Physical Principles That Influence Blood Circulation – Blood vessels and blood circulation

00:00 Finally, let’s talk about which physical principles influence the blood circulation.

00:08 We talked a little bit about this before. Remember that the blood velocity – the speed with which the blood is travelling in the blood vessels – is greatest just after it leaves the left ventricle, just above the aortic valve in the aorta. And the speed decreases progressively with decreasing vessel size or diameter. There’s a significant drop off in speed in the arterioles – remember the resistance vessels. And then there’s very slow flow through the capillaries. That’s important because that’s when all the work is being done of exchanging oxygen, nutrients and waste products. So that you can see that the speed in the aorta is 30 cm/s for the blood whereas in the capillaries it’s only 0.026 cm/s. But that’s very important because we want plenty of time for the red blood cells to give up their oxygen and get in the carbon dioxide from the cells and give off nutrients to the cells.

01:19 This is one of the physical principles that’s most important in the cardiovascular system is called the law of continuity. That is the volume of flow per second has to be constant throughout the entire system. And, of course we know that because there’s so much greater surface area in the capillaries so that, even though the flow is slower, it ends up being constant and the flow continues in a continuous circle.

01:48 You can see the little diagram below showing you the total surface area being much larger in the capillaries where the velocity is much slower. And also the blood pressure is much higher where the cross-sectional area is smaller. And the pressure is much lower in the capillaries where the blood is doing its work of exchanging oxygen and nutrients for waste products. So here we see a very important law of the capillary.

02:20 The physical principles governing capillary function are extremely important and were discovered by an English physiologist named Starling in the early part of the twentieth century. He is also the Starling who came up with the rubber-band law of the heart in which the heart contracts more when you stretch it more. In any case, the capillary law of Starling talks about fluid passage in and out of the capillary. There are two forces here: the blood pressure pushes fluid out of the capillary and the oncotic pressure made by dissolved proteins in the blood tends to pull fluid by diffusion back into the capillary.

03:04 You can see on the left near the arteriole end of the capillary that the blood pressure is 32 mm Hg and the osmotic or oncotic pressure is only 22 mm Hg, making for a net outward pressure of 10 mm Hg at the arteriole end.

03:24 When you get down to the venous end, the blood pressure has fallen. So now the blood pressure’s only 15 mm Hg but the oncotic pressure is still 22 mm Hg. So that there is a net negative pressure at the venous end of the capillary of 7 mm Hg. Now those of you who’ve been following this will know immediately, “Oh but still there’s a gain of 3 mm Hg along the length of the capillary in the favour of outward pressure, which means fluid moving out of the capillary and into the interstitial space – that is the space between the cells.” Now, if that fluid were not removed, it would result in swelling and oedema. And that’s where the lymphatic system comes in.

04:14 The lymphatic system drains that interstitial fluid so that the tissues do not become swollen with fluid.

04:23 Well, in summary then, in this lecture we’ve covered the main classes of blood vessels: