Healthy Intake
Our thirst center detects when we are low on water, and triggers the thirst response. This center is located in the hypothalamus. A mere 1% change in total body is all that is needed to activate the thirst center. The thirst center is less responsive in older adults, so they need to be extra careful to consume enough water.
The RDI, or recommended daily intake, for water in the U.S. is 3.7 liters per day for males and 2.7 liters per day for females. The idea that we should drink 'eight glasses of water per day' has no solid scientific backing. Your thirst is a better indicator of water need than a fixed number. If you increase your water intake, your kidneys will take some time to adjust to the new intake level, or vice versa.
There has been some controversy over whether or not caffeine in the amounts of a cup of coffee or tea, or even soft drinks causes a dehydrating effect. Studies have now shown that this amount of caffeine does not cause dehydration, and these beverages can contribute to our water requirements. Drinks that contain 10% alcohol or more, such as most wines, do cause dehydration, so they do not contribute to our water requirements.
Pure water and beverages containing water, such as fruit juice, soft drinks, tea, and coffee, all count towards meeting our water requirements. However, it is a good idea to restrict beverages with excess sugar because they can lead to weight gain.
Output
Our kidneys regulate fluid output. Blood comes to the kidneys to be filtered or cleaned and then returns to circulating around the body. The molecules removed during the filtering process are water, sodium, glucose, and urea , which is a waste product from the breakdown of protein. All the glucose is reabsorbed - our bodies use it for energy. The body will reabsorb as much of the water and sodium as it needs. The body has no use for urea, which along with the excess water and sodium is passed along to the bladder before being eliminated.
If you have been drinking too much water or other fluids too fast, your kidneys can't keep up. The kidneys can only filter about one liter of fluid water per hour. If the intake is more than that, they can't eliminate the excess water and the osmotic concentrate is then altered, which affects brain function by causing the cells in the brain to swell. This can lead to seizures, brain damage, coma, and even death.
The kidneys produce urine of different concentrations in order to maintain water balance. If we are low on water, more water will be reabsorbed back into the blood and the urine will be more concentrated - and a more yellowish color. If we have lots of water, less water will be reabsorbed and the urine will be less concentrated, resulting in a clearer color. Water levels in the body will vary depending on the external temperature of where we are. For example, if we live in Florida, we will sweat more and lose more water than if we live in Maine.
If we exercise, we will lose more water through sweat than if we are couch potatoes. The more fluids we drink, the more dilute our body fluids become, so the kidneys produce more urine to eliminate the excess. Sodium consumption can help to balance the fluids in our body. This is why sports drinks contain sodium, so that the water we consume when exercising won't be lost.
Lesson Summary
Water balance is the concept that the amount of water lost from the body is equal to the amount of water consumed. Keeping our water and electrolytes, like sodium, in balance is called osmotic concentration. Our thirst center can detect very small changes in the water level within our bodies. The recommended daily intake for water in the U.S. is 3.7 liters per day for males and 2.7 liters per day for females. Pure water and beverages containing water, including fruit juice, soft drinks, tea, and coffee, all count towards meeting our water requirements.
Our kidneys regulate fluid and sodium output. Blood comes to the kidneys to be filtered or cleaned and then returns to circulating around the body. The molecules removed during the filtering process include water, sodium, glucose, and urea. The kidneys can filter about one liter of water per hour. Fortunately, water intoxication is usually done on purpose. We really can't overdose on water accidentally.
Learning Outcomes
Once you are done watching the video, make a goal to try to:
- Explain what water balance is in terms of our bodies
- Describe what a healthy intake of water is important and what's considered healthy
- Discuss the output of fluids in the body
