Blood Pressure

BLOOD PRESSURE

• Lateral pressure exerted by blood column on arterial walls.
• Most commonly unit for BP – “mm Hg”.
• SI unit for measuring blood pressure – 
• Kilopascals (kPa).
• Bacause, in SI units, basic unit for pressure is newton per square meter & is called Pascal (Pa).
• Kilopascals may be converted into mm Hg by multiplying them by 7.5.
• Hence, 
• BP in mm Hg = BP in kPa x 7.5.
• 1 mm Hg= 0.133 kPa.

CARDIAC WAVES:

• Blood pressure fluctuates during cardiac cycle.
• During ventricular systole, 
• BP is higher than during diastole.
• “Systolic” BP (SBP) – 
• Maximum arterial pressure attained during systole.
• “Diastolic” BP (DBP) – 
• Minimum pressure recorded during diastole.
• Recorded from brachial artery of young adult in sitting or supine position –
• SBP – 120 mm Hg.
• DBP – 80 mm Hg.

PULSE PRESSURE:

• Difference between systolic & diastolic pressure.
• Normally about 40 mm Hg (120 minutes 80 mm Hg).

• Average pressure throughout cardiac cycle.
• Not an arithmetic mean or average of systolic or diastolic BP.
• Because systole is much shorter than diastole.
• Hence, arterial pressure is near diastolic pressure for a longer part of cardiac cycle than near systolic pressure.
• Hence, diastolic BP preferred for BP calculation.
• Mean BP is slightly less than average of systolic & diastolic BP.
• Approximately, mean BP is weighted 2:1 in favor of diastolic BP.

FORMULA FOR CALCULATION:

• Mean blood pressure – 2/3 diastolic pressure (DBP) + 1/3rd systolic pressure (or)
• Mean BP – (2 DBP +SBP)/3(or)
• Mean BP – DBP + 1/3 Pulse pressure.
• Normally, mean BP is about 93 mm Hg I.e.,(2 x 80 + 120)/3.

DETERMINANTS OF BLOOD PRESSURE:
1. Arterial blood pressure:

• Product of cardiac output & total peripheral vascular resistance (TPR).
• Mean blood pressure is major determinant of adequate blood flow to tissues.
• Mean BP = Cardiac output * Total Peripheral Resistance.

2. If cardiac output is expressed as a product of stroke volume & heart rate,

• Formula for blood pressure – Product of three variables (“Triple product”).
• Mean BP = Stroke volume * Heart rate * TPR.

3. Hence, BP affected by conditions that affect any of these factors.

• Changes in cardiac output (or stroke volume) affect mainly systolic pressure.
• Changes in peripheral resistance affect mainly diastolic pressure.

4. Heart ejects about 70 ml of blood during each systole.

• Resulting in,
• Aortic pressure rises to 120 mm Hg.
• Stretches aortic elastic wall & its immediate branches.
• Since ejection of blood occurs during ventricular systole (0.3 seconds),
• Arterial pressure falls very low during diastole (0.5 seconds).
• Maintenance of fairly high pressure during diastole (80 mm Hg) is achieved by,
• Aortic elasticity & its immediate branches (Windkessel vessels).
• Arteriolar resistance.

5. Ejection of stroke volume raising aortic pressure to 120 mm Hg is responsible for systolic BP.

6. Elastic recoil of aorta & resistance of arterioles – Maintains BP during diastole.

REGULATION OF BLOOD PRESSURE:

• Short-term & long-term regulation.

1. SHORT TERM REGULATION:

• Mechanisms acting immediately to correct BP quickly. 

1a. Baroreceptor reflex:

• Works during BP range 70-150 mmHg.

1b. Chemoreceptor reflex:

• Works when BP below 80 mmHg.

1c. CNS ischemic response:

• Only hope of survival when BP is below 40 mmHg.

1d. Hormonal release:

• Antidiuretic hormone (ADH): Increases water reabsorption in kidney.
• Angiotensin II: Causes vasoconstriction.

2. LONG TERM REGULATION:

• Mechanism taking a long period of time for regulation.
• Usually are “Renal mechanism”.
• Adjusts BP by adjusting ECF & blood volume by, 
• Pressure diuresis.
• Pressure natriuresis.
• Renin-angiotensin-aldosterone system. 
• Yet, Aldosterone II associated with short-term regulation).