Saturday, December 14, 2013

Blood Pressure

Blood Pressure

Blood pressure is the lateral force on the walls of an artery by the pushing blood from the heart. The blood pressure in the system of arteries in the body, is a good indicator of cardiovascular health.
Blood flows throughout the circulatory system because of Pressure Change. It moves from an area of High Pressure to an area of Low Pressure.

The heart's contraction force blood under high pressure into Aorta.

Systolic - is the Highest Pressure against the arteries as the heart pump. The normal systolic pressure is usually between 100 and 130mmHg.

Diastolic - Is the Pressure against the arteries as the Heart relaxes and Fills with Blood. The normal diastolic pressure is usually between 70 and 80mmHg.

Physiology of arterial blood pressure

Blood pressure reflects the interrelationship of cardiac output, peripheral vascular resistant, blood volume, blood viscosity and elasticity.

Cardiac output
A person's Cardiac Output (CO) is the volume of blood pumped by the heart during I minute.

The Blood Pressure (BP) depends on the cardiac output (CO) and peripheral vascular resistance (R) - BP = CO x R . 

When volume increases in an enclosed space, such as a blood vessel, the pressure in the space rises thus, as cardiac output increases, more blood is pumped against arterial walls, causing the blood pressure to rise.

Cardiac output can increase as result of an increase in heart rate, greater heart muscle contractility, or an increase in blood volume.

An increase in heart rate may decrease Diastolic filling time and end-diastolic volume. As a result there is a decrease in blood pressure.

Peripheral Resistance

The blood circulates through a network of arteries, arterioles, capillaries, venules and veins.

- The size of arteries and arterioles change to adjust to the needs of local tissues. For example: when blood needed by major organ, the peripheral arteries constrict, decrease their supply of blood. More blood is available to the major organ because of the resistance change in periphery.
 - Normally, arteries and arterioles remain partially restricted to maintain constant flow of blood.

- As resistance rises, arterial blood pressure rises. As vessels dilate and resistance falls, blood pressure drops.

Blood volume

The volume of blood circulating within the vascular system affects blood pressure.

Most adults have a circulating blood of 5,000mL. Normally, the blood volume remains constant. however, if the volume increases, more pressure is exerted against the arterial walls.

For example : The rapid, uncontrolled infusion of intravenous fluids elevates blood pressure. When circulating blood volume falls, as in the case of haemorrhage or dehydration, blood pressure falls.


The viscosity of blood affects the ease with which blood flows through small vessels. The haematocrit, or percentage of red blood cells in the blood, measures blood viscosity.

- When the haematocrit rises and blood flow slows, arterial blood pressure increases. The heart must contract more forcefully to move the viscous blood through the circulatory system.


Normally the walls of an artery are elastic and easily distensible. As pressure within the arteries, the diameter of vessel walls increase to accommodate the pressure change.

Factors influencing blood pressure

Factors that are influencing blood pressure: Age, Medications, Diurnal variation and gender 

Blood pressure is not constant. It is continually influenced by many factor during the day.

- One blood pressure measurement cannot adequately reflect one's blood pressure. Even under the best conditions, blood pressure changes from heart beat to heart beat. 


millimetre of mercury (mmHg) - is the standard unit for measuring pressure. it indicates the height to which blood can raise a column of mercury. 

mL = millilitre

J. Crisp & C Taylor "Fundamental of Nursing 3e"  Elsevier Morsby 2009 - Vital Sign
Better Health Channel: Blood Pressure (high) - Hypertension

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