HYPERTENSION

Hypertension:
Persistent systolic pressure, more than 140 and diastolic more than 90 mm Hg. is regarded as hypertension.

The range of pressure increases with age.

Hypertension is the single most important risk factor in both coronary heart disease and cerebrovascular accidents.

It may also lead directly to congestive heart failure (hypertensive heart disease), renal failure and aortic dissection.

About 90% of hypertension is primary and idiopathic (essential).

The remaining is secondary and mostly related to renal disease or (less often) to renal artery stenosis (renovascular), endocrine abnormalities, vascular malformations, or neurogenic disorders.

Causes of hypertension:

I. Primary, essential, or idiopathic hypertension:

II. Secondary hypertension is due to the following causes:

1. Renal :

Acute glomerulonephritis ;

Chronic renal disease ;

Polycystic disease ;

Renal artery stenosis ;

Renal vasculitis ;

Renin-producing tumors ;

2. Endocrine:

Adrenocortical hyperfunction : Cushing syndrome, primary aldosteronism, congenital adrenal hyperplasia ;

Estrogen hormones : glucocorticoides, estrogen - including pregnancy and oral contraceptive, sympathomimetics, tyramine-containing foods, monoamine oxidase inhibitors ;

Pheochromocytoma ;

Achromegaly ;

Myxedema ;

Thyrotoxicosis ;

3. Cardiovascular :

Coarctation of aorta ;

Polyarteritis nodosa ;

Aortic insufficiency ;

Increased intravascular volume ;

Rigidity of the aorta ;

4. Neurologic:

Psychogenic ;

Increased intracranial pressure ;

Sleep apnea ;

Acute stress, including surgery ;


Regulation of Normal Blood Pressure :

Blood pressure is a complex trait that is determined by the interaction of multiple genetic and
environmental factors that regulate the relationship between cardiac output and total peripheral resistance.

1. Vasoconstriction increases vascular resistance.

Vasoconstrictors include angiotensin II, catecholamines, thromboxane, leukotrienes, and endothelin.

2. Vasodilators include kinins, prostaglandins, nitric oxide, and adenosine.

3. Regional autoregulation is also important, wherein increased blood flow leads to vasoconstriction and vice versa.

4. Cardiac output is regulated by blood volume (affected by sodium load, mineralocorticoids, and natriuretic factors), heart rate, stroke volume and contractility.


Mechanism of Essential Hypertension :

Although unknown, the cause of essential hypertension at the most elemental level must be related to a primary increase in cardiac output (e.g., reduced renal sodium excretion) or to an increase in peripheral resistance (e.g., owing to increased release of vasoconstrictor agents, to increased sensitivity of vascular smooth muscle cells, or to behavioural or neurogenic factors), or both (i.e., an increase in cardiac output and an increase in peripheral resistance).

In most patients, multiple defects probably contribute to the disease.

Abnormalities in the renal mechanisms that regulate blood pressure also may contribute to essential hypertension, including:

1. The renin-angiotensin system.

2. Sodium homeostasis.

3. Production of vasodepressor substances : It is implicated in the pathogenesis of hypertension in unilateral renal artery stenosis and renal disease.

For example, studies have suggested a propensity toward hypertension in individuals with specific molecular variants of the gene-encoding angiotensinogen, the physiologic substrate for renin.

congenital anomalies & arteriosclerosis

Congenital anomalies include the following conditions:
1. Anomalous (e.g. aberrant, reduplicated) vessels are principally of interest to surgeons.




2. Berry aneurysms are outpouchings of vessels as a result of congenital focal weakness.
They occur exclusively in cerebral vessels and occasionally rupture catastrophically.



3. Arteriovenous fistula is an abnormal communication between artery and vein.
It may be congenital or secondary to trauma, inflammation, or healed ruptured aneurysm.

anatomy & physiology of the vascular system

Anatomy and Physiology of the Vascular System:

Arterial walls are thicker than veins.
Thickness of the arterial walls gradually diminishes as it becomes smaller.
Walls of the blood vessel consist of three basic structures, intima, media, and adventitia, which vary with the types of the vessels.
Blood vascular tree is a circuit that conducts blood from the heart through large- diameter, low-resistance conducting vessels to small arteries and arterioles, which lower blood pressure and protect the capillaries.
Capillaries are thin-walled and allow the exchange of nutrients and waste products between tissue and blood, a process that requires a very large area.
The circuit back to the heart is completed by the veins, which are disrensible and provide a volume biffer that acts as a capacitance for the vascular circuit.

Arteries are of three types:

1. Larger or elastic arteries (aorta and its main branches) : 
Intima is composed of lining endothelial cells, separated from the media by internal elastic lamina.
Media is composed of smooth muscle cells.
Outer limit of the media is separated is separated from adventitia by external elastic lamina. Small arterioles (vasa vasorum) pass through the adventitia into the outer one-half to two-thirds of the media to perfuse the vessel wall.
Atherosclerosis is the disease largely of elastic and muscular arteries which is associated with functional and structural changes in the muscular arteries and arterioles.

2. Muscular arteries, are branches of elastic arteries (e.g. coronary or renal arteries) regulate the blood pressure by vasodilatation or vasoconstriction under the control of the autonomic nervous system.

3. Arterioles regulate the blood flow into capillary beds by the smooth muscle of media, thus controls the systemic arterial blood pressure.

Capillaries:Capillaries are small vessels having a diameter of a red blood cell, lined by one-cell thick endothelium, a thin basement membrane, and slow blood flow.

These are ideally suited to the rapid exchange of diffusible materials between blood and extravascular tissue.

Veins:
Veins are thin-walled vessels with poorly defined internal elastic lamina and media. Hence, they often show abnormal, irregular dilation, and easy penetration by cancer cells and inflammation.

Lymphatics:
Lymphatics are identified in tissue sections as collapsed, endothelium-lined channels devoid of blood cells.