The Bad Side Of Bed Rest Horizontal Can Be Horrible For Your Body If You Stay That Way Too Long.
August 4, 1986|By DR. LAWRENCE E. LAMB, Special to the News
When you are sick, doctors often advise getting plenty of bed rest and taking lots of fluids. But there are reasons that prolonged bed rest is not so good for you. Prolonged bed rest affects the amount of fluid in your body and the pressure at which blood is pumped, making it difficult for the body to adapt to activity again. It causes changes in the size of the heart, too, and the body`s ability to replenish blood.
In short, the disadvantages of prolonged bed rest can outweigh the benefits.
CHANGES IN PRESSURE
Lying down eliminates much of gravity`s influence upon the body. When you are lying down, the blood pressure at the outlet of your left heart, just outside the aortic valve, is the reference pressure. Think of it as 120/70. The pressure will be about the same in all of your arteries, regardless of where they are in the body.
Blood flows through smaller and smaller arteries until it reaches the capillaries, the smallest blood vessels of all, and from there it drains into the veins to return to the heart. In the process, it loses its pressure. Fluid leaves your capillaries to provide nourishment and oxygen to your cells. Fluid returns to the most distant part of the capillaries that join to form the veins. The pressure in the veins in your legs, while you are lying down, will be about 10 millimetres Mercury. In the right atrium, where all the veins drain into the heart, the pressure will be about 5 millimetres Mercury.
Drastic changes occur when you stand up, and gravity asserts its influence on your body. Your blood vessels form an upright column of blood, like a water tower. The ``tower`` is the height of the distance between the top of the heart and the ankle. In the average adult male, this alone creates an added pressure of 100 millimetres Mercury. The pressure at the heart level would still be 120/70, but the pressure in the ankle would be 220/170. The leg arteries constrict to prevent all your blood from running into your legs.
Gravity also affects the blood pressure in the veins, acting on the column of blood in the veins between the top of your heart and your ankle. So venous blood pressure also will increase by 100 millimetres, Mercury. That means the pressure in the veins in your ankle will be 110 millimetres Mercury, not just 10 millimetres Mercury.
GRAVITY AND BODY WATER
All that increased pressure in your veins makes it more difficult for fluid to flow from your cells back into the veins. As a result, as you stand, you accumulate fluid in your tissues. Each leg, from the hip down, will gradually fill to capacity.
When you lie down, you eliminate that increased venous pressure. When lying down, fluid more readily flows from your tissues back into the veins. The large volume of blood that accumulated in your legs also shifts to your chest, and expands the right atrium, where blood from veins accumulates.
There are stretch receptor nerve cells in the wall of the right atrium (upper chamber of the heart). As these stretch, they send a signal to your brain. The brain, in turn, controls an endocrine mechanism that causes the kidneys to start to eliminate water. A person who is normally hydrated may lose 5 or 6 pounds of water in 24 to 48 hours of lying down.
The problem begins when you get out of bed, after the period of bed rest. The loss of water from the legs that has occurred during bed rest decreases the pressure that keeps the thin-walled veins from expanding. The veins expand readily, and a lot of blood pools in the legs. Also, the loss of water from the blood during the time of lying flat has reduced the volume of blood circulating in your body. The combination of pooling of blood in the legs and lower blood volume means that there may not be enough blood getting back to the heart, which can cause fainting.
THE HEART SHRINKS
A well-conditioned athlete, such as a distance runner, tends to have a large heart. The heart must be able to store a large volume of blood between each contraction. The large-chambered heart is able to pump even more blood during exercise. That makes it possible to deliver lots of oxygen to supply the working muscles.
The opposite happens with inactivity. The heart actually gets smaller. Since the heart is not worked at high levels, there is no requirement for it to be able to pump large amounts of blood and it loses its ability to do so.
The heart muscle behaves much like other muscles in the body. It increases in size and strength as a response to being asked to do more frequent and harder work. When it is not exercised adequately, it gets flabby or weak, and loses its capacity for work. That may lead to early fatigue of the strained heart muscle and signs of impending heart failure.