Blood Pressure ratio question

[OnceFuturePoly]

My understanding is fuzzy, so please excuse the wording of this question. I'm hoping a nurse or other medico can shed some light, please?

The body needs a difference between the two blood pressures (low - Diastolic and high - Systolic) to force oxygen through the capillaries on each heart beat.

Is there some rule of thumb for the minimum difference between the two measures?

I kinda of remember some formula...

And does age or altitude affect things, like change some number in the formula?

Thanks!

 

[shiny5437]

I'm not familiar with any clinically significant ratio between diastolic and systolic blood pressure. There are equations that relate diastolic and systolic to mean arterial blood pressure. Guidelines on BP control talk in terms of absolute pressures rather than ratios or differences between the two.

Finally it's not the difference between systolic and diastolic that pushes the blood round. It's the difference between arterial pressure on one side of the tissue and venous pressure on the other side. People on cardiac bypass have a single constant arterial pressure rather than a high/low systolic /diastolic blood pressure and the blood still goes around

Age tends to make blood pressure increase due to stiffening of the arteries (although the physiology is a bit more complex), and altitude I'm not sure about but certainly low oxygen levels and height cause hyperventilation and lower carbon dioxide these can change BP.

Why do you want to know?

 

[Bramblethorn]

Shiny is correct here. The difference between systolic and diastolic isn't necessary to make blood circulate, it's just that we evolved with a pump that works in pulses. Some folk have artificial hearts that maintain a steady arterial pressure, and they do OK with no pulse at all.

 

[OnceFuturePoly]

Yes, it's the pressure differential across the capillaries that pushes the platelets through. Yes, the high side pressure can be constant (artificial heart, bypass pump) or pulsed (normal heart).

The question here is about the net result of a pulsed arterial supply.

The graph of arterial pressure is choppy. A fast increase from low to high as the heart chamber compresses, followed by a sliding decline as the pressure built up bleeds away through the capillaries.

Overlay that with a graph of venous pressure. It will be a lot smoother, but still show an increase/decrease cycle during the time the arterial pressure is up.

Complicate that with the pulse rate, which feeds another pressure pulse into the system before the effects of the previous completely decay.

(If someone wanted want to give some college credit, it might be worth dragging out the fluid mechanics equations, fudging in the elasticity of the vessels, and drawing out the curves... Better yet, do a computer simulation. It's all basic engineering.)

Now validate things, i.e. magically measure the pressures at capillary entry and exit, and validate the assumption that pulse rate and low & high blood pressures do have a distinct and controlling effect.

Finally, add a curve showing how many platelets (rate) are moving through the capillaries. Consult the physiologists for the minimum numbers (which varies of course by physical activity). Hopefully enough platelets are moving enough of the time to meet the body's current demand!

Now play around with the parameters of the system (Systolic, Diastolic, Pulse rate). Any "rules of thumb" come up?

PS: I'm also assuming the platelets are "fully loaded" with oxygen. Adding in variable breathing rate and pulmonary efficiency is a complication...

 

[Bramblethorn]

Yes, it's the pressure differential across the capillaries that pushes the platelets through. Yes, the high side pressure can be constant (artificial heart, bypass pump) or pulsed (normal heart).

The question here is about the net result of a pulsed arterial supply.

Sorry, now I'm not quite sure what you're asking. You initially stated that "The body needs a difference between the two blood pressures (low - Diastolic and high - Systolic) to force oxygen through the capillaries on each heart beat" (which isn't correct) and then asked "Is there some rule of thumb for the minimum difference between the two measures?"

As I stated and you seem to accept, there doesn't need to be any minimum difference between those two. Constant arterial pressure works just fine.

Unless... you're not confusing "venous" and "diastolic" pressure, perhaps? Because that's not what diastolic is. Systolic and diastolic are both measures of arterial pressure, at high and low extremes.

Overlay that with a graph of venous pressure. It will be a lot smoother, but still show an increase/decrease cycle during the time the arterial pressure is up.

It's been a long time since I did physiology, and I'm willing to be contradicted by cites, but IIRC venous pressure is very close to zero (relative to ambient) and the resistance of the capillaries pretty much damps out any pressure variability coming through from the arteries.

(If someone wanted want to give some college credit, it might be worth dragging out the fluid mechanics equations, fudging in the elasticity of the vessels, and drawing out the curves... Better yet, do a computer simulation. It's all basic engineering.)

Circulation isn't my area, but I can tell you first-hand that pretty much nothing associated with modelling human physiology is "basic engineering". Biological tissues and fluids have complex properties that require getting into viscoelasticity, anisotropy, shear flow, and a bunch of other things that make modelling a pain in the neck.

Now validate things, i.e. magically measure the pressures at capillary entry and exit, and validate the assumption that pulse rate and low & high blood pressures do have a distinct and controlling effect.

When did we get into pulse rate? I thought we were talking about the pressures; rate is another thing again.

Finally, add a curve showing how many platelets (rate) are moving through the capillaries. Consult the physiologists for the minimum numbers (which varies of course by physical activity). Hopefully enough platelets are moving enough of the time to meet the body's current demand!...
PS: I'm also assuming the platelets are "fully loaded" with oxygen. Adding in variable breathing rate and pulmonary efficiency is a complication...

Oxygen is transported by red blood cells, not platelets. Platelets are about clotting.

 

[shiny5437]

******Thanks to Bramblethorn for spotting the error in the MAP equation - serves me right for doing physiology after my bedtime*****


Just going to jump into what Bramblethorn has said. This is in no specific order but I hope it makes sense. It's going to be bit of a physiology 101.

1. Oxygen is delivered by red blood cells (which contain haemoglobin). If you're interested in O2 delivery to tissues you need to think about the O2 carrying capacity of blood and cardiac output.

So O2 delivery = cardiac output * O2 content of blood
O2 content of blood = spO2*1.34*[Hb] + pO2/ 3000 (I think it's 3000 )

2. Cardiac output is related to how much the heart ejects per beat (stroke volume) and heart rate. Also related to this is the systemic resistance of the vessels. These interact according to BP. You can have a high BP but because the vascular resistance is so high little blood flows. Or vice versa a low resistance system with low pressure but the actual flow is large. This is basically Ohm's law. Mean BP = cardiac output * vascular resistance. Cardiac output = heart rate * stroke volume.

3. At the level of the tissues you can think of an effective perfusing pressure with diastolic and systolic working as an effective average / mean arterial pressure. MAP = DBP + 1/3 (SBP-DBP). On the pulse pressure graph you can also calculate this value by drawing various 'clever lines'.

4. Venous pressure is pretty much constant at the venous side of the capillaries. Check out Starling forces for this bit of the physiology. There are venous pressure waves but these are more in the larger veins due to heart filling and contraction. Increased venous pressure can reduce the flow of blood through the tissues.

5. Modelling is a pain in the neck. You can think in terms of simple laminar flow, but a lot of the pressure waveform comes actually from the elastic recoil of of the tissues and pressure wave reflections back and forth in the system. It gets complicated.

And if you really want to complicate it all you can look into Guyton curves and convince yourself that it's Central venous pressure that controls arterial blood pressure.

Why do you need to know this?
Why am I on sex website talking physiology?

 

[desertslave]

Both high and low BP differentials can indicate disease or injury. If either is evident, then the real number you would want to know about is the ejection fraction. This is very well explained (and much better than I can paraphrase) on Wiki.

http://en.wikipedia.org/wiki/Ejection_fraction

 

[OnceFuturePoly]

...
Oxygen is transported by red blood cells, not platelets. Platelets are about clotting.

Told you I was fuzzy... I was wrong to say platelets when I meant RBC.

...

3. At the level of the tissues you can think of an effective perfusing pressure with diastolic and systolic working as an effective average / mean arterial pressure. MAP = DBP + 1/3 (SBP-DBP). On the pulse pressure graph you can also calculate this value by drawing various 'clever lines'.

...

That's the formula I was trying to remember. Thank you very much.

(And yes, it has little to do with the awkward way I asked my question. This quote from Wikipedia about Mean arterial pressure is where I was headed.)

Clinical significance

MAP is considered to be the perfusion pressure seen by organs in the body.

It is believed that a MAP that is greater than 60 mmHg is enough to sustain the organs of the average person. MAP is normally between 70 to 110 mmHg

If the MAP falls significantly below this number for an appreciable time, the end organ will not get enough blood flow, and will become ischemic.

...
Why do you need to know this?
Why am I on sex website talking physiology?

Of all the places I hang where you can ask serious or technical questions, I found the best answers on the How To forum. The depth and width of people interested in Literotica is incredible!

(And the shallowness of the trolls and wankers is notorious.)

 

[shiny5437]

Thanks to Bramblethorn for spotting my mistake in the MAP equation! Thanks dude.

Wikipedia's value of 60mmHg of pressure isn't a strict 'universal' thing. For example if people have chronic high blood pressure the tissues adjust such that they need a much higher MAP than 60mmHg.

And consider that the pressure in the pulmonary artery is 9-19mmHg (according to Wikipedia) and has a blood flow equal to cardiac output!

It's not all about the pressure.

Guyton's physiology is good and actually very readable. Much better than my half - remembered fumblings.

I've gotten good advice from the forum before so nice to be able to offer something back.