Saturday, 26 May 2012

Two hearts beating as one

Two hearts beating as one

Before indulging in some data analysis I just wanted to make sure that we have a good grasp of the heart's function within the circulatory system and its control. To start with, I think it is useful to divide the circulatory system into six parts each with separate functions but physically and functionally coupled together.

Arteries and arterioles

Moving away from the heart, through the circulatory system, we have the arteries and their branching smaller arterioles. The arteries have an important function in storing blood for short periods of time just after each heart beat. The rise in pressure at the end of each heart beat (systole) pushes blood into the arteries. The rise in pressure expands the arteries and gives the blood somewhere to go since it cannot escape through the rest of the circulatory system fast enough. This expansion of the arteries reduces the peak systolic pulse pressure and their elastic recoil as the heart is relaxing maintains your blood pressure at high enough levels to perfuse your brain. Elastic arteries allow for lower heart rates and lower peak pressures - it is definitely worth trying to keep your arteries fat free and in good condition! The smaller arterioles branch off the arteries and they are innervated by nerves which generally cause them to contract. We call this sympathetic tone. This contraction causes the vessels to be small in diameter and stop the blood escaping too fast from the heart. By doing so they reduce the amount of work the heart has to do. The arterioles are the 'taps' of the circulation, they are the points at which flow is controlled.


Rather short vessels that connect the arterioles with the venules. These vessels are slightly smaller than the diameter of red blood cells (bad news for sickle cell suffers with rather stiff red blood cells...). Capillaries have walls that are one cell thick and no cell in your body is very far from a capillary. Interestingly most capillaries don't have much blood flowing through them at rest - they tend to be shut. Because they are very small they are intrinsically very strong (Laplace's Law) but they are quite permeable to water. Diffusion across capillary walls is fast and this is where most O2 is delivered and CO2 picked-up. So, the function of the capillaries is to allow for exchange of metabolic substrates and waste products.

Venules and veins

If you read the previous blog, you won't be terribly surprised to find out that these vessels are really important. At rest most of your blood is sitting in your veins doing remarkably little except staying away from your heart and stopping it from having to work too hard. Venules collect blood from the capillaries and carry it to the veins. They are also quite permeable to water and a rise in venous pressure can easily give rise to a loss of fluid into the intercellular space (the space between the cells that make up your body). This swelling occurs when venous pressure rises due to a failure of valves, fluid overload, heart failure or a loss of muscle pumping through sitting still for too long! Veins can contract and push a lot of blood into the circulatory system. When blood is pump through the heart into the arteries and the capillaries it can - if the control system isn't working - simply pool in the veins. That lack of blood flow back to the heart stops it pumping enough blood to maintain blood pressure and a faint usually results....a common circulatory failure rapidly and automatically cured by the adoption of a prone position!

Right side of the Heart

The right side of the heart accepts blood returning from the body and pushes it on to the lungs. If does this at rather a low pressure to stop water being squeezed out of the lungs. Starling's Law of the heart ensures that all of the blood returning to the heart gets pumped onwards, not by altering heart rate, but by raising stroke volume when more blood fills the ventricle. The intrinsic control prevents venous pressure rising too high. This is particularily important for blood returning back to the left hand side of the heart from the lungs.

Left side of the Heart

If you take a heart in your hands almost all of it is made up of the left hand ventricle. The right side is pretty small - not that it doesn't pump the same amount of blood - it is just the left hand side has to develop a much higher pressure than the right hand side. Blood flowing into the left side, by virtue of Starling's Law, gets pumped away through the aorta at just the right rate. This mechanism results in a matching of the amount of blood pumped by each side. The thick ventricular wall is made of muscle that can generate the high pressure necessary to push the blood onwards around your body. Rather oddly the heart muscle does not get nurished by the blood in the ventricles. Instead blood in the main artery (aorta) flows off down the coronary arteries to supply the heart muscle. Most importantly this only happens when the heart is relaxed. Here in lies the rub. The faster you ask your heart to beat, the more you make it work, the less time it is relaxing and the less blood flows through the muscle. This is a really silly bit of design and is one of the features that stops the heart beating at very high rates. (It is a bit more complicated than that since mice can have heart rates of about 1,000 beats per minute....and I measured my daughter's guinea pigs' heart rates at about 300 beats - but that is another story....). When those coronary arteries get blocked pain and then damage can result....

The control system

The default setting of the control system is to shut pretty much everything down to a minimal energy saving level. At rest only a small proportion of your capillaries are fully open and the heart is pumping only 5 or 6 L of blood per minute (or something close to that). The control system has set your arterioles to a narrow diameter to stop blood flowing away from your heart too rapidly, the veins relaxed so blood can pool in them. Because of the constricted arterioles systolic pressure is high and the arterial pressure sensors (baroreceptors) keep the heart rate low. The relaxed veins keep venous return to the heart low and thus the heart contracts relatively weakly. This is a nice energy saving setting.
When you exercise everything changes. Oddly the arterioles contract more, further reducing general capillary blood flow - but that is to keep blood pressure high. The veins contract injecting blood back into the circulatory system towards the heart. This raises the force of contraction of the heart. Heart rate gets controlled by the baroreceptor reflex to keep blood pressure in the right range. Then, the exercising muscle through a process of autoregulation begin to dilate arterioles and take the blood that they need by virtue of their metabolic activity.
Thus, you and your circulatory system are a bloody marvel. Heart, arteries, capillaries, veins, sensory receptors, brain and nerves functioning together to allow a massive increase in work rate. Fantastic.

But, as athletes all we record is heart rate....disappointing....

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