Blood Pressure Lab

Blood pressure is the pressure exerted by blood upon the walls of blood vessels. Blood pressure is pulsatile due to the rhythmic contractions of the heart. It is measured in millimeters of mercury (mmHg).

The peak of ventricular contraction is systole.

The minimum at the end of ventricular relaxation is diastole.

The difference between systolic pressure and diastolic pressure (SP - DP) is the pulse pressure.

Systolic pressure and diastolic pressure only give the range of pressure within arteries. Often, it is more useful to use a single value, the mean arterial blood pressure (MABP). MABP is the average blood pressure within arteries over one cardiac cycle.

The sphygmomanometer indicates the pressure. The first Korotkoff’s sound is the systolic pressure and the last is the diastolic pressure.

When the cuff pressure is lower than fully opened brachial artery, the sounds cease.

Pulse in an artery represents the expanding and recoiling of arterial wall.

Radial pulse rate: rate at which left ventricle contracts (used for heart rate in BPM)

BP is the force of blood moving through vessels

HR is the number of times the heart beats per minute

Cardiac output (CO): volume discharged from ventricle per minute

Stroke volume (SV): volume of blood entering arterial system

CO: SV x HR

Peripheral resistance (PR) is the force caused by the friction between the blood and the walls of the vessels


Arteries

  • Thick, strong wall, thicker than walls of veins

  • 3 layers or tunics:

    • Tunica interna (intima): innermost layer

    • Tunica media: smooth muscle and elastic tissue

    • Tunica externa (adventitia): outer layer, connective tissue

      • Transport blood under high blood pressure

      • Give rise to smaller arterioles


Veins

  • Thinner walls than arteries (3 layers or tunics)

  • Tunica media less developed

  • Carry blood under relatively low pressure

  • Function as blood reservoirs

  • Many have flap-like valves


Control of BP

BP is determined by cardiac output and peripheral resistance, such that BP = CO x PR

CO is the amount of blood pumped in one beat


Stroke volume (volume of blood ejected from the heart and entering arterial system x heart beat) can increase when blood enters the ventricles and the walls stretch = preload

The ability of the heart to change its force of contraction and therefore stroke volume in response to changes in venous return is called the Frank-Starling mechanism or law


Frank-Starling Law of the Heart

  • The higher the end-diastolic volume (volume of blood in the ventricles before contraction), the higher the preload (end diastolic pressure which stretches the ventricles to the max).

  • The higher the length of ventricular myocytes prior to contraction, the higher the force of contraction.

    • This enables the heart to push out more blood in the event of additional venous return (more blood in the heart).

      • The stroke volume increases (the amount of blood ejected from the heart) to compensate for additional venous return

  • Application: Upon exertion increase amount of blood enters heart from veins, increased distension, increased contraction, increased stroke volume, increased cardiac output


Hypertension

  • Persistently elevated arterial pressure

  • Essential (idiopathic) vs secondary hypertension