Arterial pressure Flashcards
Learning objectives
• To frame the role of the cardiovascular
system as a sophisticated “delivery
system” for the body, delivering gases
and nutrients, and taking wastes away
according to metabolic needs
• To define arterial blood pressure and
characterise the factors that influence it
• To delineate how arterial blood pressure
is regulated within tight margins by
neural, reflex and hormonal means
Framing the problem
• Not too simple to describe the cardiovascular system as a delivery system.
• Think of the characteristics of an efficient delivery system in a real life situation.
• The ideal delivery system
-Good tracks to and from delivery site
-Enough force (fuel) to get to and from delivery site
-Controlled supply to match demand
Histology of aorta, vein, artery and arteriole
Aorta (conducting, or elastic artery)
o Relative size of wall and lumen
Thick wall- elastic lamina, thick compared to lumen (generate pressure), proximity to heart
o Composition of wall- elastic lamina
o Function (Windkessel-smooth output, vessels with elastic lamina that absorbs pressure )
Main conducting artery, transports blood from heart
Artery and vein contrasted
o Relative sizes of wall and lumen
The vein has a much wider lumen, with the artery having a much thicker muscular wall- width can be controlled by nervous stimuli
o Composition of walls
Walls of arteries are muscular, veins are thin endothelium
o Function
Arteries are blood vessels responsible for carrying oxygen-rich blood away from the heart to the body. Veins are blood vessels that carry blood low in oxygen from the body back to the heart for reoxygenation.
Distributing arteriole
Arterioles are the main distribution vessels of the body. Small changes in the caliber of the arterioles cause large changes in total peripheral resistance.
o Composition of wall- nerve ending dense, very controllable
o Function
Capillaries- no muscle, just endothelium and basal lamina for easy diffusion of gases etc
Arterial blood pressure
- Blood pressure (defined as force per unit area) is the pressure exerted on the walls of the arteries.
- Main conducting arteries
- Pressure head imparts motive force for peripheral perfusion
- Peak during systole/nadir during diastole
- Values going from LV – Aorta
- Remember Windkessel function of aorta (term accounts for the shape of the arterial blood pressure waveform in terms of the interaction between the stroke volume and the compliance of the aorta and large elastic arteries and the resistance of the smaller arteries and arterioles.)
- Smooths wave out.
Effect on MAP (mean arterial pressure) and changes to it
MAP is taken as diastolic pressure + one third the difference between systolic and diastolic pressure (pulse pressure). (MAP = DBP + (pulse pressure/3).
- Posture has an effect on this pressure. When standing, pressure in any vessel above the heart becomes reduced, and conversely, pressure in any vessel below the heart increases (because of the force of gravity, there is a hydrostatic pressure due to the weight of the blood). This reduction/increase has a value of 0.77 mmHg for every cm difference from the level of the heart. Representative values have pressure in the feet at 183 mmHg (93mmHg generated by heart + 90mmHg hydrostatic component). In the head the pressure is reduced so a characteristic mean arterial pressure is 60mmHg (93mmHg generated by the heart – 33mmHg hydrostatic component). Because of the effects of age on the stiffness of the vascular tree, blood pressure climbs as one gets older, with the effects of stress, poor diet, lack of exercise contributing to this climb.
Determinants of blood pressure
• Arterial blood pressure is directly
related to
• Cardiac output
• Total peripheral resistance (mainly in the arterioles)
• Calculated by multiplying the two
• BP = C.O. X T.P.R.
• Control of BP through control of either variable
The baroreceptors
• Stretch receptors in the walls of the carotid sinus and
aortic arch
• Signals via vagal (X) and glossopharyngeal (IX) afferents
• To the medullary cardiac and vasomotor areas
• Make changes appropriate to keep BP within normal
ranges
Cardiac output and baroreceptors
- Afferents from baroreceptors synapse in the NTof the medulla
- Neurons inhibitory to sympathetic outflow
- Neurons excitatory to vagal outflow
- Net effect is to reduce cardiac output
Afferents from the baroreceptors reach the nucleus of the tractus solitarius in the medulla oblongata where they synapse with neurons that inhibit sympathetic outflow to the heart, thus reducing cardiac ouput. The baroreceptor afferents also stimulate cardiac vagal fibers to slow the heart, also reducing cardiac output.
TPR (total peripheral resistance) and baroreceptors
• Vasomotor centre, strictly speaking a misnomer
• Projects to noradrenergic sympathetic efferents
that constrict blood vessels (esp. arterioles and veins)
• TPR can be neurally controlled therefore by altering vasomotor centre activity
• Baroreceptor stimulation inhibits output from the vasomotor centre
The integrated response
- A rise in BP leads to
- ↑baroreceptor stim
- ↓ Symp output to Heart
- ↑ Vagal tone
- ↓ Vasomotor tone
- ↓ cardiac output
- ↓ TPR
- Both changes act to reduce, BP back to normal
- A fall in BP leads to
- ↓ baroreceptor stim
- ↑ Symp output to Heart
- ↓Vagal tone
- ↑ Vasomotor tone
- ↑ cardiac output
- ↑TPR
- Both changes act to increase, BP back to normal
Hospital- baroreceptor testing
- Simply standing up!
- Infusing an adrenergic agonist phenylephrine. Thereshould be a linear relationship between blood pressure and RR interval (ie as BP goes up HR should come down by the baroreceptor reflex)
- Valsalva manoeuvre (forced expiration against a closed glottis), measure BP and HR on a trace
- Initial rise due to physical pressure in the chest
- Then fall because of venous compression and ↓ CO (Starling)
- Then rise because of baroreceptor compensation
- When you stop straining pressure rises due to the restoration of cardiac output against a background of high peripheral resistance. This stimulates baroreceptors so that BP then falls back to normal
Other reflexes
- Low pressure side of the circulation
- Monitor blood volume as opposed to pressure
- Walls of atria (type A and B), entrance of superior and inferior vena cava, pulmonary veins)
- If stimulated by rises in venous return, they cause venodilation, vasodilation and a fall in BP.
- May cause HR to rise.
- Bainbridge, Bezold Jarisch reflexes
Hormones and blood pressure
- Long term BP maintenance
- Noradrenaline + Adrenaline
- Vasoconstrictors, although Adr is a vasodilator in skeletal muscle
- Angiotensin; vasoconstriction + sodium retention via axis.
- Kinins – vasodilatation
- ANP (atrial natriuretic peptide) inhibits vasoconstriction and causes natriuresis when blood volume increases
- Endothelin and EDRF
