28.Factors determining the blood pressure; pressures on different parts on the circulatory bed, measuring the blood pressure Flashcards
what should be mentioned?
Blood flow in circulation
Energetic components maintaining pressure gradient
Resistance and perfusion (flow rate) relations
Narrowing of the blood vessels
Viscosity of the blood
Blood pressure at different part of the circulation
Arterial blood pressure
CO– Blood Pressure relationship
Work of the heart Increased Frequency
Types of Flow
Capillaries
Veins
Blood pressure
Measurment of blood pressure
Pressure gradient
Blood flows in the vessel saccording to its pressure gradient
– Arteries → capillaries→ veins
Energetic components maintaining pressure gradient
- *Potential energy**
- pressure exerted onto the wall of the vessels.
Kinetic energy
1⁄2 m x v2 (m=mass; v= velocity)
During, the flow friction also occurs, creating energy loss, so the total energy is less at the outlet than at the inlet of the tube.
- The resistance is determined by flow rate.
- If the opening of the probe is perpendicular to the flow, than the height of the fluid indicates the measure of the potential energy at the insertion point.
- If the opening is parallel to the flow, the height of the fluid in the probe gives the total energy at the insertion point.
- The difference of the fluid levels in these two probes is the measure of the kinetic component.
Resistance and perfusion (flow rate) relations
Resistance is determined byflowrate,in other words a, the geometry of the tube and b, the properties of the fluid (viscosity).
-As for geometry of the tube:
Resistance is inversely related to the radius of the 4th power(r4)! Therefore tiny enlargement of diameter will tremendously increase the perfusion (while R decreases) and vice versa.
(Perfusion is the passage of fluid through the circulatory system)
Narrowing of the blood vessels
Under standard flow conditions narrowing causes an increase of kinetic, and decrease of potential component.
Example: coronary stenosis (narrowing of coronary diameter) – results in dramatic increase of flow-rate (kinetic energy) with dangerous drop in potential energy (pressure onto the wall), so the coronary may collapse.
Viscosity of the blood
Viscosity is higher in vessels of about 200 micrometer diameter, then in the very small vessels (<200μm), where it drops dramatically (Fahraeus-Lindquist effect, makes red cells flow much faster through small arteries ).
-Viscosity of the blood is a function of its hematocrit.
Normal hematocrit values (Ht=40) the relative viscosity of the blood is 3.5.
- An increase of the hematocrit value to 50 (which easily happens during hemoconcentration) raises the viscosity to 7.0!
- Hemoconcentration is enormously burdening the heart. It has to work at much higher level to maintain constant pressure.
Blood pressure at different part of the circulation
(3)
- Arterial blood pressure
- Pressure in the capillary system
- Venous blood pressure
Arteries:
(6 long points)
(blood pressure)
1. Work of heart
Main parameter describing the work of heart: Cardiac Output (CO) = the amount of blood going into the aorta from the left ventricle per unit time.
Increased Frequency also increases blood pressure, since increased blood volume is forwarded from venous reservoir to the arterial resistance system.
- *Artificial increase** = (pacemakers)
- *Natural increase** = Sympathetic activation)
Runoff can not match up to the increase immediately, so the consequently increased arterial volume (Va) creates increased blood pressure.
Runoff- it is the amount of blood, which moves from the arterial to the venous part.
2. Total peripheral resistance (TPR)
TPR ↑ → blood pressure ↑
-Increased peripheral resistance (arterial contraction) will also increase the arterial blood pressure.To keep the elevated blood pressure, the heart needs extra work.
- *3. Role of the structure of the vessels**
- Arterial distensibility and compliance
- Vessels are not rigid tubes, but distensible ones due to elastic elements and myogenic elements. These keep the vessels in contracted state in resting.
- Increased pressure creates sudden increase of flow due to the increasing diameter of the distensible wall.
- Dropping pressure, causes inproportional decrease in vessel diameter, due to the intrinsic-myogenic and vasoconstrictor tone.
- The pressure where the vessels are is called: critical closing pressure.
- Distensibility is used, if different circulations are compared.
- Compliance is used, when the absolute change is of interest.
- *4. Distribution of blood in the circulatory system – (volume of blood in the arterial system)**
- Distribution is determined by total cross sectional area of vessels and the structure of the wall.
- Physiological relevance: cardio-vascular regulation may „rearrange” the blood distribution according to the need of the organism.
- Effect of arterial volume on blood pressure
arterial volume ↑ → blood pressure ↑
• The increase of SV (hence the increase of arterial volume change): will cause a sudden increase of average arterial pressure and pulse pressure.
-This is characteristic to the healthy animals, where the compliance is constant.
5. Location of the organs in the circulation – Paralelly attached organs in circulation
The circulatory bed of the individual organs are parallely attached to each other.
-This makes it possible:
that the changes in the perfusion of one organ does not suddenly changes the work of the heart
6. Types of blood flow
Laminar
-sheets of fluid move above and next to each other.
Turbulent
-due to the high velocity of the flow, turbulence occurs. Which can be characterized by the Reynold’s number.
Physiological relevance of laminar flow
-Resistance is smaller, so less work of the heart (Stokes-
law)
Runoff (Qr)
(picture)
Capillaries:
Maintaining the capillary blood pressure.
-Microcirculation
Veins
Factors maintaining the venous blood pressure:
- „vis a tergo” the work of the heart (force from behind)
- Gravitation
- Venous valves
- Skeletal muscle pump
- Changing pressure in chest and in abdomen
- CVP (central venous pressure)
Skeletal muscle pump
- Rhytmic changes of muscle tension „pumps” the blood, direction of flow is always centripetal due to the valves; in case of weak muscle tension, blood accumulates in the vein, creating retrograd capillary flow – increased pressure – leading to edema!
- Changing pressure in chest and in abdomen
- „chest pump”; during inspiration intrathoracal pressure decreases and facilitates the filling of right atrium (RA), while abdominal pressure changes the transmural pressure of veins running through the abdomen.
Blood Pressure
Blood flow is predominantly maintained by blood pressure (beyond the work of the heart).
- Blood pressure is maintained by the work of the heart and the peripheral resistance.
- Blood flows in the vessels according to its pressure gradient.
- The peripheral blood pressure is kept in a narrow range by the cardiovascular regulatory mechanism.
- That guarantees the migration and transport of substances in the territory of microcirculation.
- The blood pressure is higher in systole than in diastole. The
mean arterial pressure is the weighted average of the systolic and diastolic pressure.
Blood pressure at the different part of the circulation
- *Arterial blood pressure :**
- Systolic pressure
-Diastolic pressure
-Pulse pressure
Difference between systolic and diastolic pressures.
-Mean arterial pressure
During normal cardiac cycle, diastole is longer thensystole (cca 2 times). Mean arterial pressure is the average of the systolic and diastolic pressure (due to the difference in duration).
Static pressure (can be measured postmortem)
Capillary blood pressure
Venous blood pressure
Central venous pressure (CVP)
Due to gravity, the arterial pressures are lower above the heart and higher below the heart.
Relationship between blood pressure and body size
There is no direct correlation between the size of an animal and the arterial blood pressure, however mammals with a large vertical distance between the heart and the brain exhibit the highest pressure.
- The giraffe for example has 300 mmHg average arterial pressure at the level of heart in order to maintain 100 mmHg at the brain level, which provides the appropriate blood supply to the brain, even when the head is raised.
- In cow however, 100 mmHg at the heart level is enough to create 50 mmHg at the brain level, in order to supply the brain with the necessary amount of blood.
Measurement of blood pressure
1.Direct method
The direct method has been originally used by Hales in horse, in 1770.A glass catheter was inserted to the carotis and height of the fluid column was measured.
It can be used even today in 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.
2.Indirect methods:
Palpation method
- During indirect blood pressure measuring methods an artery (usually the brachial artery) is first compressed then released slowly.
- If we raise the cuff pressure over the systolic pressure, circulation will cease in the compressed brachial artery.
- Then, if the pressure in the cuff is decreased slightly below the systolic pressure, at the peak of the systole a small volume of blood will slip through between the walls of the artery before they clap together again.
- At this point pulse on the radial artery my be felt again (=systolic blood pressure)
Ausculatation method
If the pressure in the cuff is decreased slightly below the systolic pressure, at the peak of the systole a small volume of blood will slip through between the walls of the artery before they clap together again. Therefore, a clear tapping sound is heard at every pulse beat. The appearance of this sound indicates the value of systolic blood pressure.
-If we decrease the cuff pressure further, the sound will become louder. If the cuff pressure drops below the diastolic pressure, i.e. if blood circulates in the arteries also during diastole, the walls of the artery will not clap together. From this time on the blood flow in the artery will be laminar and the tapping sound will disappear. The disappearance of the tapping sound indicates the value of the diastolic pressure.
