for the heart to do its work will become deficient when the coronary arteries are damaged. Then there will be a sensation of pressure, tightness and heaviness in the chest and arm. Usually this sensation last for five minutes and goes away with rest. This sort of pain is called angina. At first with exercise or strenuous work. Later with very little activity or with excitement or emotional upset. 
When blood supply to the part of the heart is blocked permanent damage or myocardial infarction can occur. Here chest pain is longer when compared to angina. (15 minutes and often much longer) it may be associated with nausea and sweating.
Brain: Stroke.
Kidney: Nephrosclerosis may leads to renal failure. 
Arteries of leg: cramps of leg and intermittent claudication. Later very severe pain even at rest – may leads even to gangrene.  
Severely elevated blood pressure can cause the blood vessels either to dilate (Aneurysm) or to burst (hemorrhage and stroke). 
Malignant hypertension and accelerated hypertension 

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