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Blood Pressure

by demiguin

 What is BP?

The blood pressure is generated by cardiac contraction, which produces its maximal pressure, the systolic blood pressure , during ejection.  This is followed by cardiac relaxation, which results in a fall of blood pressure to its nadir level, the diastolic blood pressure.

The mean arterial pressure, MAP, lies between the systolic and diastolic prssure and is determined by cardiac output and systemic vascular resistance.  This is maintained so as to provide adequate tissue perfusion but minimized vascular trauma.


What maintains BP?

  1. Baroreceptor reflex receptors in the carotid  sinus (a vessel near the heart) detect changes in the blood pressure, then relays it to the brain and the brain tells the heart to beat faster/slower, increase/decrease contraction, increase/decrease resistance in blood vessels. Thus returning BP to normal.
  2. Renin-angiotensin system kidney produces renin (an enzyme) that forms angiotensin I, this goes to lungs to become angiotensisn II.  Angiotension II does two things, it causes vasoconstriction so increasing resistance and BP, and it tells adrenals to make aldosterone.  Aldosterone promotes absorption of salt and water from the kidneys. Thus higher salt and water, the higher the BP.
  3. Plasma volume if amount of fluid in the blood vessels in high (high BP) water and salt are excreted in the kidneys, then BP becomes lower. If BP is low, salt and water is reabsorbed to increase BP.


What is being done while taking BP?

The blood pressure in a human being is usually taken using a sphygmomanometer.  In the common sphygmo, a rubber cuff (armlet) is applied to the bare upper arm of the subject.  The cuff has two openings, on opening connected to a bulb forces air into the cuff or releases it, and another opening connected to a gauge (aneroid or mercury) telling the amount of pressure.

The arm band is placed around the left arm just above the elbow.  The brachial artery is found just below the band. A stethoscope is placed on the artery and cuff is inflated until the sound disappears. This is the systolic pressure.  The cuff is slowly released until the sound returns and eventually softens, and eventually disappears.  One can use the softening or the disappearing as the diastolic pressure.

As the cuff is inflated, we are cutting of circulation to the arm by compressing the blood vessels. When we release the cuff, we are noting the first sound than is produced.  This the systolic pressure which represents the maximum amount of pressure the heart exerts.  The last sound is the diastolic where the heart is relaxed.


What happens if we have high BP?

Very high blood pressures can result to target organ damage.  The organs affected include the brain, the eyes, the heart, the kidney and the blood vessels.

  1. Brain -  hemorrhages, infarcts, encephalopathy
  2. Eyes edema of the optic disc, exudates, hemorrhages in the fundus of the eyes
  3. Heart congestive failure, enlarging of heart, myocardial infarct, angina (chest pain)
  4. Kidney nephrosclerosis (destruction of kidney structure)
  5. Vessels aortic dissection (rupture or bleeding due to high pressure)


What is the relationship of salt intake to BP?

It is important to note that where the salt is, the water will follow.

BP = cardiac output x peripheral resistance

When we eat, part of the food is salt, this is absorbed through the body and the body decides whether to excrete the salt or store it depending on the needs of the body.  If we take in a large amount of salt, the kidney cannot immediately dispose of this thus salt is maintained in the individual.  So we have two things here 1. we have increased intake 2. we have decreased excretion.  Both of this will cause high amounts of salt in blood.  An where salt is, water follows.  Increasing fluid volume in the vessels will also increase the amount of preload (blood brought into the heart), higher preload will give a higher cardiac output (amount of blood the heart ejects), thus this will yield a higher BP.

 Newer studies show that this is not the only way sodium increases BP, increased salt intake will also increase pressor mechanisms which causes vasoconstriction, vasoconstriction will increase peripheral resistance, thus increasing BP.

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