The Maternal Cardiovascular System
Kelsey McLaughlin, PhD
Pregnancy is a time of enormous change, including changes to mom's heart and blood vessels.
Keep reading to learn more about what happens to your cardiovascular system during pregnancy!
What exactly is the cardiovascular system?
The cardiovascular system is one of the body’s organ systems; it is responsible for moving blood around your body to provide your tissues with oxygen and nutrients. The heart is the central organ of the cardiovascular system, located in the thoracic cavity. The heart is commonly referred to as the ‘pump’ of the body, as it generates the force necessary to move blood through all of your blood vessels. The left and right side of the heart each contains an upper section (or chamber) called an atrium and a lower chamber called a ventricle. The left and right sides of the heart have very different functions.
Side note - the left and right labels of the heart are in reference to your own body. If you were looking at someone else’s heart, the left side of the heart would be their left, not yours.
The veins return blood from the body to the right side of the heart; this returning blood is poor in oxygen (de-oxygenated) and has high levels of carbon dioxide. This de-oxygenated blood enters the right side of the heart into the right atrium (the upper chamber) and is then pushed into the right ventricle (the lower chamber). From the right ventricle, the heart then pumps this de-oxygenated blood into the pulmonary arteries - these vessels bring the blood into the lungs. In the lungs, the blood from the heart runs through very small blood vessels in the lungs, called capillaries. These capillaries are located in the parts of the lung that interact with the air we inhale, bringing the de-oxygenated blood very close to this air (approximately one-thousandth of a millimetre). When we inhale, carbon dioxide in the de-oxygenated blood is transferred to the air, while oxygen from the air moves into the blood.
This blood now has high levels of oxygen; the blood returns to the heart, entering the left side at the atrium then the left ventricle. The left ventricle is a muscular machine - it generates incredible force to push the oxygenated blood from the heart to all the arteries and capillaries around the body.
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.
Overall, maternal blood pressure drops by approximately 10% early in pregnancy before returning to pre-pregnancy levels by the third trimester. Despite huge changes in blood volume and heart function, the maternal cardiovascular system is able to maintain blood pressure.
Wait - am I technically an elite athlete?
How does your heart adapt and remodel to such drastic changes during pregnancy? The heart gains more muscle, gets bigger and improves its ability to contract. The diameter, volume and mass of your heart changes. Changes are especially obvious to your left ventricle, which is now responsible for pumping higher volumes of blood more frequently.
Do you know which other populations see changes like this to their heart? Elite athletes.
In a study from the American Heart Association, young adults were intensively trained for a marathon to observe the adaptation and remodeling of the heart. Similar to elite endurance athletes, these participants exhibited increases in left ventricular mass of approximately 20%.
In comparison, pregnant women in normal pregnancy exhibit increased left ventricular mass of approximately 40% over 9 months, from 88 grams to 123 grams. That's very impressive - just one chamber of the heart increases by 35 grams throughout pregnancy.
The maternal cardiovascular system undergoes incredible function and anatomic changes to accommodate a growing baby, uterus and placenta without sacrificing proper blood flow to the rest of your body. As your baby grows, so does your left ventricle!
Every woman and every pregnancy is unique. Pregnant women should speak to their healthcare provider to ensure maternal and fetal safety. This article is meant to provide readers with current information and opinions. All medical and treatment decisions should be discussed with your healthcare provider.