28. Factors determining the blood pressure: pressure on different parts of the circulatory bed, measuring blood pressure

Question 1

Blood pressure

Answer 1

Blood pressure maintains the flow of blood in the circulatory bed. The filtration/resorption and the transport of the nutrients, and the transport of the metabolites from the tissues are basically determined by the blood pressure.

During systole the blood pressure is higher than during diastole. The movement of blood in the vessels is maintained by the mean blood pressure.

The work of the heart and the resistance of the periphery determine the blood pressure.

Runoff: The amount of blood that is forwarded from the atrial side of the circulation into the venous part during systole of the heart. This is not equal to the stroke volume. The difference between the stroke volume and the runoff determines the actual change in the arterial volume during systole.

Question 2

Factors determining blood pressure

• Physiological effects:

Answer 2

Physiological effects:

• Cardiac output:
  
  • If the C.O. is suddenly increased experimentally the mean arterial pressure increases to a value that is able to forward the increased volume of blood appearing in the arterial system.
  • After some cardiac cycles the increased C.O. increases the blood pressure, which then forwards the increased amount of blood to the venous side.
• Increase of heart rate:
  
  • Increases the blood pressure because blood is forwarded from the venous reservoir system into the arterial resistance system.
• Total peripheral resistance (TPR):
  
  • A sudden increase in the TPR, while other parameters remain constant, decreases the runoff.
  • The arterial blood volume grows and a consequent increase of the mean arterial pressure helps to forward the stroke volume towards the increased resistance.

Question 3

Factors determining blood pressure

• Physical effects

Answer 3

• Arterial blood volume:
  
  • From the physical point of view the major determinant of the blood pressure is the arterial blood volume (Va).
  • Va changes at each cardiac cycle as a result of the difference between stroke volume and runoff.
  • Under constant compliance and peripheral resistance the increase of the Va causes the increase of both the pulse pressure and the mean arterial pressure.
• Arterialdistensibility andcompliance:
  
  • Vessels are capable to take up more blood when the pressure increases because their wall is distensible.
  • Distensibility = the capacity to expand/stretch under pressure
    
    • Def: The volume change related to the unit change of pressure and to the original volume.
    • D=∆V/∆PxVa
  • Compliance = flexibility
    
    • Def: If the original volume of the system is ignored, and only the relative volume change per unit pressure change is observed
    • C=∆V/∆P
• o Distensibility is used when one
• compares circulations
• o Compliance is used when the absolute
• change is of interest.

Question 4

Pressures on different parts of the circulatory bed

Answer 4

Blood pressure values: systole/diastole

Question 5

Pressures on different parts of the circulatory bed

• Standard data

Answer 5

Question 6

Measurement of blood pressure

Answer 6

• Direct method
  
  • It can be used on fully anesthetized animal inserting a fluid-filled catheter into the carotis, which is connected to a pressure transducer that converts the oscillation of the arterial pressure into recordable electrical signals.
• Indirect method
  
  • Palpation
  • Ausculation