Tag: Blood Pressure

Regulation of cerebral blood flow


Regulation of cerebral blood flow


Major Regulating factors – Blood pressure, PCO2 & Temperature.

The mechanism involved in each one of the factors are:

1.Blood pressure Fluctuation Bt. 65 – 140 mm Hg NO CHANGE in blood flow

AUTOREGULATION

2. PCO2

↑ PCO2 -↑ Blood Flow

3. Temperature

On ↓ Body Temp. by 1° C -↓ cerebral flow by 7% – INDUCED HYPOTHERMIA in

neurosurgical procedures – Minimizes blood loss

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.
Don’t Forget to Solve all the previous Year Question asked on Blood Pressure

Blood Pressure

BLOOD PRESSURE

Q. 1

Which of the following statements regarding stroke volume is TRUE?

 A

Decreases by increase in heart rate

 B

Determined by pre-diastolic volume

 C

Determined by afterload

 D

Is equal to cardiac output

Q. 1

Which of the following statements regarding stroke volume is TRUE?

 A

Decreases by increase in heart rate

 B

Determined by pre-diastolic volume

 C

Determined by afterload

 D

Is equal to cardiac output

Ans. A

Explanation:

Stroke volume = cardiac output/heart rate, i.e it is inversely proportional to heart rate. As a result when heart rate increases the stroke volume decreases. This reduction in stroke volume at high heart rates is due to a decrease in the length of time the heart spends in diastole and thus a reduction in time available to the heart for filling.

Stroke volume refers to quantity of blood ejected with each heart stroke. According to Starling’s law of heart, stroke volume is determined primarily by the preload. As the preload increases, stroke volume also increases until it reaches a plateau. Stroke volume can also be increased by sympathetic stimulation of the heart.

Ref: Physiology Secrets By Hershel Raff, 2nd edn, page 74


Q. 2

Blood pressure is defined as the product of:

 A

Systolic pressure x pulse

 B

Diastolic pressure x pulse rate

 C

Pulse pressure x pulse rate

 D

Cardiac output x peripheral resistance

Q. 2

Blood pressure is defined as the product of:

 A

Systolic pressure x pulse

 B

Diastolic pressure x pulse rate

 C

Pulse pressure x pulse rate

 D

Cardiac output x peripheral resistance

Ans. D

Explanation:

D i.e. Peripheral resistance x cardiac output

The overall blood flow in circulation of adult at rest is about 5 liter/min which equals to cardiac output because it is the amount of blood pumped by heart per unit time. So

Blood pressure =

Cardiac output x peripheral resistanceQ or Blood flow (Q) x resistance (R)

 

Venous returnQ =

MSFP — RAP                       MSFP = Mean systemic filling pressure

 

 

 

 

= Right

RAP                                                 atrial pressure

RVR                                          RVR    = Resistance to venous return

 

Q. 3

SI unit for measuring Blood Pressure is :

 A

Torr

 B

mmHg

 C

kPa

 D

Barr

Q. 3

SI unit for measuring Blood Pressure is :

 A

Torr

 B

mmHg

 C

kPa

 D

Barr

Ans. C

Explanation:

C i.e. KPa


Q. 4

Blood pressure is defined as the product of.

 A

Systolic pressure x pulse

 B

Diastolic pressure x pulse rate

 C

Pulse pressure x pulse rate

 D

Cardiac output x peripheral resistance

Q. 4

Blood pressure is defined as the product of.

 A

Systolic pressure x pulse

 B

Diastolic pressure x pulse rate

 C

Pulse pressure x pulse rate

 D

Cardiac output x peripheral resistance

Ans. D

Explanation:

D i.e. Peripheral resistance x cardiac output:

Blood pressure is defined as the product of cardiac output and peripheral resistance.

Blood pressure =  Cardiac output x peripheral resistance or Blood flow (Q) x resistance (R)


Q. 5

SI unit for measuring Blood Pressure is:

 A

Torr

 B

mmHg

 C

kPa

 D

Barr

Q. 5

SI unit for measuring Blood Pressure is:

 A

Torr

 B

mmHg

 C

kPa

 D

Barr

Ans. C

Explanation:

Answer is C (kPa)

Pressure in SI units is measured in Pascal

Pal Blood pressure in SI units is measured in kPa


Q. 6

Mean BP is ‑

 A

CO x TPR

 B

CO x heart rate

 C

Heart rate x TPR

 D

Stroke volume x TPR

Q. 6

Mean BP is ‑

 A

CO x TPR

 B

CO x heart rate

 C

Heart rate x TPR

 D

Stroke volume x TPR

Ans. A

Explanation:

Ans. is ‘a’ i.e., CO x TPR

  • Arterial blood pressure is the product of the cardiac output and the total peripheral vascular resistance (TPR). Mean blood pressure is the major determinant of adequate blood flow through the tissues.

Mean BP = Cardiac output x Total peripheral resistance

  • If cardiac output is expressed as a product of stroke volume and heart rate, the formula blood pressure can be expressed as the product of three variables (the triple product) : –

Mean BP = Stroke volume x Heart rate x TPR

  • Blood pressure is therefore affected by conditions that affect any of these factors. Changes in cardiac output (or stroke volume) affect mainly the systolic pressure while changes in peripheral resistance affect mainly the diastolic pressure.


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