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ROLE OF BARORECEPTORS DURING STANDING

HIGH PRESSURE RECEPTORS that will detect blood pressure lowering during standing. Many people think that blood pressure is lowering during standing, but this is not necessarily so.

LOW PRESSURE RECEPTORS 49 CirculationThe low pressure receptors are important for standing because central venous pressure will fall and the filling pressures of the heart will fall and it means that we always need the response of these low pressure receptors (cardiopulmonary baroreflex) to stand. Lower the filling pressure of the heart, for instance, haemorrhage a large loss of blood, do the same and also haemorrhage will have a sympathetic response on this basis. A-fibers fire in synchrony with atrial contraction; they monitor heart rate. B-fibers fire in synchrony with ventricular systole; they monitor atrial filling & CVP.

Fast reaction (seconds –minutes): autonomic nervous system maintains the circulation by sympathetically induced vasoconstriction:Baroreceptor

feedback loop – that happens during standing up is always a decrease of right atrial pressure. This must happen because these are the laws of gravity and anyway, that will activate the cardiopulmonary baroreflex and if arterial blood pressure goes down as well, then the arterial baroreflex is also activated and both reflexes work in the same way and they will increase sympathetic outflow and decrease parasympathetic outflow. This has consequences for systemic vascular resistance (SVR) and for the cardiac output (CO), vasoconstriction and all together this will more or less compensate part of the blood pressure. This reaction is fast, it happens in seconds and during several minutes before the event in angiotensin aldosterone system comes into play, it is the only compensatory mechanism that we have. A sudden decrease in arterial pressure, as occurs when a person suddenly stands up from a supine position, decreases baroreceptor firing, activating sympathetic nerves and inhibiting.parasympathetic (vagal) nerves.
This change in autonomic balance increases (+) cardiac output (CO) and systemic vascular resistance (SVR), which helps to restore normal arterial pressure.
Lower Body Negative Pressure - The effects of standing are sometimes simulated by applying lowered body negative pressure. So there is negative pressure around your abdomen and legs. In this way, the responses to standing can be imitated and very precisely controlled by the amount of lower body negative pressure.
Splanchnic and forearm blood flow decrease with right atrial pressure LBNP = lower body negative pressure; MP = mean pressure; PP = pulse pressure; RAP = right atrial pressure; HR = heart rate; SBF = splanchnic blood flow; FBF = forearm blood flow.
In an old publication, we see exactly what happens during lower body negative pressure. It is an old publication, but it is an important publication because here the pressure in the aorta and in the right atrium have been measured by invasive method (by a)

catheter) and nowadays these measurements would be forbidden because of ethical reasons. So, publication shows us what happens when blood pulling in the legs progressively increase with lower body negative pressure.

50 Circulation

In LBNP we see the pressure go down slowly till - 50 mmHg. The Aortic MP (mean pressure), and this is very striking, is staying constant over a long trajectory only at the end it's going down. The Aortic PP (pulse pressure) is also remaining constant over a certain trajectory, but then it goes progressively down. What we see and this is an important observation is that immediately the right atrial pressure (RAP) starts to go down when there is low body negative pressure applied.

The heart rate is only going up in a in a later stage and there is a reason for that, because we can safely assume that it also decreases, which is a proof of sympathetic activation and vasoconstriction. The reason that the heart rate is not going up immediately is because the increased

sympathetic outflow is compensated by the decrease in the right atrial pressure and when the RA pressure decreases, the intrinsic heart rate goes down. So, over a certain trajectory, nothing happens to heart rate and then it starts to increase. The SBF (splenic blood flow) is more difficult to measure but also there can you see that it immediately goes down when the lower body negative pressure starts, and this is the proof that the baroreflexes work perfectly because the aortic mean pressure and even the pulse pressure of a certain trajectory remain more or less constant. The right atrium pressure changes or the right atrial pressure is inducing the cardiopulmonary baroreflexes that keeps all other pressures okay and keeps heart rate okay, while at the mechanism of this is vasoconstriction, then we see it here as proof because the FBF decreases immediately from the beginning. So this is the physiology of lower body negative pressure but it is also the physiology of standing. Role of humoral.

control during standing - Late reaction (minutes-hours): RAA System induces vasoconstriction and fluid retention: maintain right atrial pressure and cardiac output! Formation of angiotensin II and its effects on renal, vascular, and cardiac function. Renin is released by the kidneys in response to sympathetic stimulation, hypotension, and decreased sodium delivery to distal tubules. Renin acts upon angiotensinogen to form angiotensin I (AI), which is converted to angiotensin II (AII) by angiotensin converting enzyme (ACE). AII has several important actions: stimulates aldosterone release, which increases renal sodium reabsorption; directly stimulates renal sodium reabsorption; stimulates thirst; stimulates release of antidiuretic hormone (ADH); produces systemic vasoconstriction; activates the sympathetic nervous system; and causes cardiac and vascular smooth muscle hypertrophy. The overall systemic effect of increased AII is increased blood volume, venous pressure, and arterial pressure.

reaction (minutes-hours): RAA System induces vasoconstriction and fluid retention: maintain rightatrial pressure and cardiac output! The RAA system takes in after a few minutes, possibly 10 minutes or 15minutes after standing started and will last for the time that we are standing so it can remain active for hours.The reason that it begins is because of the elevated sympathetic stimulation that started immediately whenwe started standing. All the reactions of the RAA system are trying to keep the blood pressure filled and thereis also extra sympathetic activation, extra systemic vasoconstriction. The system seems to control arterialpressure, but of course it controls also the right atrial filling pressure, and this is what is necessary to maintaincardiac output. 51 CirculationRole of activity in the upright position – Standing causes high hydrostatic pressures and when there are highhydrostatic pressures in the lower legs edema can easily occur and actually edema has loss of

circulating volume into the interstitial. So that is due to the fact that high hydrostatic pressures increase the capillary filtration. This is not what we want because the volume has to be in the circulation and not in the interstitium. So edema is disadvantages for the circulating volume and in this way, also disadvantages for a correct Right Atrial pressure and the heart has to fill the right atrial pressure has to be sufficiently high to fill the heart.

Any activity, slow walking already, reduces enormously the venous pressure in the lower legs and will help to promote venous return and to prevent the development of edema, so a little movement is much better than standing completely still.

Muscle pump - Muscle pump reduces venous pressure by 70 mmHg, by temporarily reducing the venous blood column that rests on the foot. Muscle pump is not only useful to cause venous return or to help in the generating venous return, but it's also very useful to decrease the lower leg venous pressures.

What happens is that if you are standing still, the amount of venous blood that is appearing in the circulation is slowly opening all valves and when the valves open, the total column rests on the feet and that gives a very high venous pressure. One movement of the muscles, one step forward, for instance, would squeeze all the blood out of this compartment. And when the muscle releases again, the valves close and this means that the venous pressure is now tremendously reduced. This is the effect, and this helps to prevent an edema. What will be the final situation during standing? With low-compliant vasculature, not too much blood will pool in the legs, CVP and CO will not decrease too much. Compliant vasculature causes blood pooling, and CVP and CO, and possibly MAP will fall dramatically. Sufficient vasoconstriction is necessary!
  1. A: we have the supine situation with a certain central venous pressure, a certain mean arterial pressure.
  2. B and C: what happens in a rigid circulation (a low-compliant vasculature).

compliant circulation) and compliant circulation? A rigid circulation could be an old person and a compliant circulation could be a young person.

What happens in the rigid circulation is that the pressures remain about the same because there is no pooling of blood possible in the legs because the person is low compliant. Compliant persons will have a lowered central venous pressure and as a consequence, the cardiac output will decrease and likely not always sure because we have a baroreflex, the mean arterial blood pressure will go down as well.

So, we have, in a way, sufficient vasoconstriction, we need it. Certainly when we have a compliant circulation, which is a good thing by itself, we need sufficient vasoconstriction also to keep up the mean arterial blood pressure.

52 CirculationDistribution of orthostatic systolic blood pressure response among participants of the Malmö Preventive Project (n = 32 699) – In a large study in which almost 33000 individuals were included, the

Malmo PreventiveProject. They have measured the blood pressure response to standing. What happens to the systolic blood pressure when they stand up? For healthy persons we see that response in systolic blood pressure centres around zero, it means that most people have no change of systolic blood pressure. There is some variability: it happens that systolic blood pressure drops 10-20mm or increases 10-20mm upon standing, and this means that is not necessarily an hypotensive or high hypertensive response. The idea is that the sympathetic excitation gives so much vasoconstriction that blood pressure can easily remain the same or even a little bit more. Also, the compensation can be a little incomplete and then there is a little blood pressure lowering. This is absolutely not relevant for the way people are experiencing standing, because what counts is the cardiac output and not directly the blood pressure.

Fainting - Fainting is a well-known complication of standing. This is called syncope Transient

Loss of Consciousness (TLOC) occurs due to cerebral hypo-perfusion. It is called transient loss of consciousness because when so.

Dettagli
Publisher
A.A. 2020-2021
67 pagine
SSD Scienze mediche MED/23 Chirurgia cardiaca

I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher maria456789 di informazioni apprese con la frequenza delle lezioni di Physiological signal processing and modelling in cardiology e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Università Politecnica delle Marche - Ancona o del prof Swenne Cees A..