Blood Pressure

What is it?
The force exerted by the heart against any unit area of the vessel wall. It is measured in millimeters mercury; mercury is used as a standard reference. Systole is the maximum pressure during contraction and diastole is the minimum pressure in the arteries while the heart is relaxing or filling up with up with blood, right before contraction.

Eg: 120mm/Hg means that the force that is exerted is enough to push a column of mercury against gravity up to 120 mm high.

A high diastole may indicate that pressure in the arteries is high, perhaps due to a narrower lumen or pathology such as atherosclerosis.

A high systole may indicate a forceful contraction, if it is too high, that may mean that extra force is needed to pump blood out, which might mean that there is more resistance or viscosity in the blood.

Poiseuille’s relationship shows that resistance is inversely related to power (the more resistance, the less power)

Blood pressure is determined by two factors: 1) The amount of blood pumped by the left ventricle of the heart into the arteries, and 2) the resistance to the flow of blood caused by the walls of the arterioles (smaller arteries). Resistance could also be caused by high viscosity of the blood which is a result of increased hematocrit. Usual amounts for women is about 38 and 42 for men.

Mechanisms to control blood pressure (http://www.medicinenet.com/low_blood_pressure/page3.htm)
•The heart rate can increase or decrease depending on the body’s needs.
•Veins can narrow to return more blood to the heart for pumping if there is low blood pressure
•Blood flow to the kidneys can increase or decrease to alter the formation of urine and thereby increases or decreases the volume of blood in the arteries and veins.
•Arterioles may narrow to increase resistance to blood flow (if there is low blood pressure) or widen to decrease resistance.

Normal values
These can depend on age, gender and height and normal values can depend on studies where the average blood pressure taken from a sample of people without any history of hypertension can be used as an indicator. (http://www.nhlbi.nih.gov/guidelines/hypertension/child_tbl.htm)

The risk for cardiovascular increases progressively from 115/75 mmHg.
In the UK, blood pressure is considered to be normal up till 140/90 mmHg. (http://www.nice.org.uk/nicemedia/pdf/CG034NICEguideline.pdf)

Classification

Category Systolic (mmHg) Diastolic (mmHg)
Hypotension <90 <60
Normal 90-119 60-79
Pre hypertension 120-139 80-89
Hypertension Stage 1 140-159 90-99
Hypertension Stage 2* >160 >100
*Hypertension that results secondarily from renal disease, endocrine disorders, or other identifiable causes.

Measurement of Blood Pressure:
1st Korotkoff sound is heard after pressure is slowly released from a completely occluded artery and is the sound of blood flowing back. This is the measurement of the systolic pressure. This sound will gradually fade and the when it completely disappears, this indicates that diastolic pressure.