Water retention (medicine)
Water is found both inside and outside the body’s cells. It forms part of the blood, helping to carry the blood cells around the body and keeping oxygen and important nutrients in solution so that they can be taken up by tissues such as glands, bone and muscle. Even the organs and muscles are mostly water.
The body uses a complex system of hormones and hormone-like substances called prostaglandins to keep its volume of fluid at a constant level. If one were to intake an excessive amount of fluid in one day, the amount of fluid would not be affected in the long term. This is because the kidneys quickly excrete the excess in the form of urine. Likewise, if one did not get enough to drink, the body would hold on to its fluids and urinate less than usual. Imbalances in this system can lead to water retention, which can range from mild and unnoticeable to symptomatic with swelling.
Where water retention occurs
Fluid rich with oxygen, vitamins and other nutrients passes all the time from the capillaries (the smallest blood vessels) into the surrounding tissues, where it is known as tissue fluid or interstitial fluid. This fluid nourishes the cells and eventually should return to the capillaries. Water retention is said to occur as a result of changes in the pressure inside the capillaries, or changes that make the capillary walls too leaky (see edema and vascular permeability). If the pressure is wrong, or the capillaries are too leaky, then too much fluid will be released into the tissue spaces between the cells. Sometimes so much fluid is released that it cannot all return to the capillaries and remains in the tissues, where it causes the swelling and waterlogging which is experienced as water retention.
Another set of vessels known as the lymphatic system acts like an "overflow" and can return a lot of excess fluid back to the bloodstream. But even the lymphatic system can be overwhelmed, and if there is simply too much fluid, or if the lymphatic system is congested, then the fluid will remain in the tissues, causing swellings in legs, ankles, feet, abdomen or any other part of the body.
The pumping force of the heart should help to keep a normal pressure within the blood vessels. But if the heart begins to fail (a condition known as congestive heart failure) the pressure changes can cause very severe water retention. In this condition water retention is mostly visible in the legs, feet and ankles, but water also collects in the lungs, where it causes a chronic cough. This condition is usually treated with diuretics; otherwise, the water retention may cause breathing problems and additional stress on the heart.
Another cause of severe water retention is kidney failure, where the kidneys are no longer able to filter fluid out of the blood and turn it into urine. Kidney disease often starts with inflammation, for instance in the case of diseases such as nephrotic syndrome or lupus. Once again, this type of water retention is usually visible in the form of swollen legs and ankles.
Other causes of swollen legs
Swollen legs, feet and ankles are common in late pregnancy. The problem is partly caused by the weight of the uterus on the major veins of the pelvis. It usually clears up after delivery of the baby, and is mostly not a cause for concern, though it should always be reported to a doctor.
Lack of exercise is another common cause of water retention in the legs. Exercise helps the leg veins work against gravity to return blood to the heart. If blood travels too slowly and starts to pool in the leg veins, the pressure can force too much fluid out of the leg capillaries into the tissue spaces. The capillaries may break, leaving small blood marks under the skin. The veins themselves can become swollen, painful and distorted – a condition known as varicose veins. Muscle action is needed not only to keep blood flowing through the veins but also to stimulate the lymphatic system to fulfil its "overflow" function. Long-haul flights, lengthy bed-rest, immobility caused by disability and so on, are all potential causes of water retention. Even very small exercises such as rotating ankles and wiggling toes can help to reduce it.
Protein attracts water and plays an important role in water balance. In cases of severe protein deficiency, the blood may not contain enough protein to attract water from the tissue spaces back into the capillaries. This is why starvation often shows an enlarged abdomen. The abdomen is swollen with edema or water retention caused by the lack of protein in their diet.
When the capillary walls are too permeable, protein can leak out of the blood and settle in the tissue spaces. It will then act like a magnet for water, continuously attracting more water from the blood to accumulate in the tissue spaces.
Certain medications are prone to causing water retention. These include estrogens, thereby including drugs for hormone replacement therapy or the combined oral contraceptive pill, as well as non-steroidal anti-inflammatory drugs and beta-blockers. Premenstrual water retention, causing bloating and breast tenderness, is common.
The treatment of water retention depends on whether or not the primary cause is excessive leakiness of the capillary walls. If this is not the cause, as in cases of heart or kidney disease, then diuretic medicines (diuretics) may be an appropriate treatment.
If, on the other hand, the capillaries are very leaky, and tissues are therefore retaining both water and protein, then diuretics can do more harm than good. They will cause the kidneys to remove fluid more rapidly from the blood while at the same time the protein in the tissue spaces will be drawing fluid from the blood into the tissues. The result can be dehydration of the blood. Over time, the use of diuretics can aggravate water retention of this type, as the body will learn to hold on to water in an attempt to avoid becoming dehydrated.
When presented with water retention, a doctor may sometimes find it difficult to distinguish between these two different types. Many doctors will automatically prescribe diuretics. However the most appropriate treatment for the protein-related water retention is to address the cause of the leaky capillaries while at the same time giving remedies which will help to break up the protein.
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