Blood Pressure

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).
MEAN BLOOD PRESSURE:
  • 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).
Exam Question
 

BLOOD PRESSURE

  • SI unit for measuring blood pressure – Kilopascals (kPa).
  • Basic unit for pressure is newton per square meter & is called Pascal (Pa).
  • BP in mm Hg = BP in kPa x 7.5 or 1 mm Hg= 0.133 kPa.
  • Cardiac waves recorded from brachial artery of a young adult in sitting or supine position.
  • Pulse pressure is difference between systolic & diastolic pressure.
  • 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.
  • Arterial blood pressure is product of cardiac output & total peripheral vascular resistance (TPR).
  • Changes in cardiac output (or stroke volume) affect mainly systolic pressure.
  • Changes in peripheral resistance affect mainly diastolic pressure.
  • Maintenance of fairly high pressure during diastole (80 mm Hg) is achieved by aortic elasticity & its immediate branches (Windkesselvessels) and arteriolar resistance.

REGULATION OF BLOOD PRESSURE:

1. SHORT TERM REGULATION:

  • Antidiuretic hormone (ADH): Increases water reabsorption in kidney.
  • Angiotensin II: Causes vasoconstriction.
  • Yet, Aldosterone II associated with short-term regulation.
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