At this point, the blood enters the blood vessels of the rest of the body (not just the heart and lung vessels) for distribution to the whole body. Blood exits the heart through the aorta, which is a strong, muscular blood vessel. The aorta absorbs and stabilizes the pressure of the blood exiting the heart before the blood moves through smaller arteries, arterioles and capillaries. As the blood vessels get smaller, they become less muscular and the blood pressure gets lower. Blood eventually enters the incredibly thin capillaries, allowing oxygen from the blood to diffuse into the surrounding tissues and muscles, where it is exchanged with carbon dioxide before entering the veins.

​

Veins, on the other hand, are not muscular or elastic vessels. Veins moves de-oxygenated blood back to the heart, sometimes holding the blood for a little bit before it enters the heart.

​

This is a good time to think of what the overall goal of the cardiovascular system is - the overall goal is to make sure that all the organs and tissues of the body receive proper amounts of blood so they get the oxygen and nutrients necessary to function. This means maintaining a good blood pressure level. The blood pressure leaving the heart is high, with the force generation from the left ventricle, let’s say around 95 mmHg.

​

By the time blood reaches the capillary level, the pressure has dropped significantly, to approximately 20 mmHg. Pressures remain low as the blood moves through the venous system, returning to the heart.

In cases where blood pressure is too high, damage can be inflicted on the blood vessels, especially the delicate capillaries. Conversely, if blood pressure is too low, tissue and organs may not receive proper blood flow. Maintaining proper blood pressure is of critical importance.

​

​

What happens to the cardiovascular system during pregnancy?

Let’s first think about the demands on the cardiovascular system during pregnancy. In addition to meeting the needs of mom's tissues and organs, the system of a pregnant women now needs to also supply blood to a growing uterus and fetus, as well as a new organ - the placenta.

And is this organ demanding.

During pregnancy, the volume of blood in your body drastically increases. The plasma volume (the liquid component of blood) increases by approximately 50% by the end of pregnancy. As discussed above, the goal of the cardiovascular system is to maintain normal blood pressure levels. In order to accommodate large increases in blood volume, the maternal system must adapt to avoid increases in blood pressure.

Imagine a hose with a fixed shape - when you turn the stream of water up, the pressure becomes higher and shoots through the hose at a faster rate. If you were to then use a wider hose, the pressure of the water would be lower than in the smaller hose.

​

In the same way, the maternal blood vessels begin to relax in early pregnancy, in order to accommodate increases in blood volume and maintain normal blood pressure. Clinically, this relaxation is known as total peripheral resistance, representing the combined resistance of all your blood vessels. In normal pregnancy, the maternal resistance falls by approximately 25%, compared to pre-pregnancy levels. This change is hypothesized to be caused by a number of factors, including hormones and proteins produced by the placenta.

To accommodate this fall in resistance, the maternal heart then begins to beat more frequently and more forcefully. Maternal heart rate, representing the number of beats by the heart per minute, increases by 15%, from 71 to 85 beats per minute. The amount of blood expelled from the left ventricle with each beat, known clinically as stroke volume, increases by 18%, from 70 to 83 mL per beat. Cardiac output is reported as an indicator of both heart rate and stroke volume - it represents the volume of blood pumped by the heart per minute. Over the course of normal pregnancy, maternal cardiac output increases by 38%, from 5 to 7 litres per minute.

That’s crazy - a pregnant woman's heart pumps an extra 2 litres of blood per minute.