At this point it is fairly common knowledge that high blood pressure (hypertension) is not good for our health. At almost every doctor visit blood pressure measurements are an automatic standard no matter the patient’s age or risk factors. But why is it so critical? Why is blood pressure something that is measured and monitored so regularly?
67 Million Americans have high blood pressure- that’s roughly a third of the population! What’s worse? Another third of the population has pre-hypertension; higher than normal blood pressure levels that serve as a “warning” for a higher risk of heart attack, stroke, and kidney disease. Once your blood pressure rises above the normal range your risk for these diseases goes up as well. But what’s the connection? How is blood pressure related to the heart and kidneys?
Blood pressure tells us how hard your heart has to work to pump blood to the rest of your body. To understand this, picture filling a balloon with water. The more and more water you add to the balloon, the larger and larger it stretches. The water you add to the balloon creates more stress, or pressure, on the rubbery material of the balloon. Pressure goes up. This is exactly what happens to our blood vessels; a greater volume of blood will stretch our arteries and exert a higher pressure. The difference between our vessels and the balloon is that our vessels won’t burst; they have tiny little spaces between cells that allow fluid to move out if it gets too full.
Blood carries all sorts of nutrients and oxygen to our tissues and then carries waste and carbon dioxide away from the tissues- in this way, we are able to live and breathe. When blood pressure is high your heart needs to work much much harder to push blood out into your body. As it turns out, the microscopic muscle cells of our hearts cannot divide and multiply. In order to increase their ability to pump harder, they grow larger (this is called “hypertrophy”). Larger cells need more oxygen and it becomes difficult for the arteries around the heart to keep up the oxygen supply that is required….without oxygen the cells of the heart can eventually die, causing a heart attack. Alternatively, the heart might also lack the ability to pump enough blood against a pressure that is too high. When this happens, blood gets “backed up” in the heart, which can then “back up” in the lungs. The back up in the lungs will also increase pressure in the vessels of the lungs- this high pressure can cause fluid to “leak” out between the vessel cell spaces and enter the spaces in the lungs where air is supposed to be…this is called “pulmonary edema,” and for all intents and purposes is similar to drowning. The important thing to understand is that it takes a very long time to get to this point. It’s a spectrum- which is why we want to treat high blood pressure as early as possible.
Now, there are times when high blood pressure is a good thing. These are the times when our body needs more blood. Most notably- with exercise. When we exercise our tissues demand more oxygen; in order to get more oxygen, the body will work to pump more blood out of our veins. More blood volume = more blood pressure. Thus, blood pressure increases when we are active. What’s fascinating about this is that (again, over a long period of time) our heart cells will also get bigger. The difference is that something about exercise also increases the formation of new blood vessels around the heart. So an athlete may have a heart the same size as a person with heart disease, but the athlete’s heart has enough blood vessels to keep up a good oxygen supply, whereas the person with heart disease doesn’t have these extra vessels and won’t get enough oxygen. Interesting, right?!
So now the question is: how do people end up with a higher blood volume (and thus higher blood pressure) if they don’t exercise?
This is where the kidneys come in. Our blood volume is dictated largely by our kidneys. When we eat food our bodies absorb a ton of nutrients (sugars, proteins, and carbs) but we also absorb sodium (salt), potassium, and other vitamins/minerals. A significant portion of this gets filtered into the kidneys and then reabsorbed into the blood. This means that much of the salt we eat will eventually end up in the blood.
Our bodies have a very strict “electrolyte” balance. In other words, the concentrations of sodium, potassium, chloride, and hydrogen are absolutely critical for cell function and communication. If the concentration falls outside a tightly controlled range, then our bodies can end up in big trouble (including but not limited to heart failure, muscle spasm, and hallucinations). I won’t go into detail about this now, but you can read more here.
The culprit most closely related to blood volume and blood pressure is sodium. What happens when we end up with too much sodium is that the concentration of sodium increases (there is more salt in a fixed amount of liquid). In order to counter this, the body will pull in more water from our kidneys. The water dilutes the sodium and brings the concentration back to normal (more salt in more liquid). It’s a safety mechanism. The result? Well now we have a bigger volume! And this volume eventually gets added to the blood. More blood volume = more blood pressure, and we’re back to square one; increased likelihood of heart disease, decreased efficiency of heart to pump, and possibility of pulmonary edema and/or heart failure.
And there you have it! Hopefully you now understand a bit more about how blood pressure works. Feel free to ask more questions in the comments if you have them. This is just a snap shot of why it’s so important to keep our blood pressures under control! A well balanced diet and physical activity go a long way in preventing these issues.
**Note: this is a very simplified explanation of how blood pressure control works in the body. It is a very complicated topic with intricate mechanisms, some of which are still incompletely understood. Blood pressure levels are thought to have a strong genetic component as well and high blood pressure can be caused issues other than changes in volume.