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Fluid intake and urine output

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Aim: To find out if there is a relationship between fluid intake and urine output, by measuring my daily fluid intakes and urine outputs.

Hypothesis: The volume of urine I will produce will be at least half of the volume of fluids I will drink, due to the body’s ability to carry out a water balance just as one would have on oxygen, carbon dioxide to maintain a healthy and working body. For example if I drink 1000cm3 of fluids then the expected amount of urine I should produce would be around 4000 cm3 to 6000 cm3 in a temperate condition in London, whereas if I were to carry out my experiment in another country such as Australia, my urine output will be considerably lower than that of my fluid take because of it is lost through sweat.



As the main substance found in fluids, which is essential for all human beings to drink and a substance found in urine, water will be a key issue in my experiment.

Water is a fundamental part of our lives and is highly dependant on human survival, ranked by experts as second, after oxygen as essential for life, it is only for this reason that it constitutes to 2/3 of our body weight. With the average body being 55 to 75% water, it is important to know what waters function in the body is and they consists of the following:

– aiding in digestion and absorption of food

– regulating body temperature and blood circulation

– transport medium

– carrying nutrients and oxygen to cell

– removing toxins and other wastes

– cushions joints and protects tissues and organs

– satisfy metabolic demand

– moistens oxygen for breathing

Inadequate intake of water compromises cell functions by contributing electrolyte imbalances, contraction of plasma volume, and inability to regulate body temperature.

The water balance in the body (fluid balance)

Water balance is the terminology given to the difference between the amount of water taken into the body and the amount excreted or lost. Water regulation is essential to homeostasis and what we mean by that is the ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its physiological processes. If water levels rise or fall beyond normal limits, many bodily functions fail to proceed at their normal rates and thus becoming a life threatening situation.

For water balance there are 2 ways in which we can gain water, one way being through our mouths, by drinking water which is how 60% of all gained water is received, because it is the most convenient and quickest way to obtain water for the body. The mouth also allows water to be taken from eating moist foods, which is where 30% of all gained water received and this water is mainly absorbed through the large and small intestines. The second way we can gain water is as a by product of cellular respiration, constituting 10% of all gained water.

Oxygen + glucose carbon dioxide + water (+ energy)

Contrasting the ways we can gain water, there is 4 ways in which we can lose water. The first being through sweating, which constitutes to 6% of how we lose water. We have to lose water through sweat in order to maintain our body temperatures and this works by the blood capillaries vasodilating, thus allowing the blood to circulate near the skin and allowing us to release the heat via sweat. Because sufficient amount of heat is required to raise the water and evaporate it off from the skin via the blood capillaries, a considerable amount of heat is taken with it when sweating allowing the body to cool down. The seconds way is through our faeces, which constitutes to 6% of water , and this is because water is need to form the faeces and allow it exit via the anus.

The Third way is by evaporation as we breathe and through our skin, which constitutes to 28% of water loss and this is because we live in environments that are less humid than our own bodies. The fourth way is through our urine, which constitutes to 60% of our water loss.

The Kidneys

The kidneys are the organ which regulates the amount of water we lose from our body. It can directly control the volume of bodily fluids by the amount of water excreted in the urine. Either the kidneys can conserve water by producing urine that is concentrated relative to plasma, or they can rid the body of excess water by producing urine that is dilute relative to plasma and this constitutes to 60% of all water output. This process is influenced by antidiuretic hormone (ADH), also called vasopressin, which is produced in the hypothalamus (a part of the brain that regulates many internal functions) and stored in the nearby pituitary gland.

Receptors in the brain monitor the blood’s water concentration. When the amount of salt and other substances in the blood becomes too high, the pituitary gland releases ADH into the bloodstream. When it enters the kidney, ADH makes the walls of the renal tubules and collecting ducts more permeable to water, so that more water is reabsorbed into the bloodstream. Therefore the amount of urine produce would be less, and the concentration of water in the urine will be less. In the case of too much water being present in the body, less ADH will be present in the bloodstream, thus allowing the kidney to let the water be filtered quickly out the body.

With all these means of controlling the water balance in the body, it is this reason why the body maintains its percentage of water in the body and removes excess water while as it take in water, when needed to.

In relation to the kidney, the kidney is one of four other components in the Urinary system, consisting of the, urethras, urinary bladder, and urethra. The human body has two kidneys, one on either side of the middle back, just under the ribs. Each kidney contains thousands of small filters called nephrons. Each nephron has a mesh of capillaries, connecting it to the body’s blood supply. Around 180 litres of blood sieve through the kidneys every day. The main functions of the kidney include:

* Regulating the amount of water and salts in the blood

* Filtering out waste products

* Making a hormone that helps to control blood pressure.

Each kidney has a tube called a urethra. The filtered waste products (urine) leave the kidneys via the urethras and enter the bladder.

The bladder is a hollow organ that sits inside the pelvis. It stores the urine. When a certain amount of urine is inside the bladder, the bladder ‘signals’ the urge to urinate. Urine contains water and waste products like urea and ammonia.

The urethra is the small tube connecting the bladder to the outside of the body. The male urethra is about 20 centimetres long, while the female urethra is shorter, about four centimetres. At the urethra’s connection to the bladder is a small ring of muscle, or sphincter. This stops urine from leaking out.

Normal daily turnover of water is approximately 4% of total body weight in adults. In a 70kg adult this is equivalent to 2500 to 3000 ml per day. Water gained via food is in the region of 1 litre (based on a 2900kcal diet eaten by a 70kg man) and metabolic water contributes a further 250ml. Thus 1650ml is required by the 70kg man. From the data collected, I can assume that my body weight of 70kg would require me to drink 1650ml per day.

Vertical section through mammalian kidney

Dehydration and Overhydration

Dehydration is defined as a lack of adequate blood volume in blood stream (isotonic fluid losses) and a lack of water leading to an excess of salts in plasma (i.e. hyper-osmotic environment). Water is reabsorbed by the kidney leading the urine to become very concentrated, low in volume & dark in color. This can occur in situations such as excessive exercising. Water is lost through sweat to control body temperature, and that water lost is not compensated.

Over-hydration is when the person water balance is uneven, having therefore an excess water content in blood, leading to a low sodium level (Na+) in blood. This may occur to marathon runners, who drink excessive amounts of water.

In relation to our body being able to control its water balance, it is also of interest to know how the body knows when to consume water, and this is by having a “thirst centre” in our hypothalamus. This thirst centre contains osmoreceptors, which are simply sensory receptors that detect changes in fluid concentrations. From the principle of negative feedback, the body detects the level and then responds in an appropriate way. For water there is a level called a ‘norm’, of which it must follow. If the level gets too high this triggers the body to lower the amount of water. If the level gets too low this triggers the body to raise the amount of water. In the case of losing too much water, our body fluids begin to become more concentrated.

As some of these fluids change in their concentration, osmotic pressure pushes on the osmoreceptors and they become active. Once the osmoreceptors are activated within the hypothalamus of a person, that person gets the sensation of being thirsty. This sensation goes away if that person drinks enough fluid to distend their stomach. All that is needed to activate the osmoreceptors is a 1% change in total body water. Considering that we have approximately 40 litres of body fluid, 1% is a change of 0.4 litres, or 400 cm3. 400 cm3 is the equivalent of 1.6 cups of water (250 cm3 = 1 cup). And, with a typical water intake of 2500 cm3 per day, 400 cm3 is only 16% of our total daily water intake.

This table below is an example of water balance in the body in a temperate climate.

This table will give me a good idea of what will the fluid intake and urine output will be.

Water intake (average intake in ml)

Fluids 1400

Water in Food 700

Water from oxidation in Food 200

Total 2300

Water output (average output in ml)

Normal Temp. Hot Weather Prolonged Exercise

Urine 1400 1200 500

Water in Faeces 100 100 100

Skin (perspiration) 100 1400 5000

Insensible Loss – – –

Skin 350 350 350

Respiratory Tract 350 250 650

Total 2300 3300 6600


* Excessive exercise- exercise is a factor which may affect the amount of fluid drank and urine produced. This is because when someone goes about exercising, they increase their body temperature therefore they increase the amount of sweat produce to maintain their body temp. Therefore if the person does not replenish his loss of water via the sweat his loss, his kidney will retain the water from the bladder, thus a lower amount of urine will be given off.

* Environmental conditions- if for example the environment is very hot, then behavioural and scientific attributes may affect the fluid intake and urine output. In a hot environment, a person may drink cold drinks to quench their thirst, coinciding with factors of sweating more and thus retaining more water. Living in London, the temperatures are not dramatics in the terms of there not being a very hot country or a cold country. The experiment I will do will be done in February, where average temperatures are 14-18 degrees, therefore a constant temperature not resulting in excessive heat loss.

* Types of fluids with having a variety of fluids to drink, the body needs to extract the water from it and the other useful substances. Those fluids with a higher percentage of water will contribute to a normal volume of urine output, whereas a fluid containing a lower percentage of water will have a lower volume of urine output in relation to the volume of fluid.

* Disease- certain disease or illness, which may raise or lower the body temperature, will have an effect on the urine output.

* Medicine- certain medicines will inhibit to the loss of more water or intake of more water.

* Alcohol- this substance will lead the body to dehydrate.

* Food- foods such as apple have a high percentage of water in it compared to a piece of bread. Thus the types of food I will eat will effect the urine out put.

Safety- Urine is a sterile liquid and proposes no biological hazard because it is filter by the body, via the kidneys. However if left to dry, the substances left will smell. As I am carrying out an experiment I should keep my experiment as safe as possible, being simple as it is there should be no serious safety hazards which may arise in my experiment apart from where I discard my urine. All urine I do use for my experiment will be discarded down the loo. I will also do the following:

* I will wash my hands after finishing measuring my urine put to avoid any contamination or spread of any hand held diseases.

* I will wash all the containers thoroughly, to avoid any chances of falling ill through the fluids I drink from the measuring jug.

Apparatus- Measuring jug (ml) – I will be use this as it is accurate to 1cm3 and is a quick way of measuring the volume of fluids I drink.

Measuring cylinder (ml) – I will use this as it is accurate to 1cm3 and is an easy way of measuring the volume of urine I will produce.

I will also use a pen and paper to note down my results.

Fair test- in this experiment I will be the only participant in measuring the fluid intake and urine put, therefore it will only be my results. Because it will only be me taking part in the experiment, it will be very difficult to ensure reliability on my results as my experiment will only measure my lifestyle, therefore to ensure there is a fair test I will do the following:

* Keep my sleep routine the same, by going to bed at 10.00 p.m. and waking up at 7.00 a.m.

* To maintain a healthy body I will exercise regularly for the same amount of time, consisting of half an hour.

* I will keep my food routine the same, by eating the same types of food, as food is a way which water can be gained to the body and may alter my results.

* 12 hours before taking part in the experiment, I will only drink only 200 cm3 of water and take part in no physical activity.

Procedure: By using a measuring jug, I will measure the amount of fluids I will drink daily and note down the results. With the measuring cylinder I will measure the amount of urine I will produced daily and take down the results.

Conclusion: By looking at my results it can be seen that my results do fairly support my hypothesis and my background knowledge. It does this in the sense that it shows a certain pattern between the fluid intake and urine output.

By looking at the results table and the graph, I can see that the amount of fluids I drink does in effect the volume of urine I produce, producing about 55-65% of the fluid I drank repeatedly, which is about 750cm3 – 800cm3 volume of urine produce after drinking under 1400 cm3 of fluids. This fairly supports my hypothesis ‘The volume of urine I will produce will be at least half of the volume of fluids I will drink, due to the body’s ability to carry out a water balance’.

Although my results are not all equal, in terms of volume of fluid intake and urine output being the same, it does show that however a certain pattern to prove my theory of how water enters and leaves the body. The urine output can be considered fairly higher then expected from the results compared to initial theory of it being at lest half of the fluid intake. Reasons for this are what were stated in my theory, and that was of how water is obtained in the body, which was via the mouth and as a by-product of respiration. The point being that as an individual my body’s water demand and metabolic rate is different to everyone else, thus my individuality may have affected my results. Again the ways in which water can be loss through the body stated in my theory, of that water can be loss through sweat, urine and faeces. Again my body has its own functioning of losing water, and for example some people tend to sweat more than others, thus contributing to a difference in urine output. For this reason, I believe that the sets of result do show substantial support for areas stated in my theory.


By looking at my experiment I can see that there are a lot areas, which could do with some improvements to make results more reliable in supporting my hypothesis.

Due to the results only being mine, the results don’t show substantial support to justify my hypothesis. Therefore if I had to do my experiment again I would of done it on a larger scale, because it is difficult in some aspects in getting a perfect result, as it is very hard to control humans, which may alter the results, whether it be due to their lifestyle or emotions. By getting more people, I will therefore get more data which can be plotted in a scatter graph, which can be analysed to show if there is a strong or weak correlation/relation between fluid intake and urine out put. When getting more people to do my experiment, I could go further into getting a matched group of people, by controlling things such as age, weight, height and lifestyle. By doing this, I improve the reliability as well controlling some variables leading me to justify why certain people produce more or less amounts of urine, or drink certain amounts of fluids, which will lead on getting more statistical analysis.

Another idea is that of a close analysis, which involves the participants to carry out the experiment in a laboratory and closely analysing and measuring their fluid intakes and urine outputs. This type of experiment is far better because variables can be controlled, such as control of time, sleep, food intake, and movement, leading to an independent variable which will only be the fluid intake and urine outputs, which can be closely measured off. The results from these will be far more reliable as you would see a repeated set of results being produce, thus providing a reliable set of results, which can be analysed to support my hypothesis.

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