This lecture covered quite a bit of high school physics level stuff, but unfortunately I didn't do high school level physics (at least not year 11/12). Oh well.
You've probably had it drilled into you over and over again (possibly a few times by this blog) that blood flows from a region of higher pressure to a region of lower pressure. Well, now I'm going to blow your mind by telling you that that's not strictly true. Blood actually flows down an energy gradient rather than a pressure gradient. If the pressure gradient was the be-all-and-end-all, then certain phenomena would not be possible, as we will soon explore.
Energy loss and pressure loss often come hand in hand, as pressure is a form of energy. When fluid flows against a resistance, energy is lost as heat, manifesting as a decrease in pressure. When resistance is increased, like in stenosis (narrowing of the arteries), pressure decreases further. Pressure goes back up once you get past the stenosis, but blood is still able to flow up the pressure gradient as the energy gradient is still decreasing (provided that the vessel is not completely blocked). But how?
Well, we need to consider another interesting thing that happens in stenosis. Kinetic energy increases in stenosis, as velocity of a fluid increases as cross-sectional area of the vessel decreases, and kinetic energy is equal to (1/2)mv2. This means that even though the pressure in a stenosis decreases, the rise in kinetic energy is enough to give the stenotic area more energy than the area after the stenosis. Therefore, blood can keep flowing past the stenotic area.
Effects of Gravity on Blood Pressure
Hydrostatic pressure, or pressure in the fluid due to gravity, is equal to (rho)gh, where rho is the density of the fluid in kg/m3, g is the gravitational constant (roughly 9.8 m/s2) and h is the depth in m. Applying this formula, the pressure at the bottom of a 1m column of water is 1000 kg/m3 * 9.8 m/s2 * 1m = 9800Pa (Pascals). As 1Pa = 0.0075mmHg, this can be converted into 73.6mmHg.
Now, we don't study biology just to look at columns of water, so let's look at animals and humans! Let's start off with giraffes with their long necks. The head of a giraffe is roughly 1.5m above its heart, which equates to a pressure difference of around 115mmHg. The significance of this is that the heart needs to pump blood at a high enough pressure to keep the brain perfused as well. It's no good for the heart to provide a pressure of 115mmHg, because while the blood would be able to leave the heart, it would have a pressure of 0mmHg by the time it reached the brain, which is clearly not enough to keep a giraffe's brain going. Hence, a giraffe's arterial pressure is quite high (around 200mmHg). Fun fact: you can also get an idea of an animal's blood pressure by looking at the size and thickness of the left ventricle. So if you can't be bothered using your knowledge of physics to make an estimated guess, cut out their hearts instead :P
Hydrostatic pressure, or pressure in the fluid due to gravity, is equal to (rho)gh, where rho is the density of the fluid in kg/m3, g is the gravitational constant (roughly 9.8 m/s2) and h is the depth in m. Applying this formula, the pressure at the bottom of a 1m column of water is 1000 kg/m3 * 9.8 m/s2 * 1m = 9800Pa (Pascals). As 1Pa = 0.0075mmHg, this can be converted into 73.6mmHg.
Now, we don't study biology just to look at columns of water, so let's look at animals and humans! Let's start off with giraffes with their long necks. The head of a giraffe is roughly 1.5m above its heart, which equates to a pressure difference of around 115mmHg. The significance of this is that the heart needs to pump blood at a high enough pressure to keep the brain perfused as well. It's no good for the heart to provide a pressure of 115mmHg, because while the blood would be able to leave the heart, it would have a pressure of 0mmHg by the time it reached the brain, which is clearly not enough to keep a giraffe's brain going. Hence, a giraffe's arterial pressure is quite high (around 200mmHg). Fun fact: you can also get an idea of an animal's blood pressure by looking at the size and thickness of the left ventricle. So if you can't be bothered using your knowledge of physics to make an estimated guess, cut out their hearts instead :P
Consequences of Gravity on the Dependent Vasculature
"Dependent vasculature" refers to basically all the vessels below the heart. This is particularly important to us as bipeds, as quite a large proportion of our vasculature is below the level of the heart.
First, let's consider a U-shaped tube lying flat on a table. If it has an inlet pressure of 100mmHg and an outlet pressure of 0mmHg (all relative to atmospheric pressure), the pressure halfway along the tube will be 50mmHg.
Second, let's consider a U-shaped tube that is upright, with inlet and outlet pressures of 0mmHg (again, all relative to atmospheric pressure). The pressure at the bottom of the tube (where the bend is) will depend on the hydrostatic pressure equation (P = (rho)gh). For the sake of this example, let's just pretend that this pressure is 80mmHg.
Now, let's combine our two examples by simply adding the two together! This gives us a new, upright tube with an inlet pressure of 100mmHg, an outlet pressure of 0mmHg and a pressure of 130mmHg in the bend (50mmHg + 80mmHg). Note, however, that despite this higher pressure at the base, the pressure difference (100mmHg to 0mmHg) is still the same, meaning that the flow is still the same.
The increased pressure at the base is not insignificant, however. Our feet have a higher venous pressure than our legs, which have a higher venous pressure than our heart, and so on. This means that our blood will tend to pool in the veins (and I say the veins specifically as they are more compliant and better at storing blood than the arteries). This has two main effects.
Firstly, as I mentioned here, the main force pushing blood out of the capillaries is the hydrostatic pressure of the capillaries. If this increases, then much more fluid will leak out of the capillaries, causing oedema over time. (This can be prevented by wriggling your feet and lower limbs a little bit.)
Secondly, a combination of blood pooling in the lower limbs and blood leaking out of the capillaries leads to a reduction in venous return. Reduced venous return leads to reduced stroke volume, which leads to reduced cardiac output, which leads to reduced blood pressure, which would lead to fainting if our bodies didn't do anything. Thankfully, our bodies have a trick up their sleeves!
"Dependent vasculature" refers to basically all the vessels below the heart. This is particularly important to us as bipeds, as quite a large proportion of our vasculature is below the level of the heart.
First, let's consider a U-shaped tube lying flat on a table. If it has an inlet pressure of 100mmHg and an outlet pressure of 0mmHg (all relative to atmospheric pressure), the pressure halfway along the tube will be 50mmHg.
Second, let's consider a U-shaped tube that is upright, with inlet and outlet pressures of 0mmHg (again, all relative to atmospheric pressure). The pressure at the bottom of the tube (where the bend is) will depend on the hydrostatic pressure equation (P = (rho)gh). For the sake of this example, let's just pretend that this pressure is 80mmHg.
Now, let's combine our two examples by simply adding the two together! This gives us a new, upright tube with an inlet pressure of 100mmHg, an outlet pressure of 0mmHg and a pressure of 130mmHg in the bend (50mmHg + 80mmHg). Note, however, that despite this higher pressure at the base, the pressure difference (100mmHg to 0mmHg) is still the same, meaning that the flow is still the same.
The increased pressure at the base is not insignificant, however. Our feet have a higher venous pressure than our legs, which have a higher venous pressure than our heart, and so on. This means that our blood will tend to pool in the veins (and I say the veins specifically as they are more compliant and better at storing blood than the arteries). This has two main effects.
Firstly, as I mentioned here, the main force pushing blood out of the capillaries is the hydrostatic pressure of the capillaries. If this increases, then much more fluid will leak out of the capillaries, causing oedema over time. (This can be prevented by wriggling your feet and lower limbs a little bit.)
Secondly, a combination of blood pooling in the lower limbs and blood leaking out of the capillaries leads to a reduction in venous return. Reduced venous return leads to reduced stroke volume, which leads to reduced cardiac output, which leads to reduced blood pressure, which would lead to fainting if our bodies didn't do anything. Thankfully, our bodies have a trick up their sleeves!
Physiological Response to Potentially Detrimental Effects of Gravity
BARORECEPTOR REFLEX.
Also just a note on how our bodies respond to an increased/decreased gravitational force as compared to usual. These are conditions that might be experienced by astronauts and/or stunt pilots. When G (gravitational force) is negative, blood moves to the head, causing "redout" from broken capillaries in the eyes. When G is larger than usual, blood moves to the feet, causing "blackout" from loss of blood to the head.
BARORECEPTOR REFLEX.
Also just a note on how our bodies respond to an increased/decreased gravitational force as compared to usual. These are conditions that might be experienced by astronauts and/or stunt pilots. When G (gravitational force) is negative, blood moves to the head, causing "redout" from broken capillaries in the eyes. When G is larger than usual, blood moves to the feet, causing "blackout" from loss of blood to the head.
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