Water 1 Flashcards
How much water makes up an adult athletes body mass? How tightly regulated is it?
Why is different to other nutrients?
How is it measured?
What is the difference between absolute and relative proportion?
How does water content differ in obese people?
- Varies widely between individuals - nutrient that is consumed and lost in the largest amount daily. making up 60–70% of an adult athlete’s body mass
- tightly regulated, with normal daily variation considered to be a change of≤1% of body mass
- Only nutrient that results in death if consumed too little or too much in a matter of day/hours(during exercise or rest)
- Absolute(varies person to person in Litres) or relative(in percentage of body composition) - largest proportion of the body
- Can be measured using tracer technologies (e.g. deuterium) - new-borns have largest body water concentration than other humans
- Mainly a function of adipose tissue content, really low water content, lower adipose tissue = less water
○ Obese people have the lowest relative adipose water content, because of higher relative fat content
How can water cause CNS dysfunction?
When water is moved from the interstitial to the intracellular space, resulting in cell swelling, which may lead to central nervous system dysfunction
What are the types of body water content?
- Distributed into 2 distinct compartments
- Intracellular fluid (ICF)- 2/3 of TBW(total body water)
- Extracellular fluid (ECF)- 1/3 of TBW
- ECF distributed between 2 compartments
- Interstitial fluid (ISF)- 3/4 of ECF
- Intravascular fluid (IVF)-1/4 of ECF - all water in blood cells, 5% of body weight is in blood
Describe the tracer method used to measure total body water:
Day 0 evening: 10g of blood and urine sample are taken
Day 1morning: urine sample to measure deuterium concentration in the urine relative to day 0 sample
- From change in enrichment we can measure the absolute volume
- Requires technical kit
Describe the mechanisms for water balance:
- Fluid intake (1600)
- Food intake (1000)
- Metabolic water (400) - made internally, respiration end products are Co2 and H2O
Unavoidable, always happen - Urine (1400)
- Expired air (320) - insensible loss, cannot feel it
- Faecal loss (100)
- Skin loss (530) - insensible
Sweat loss (650) - sensible
Define:
- euhydration
- hyperhydration
- dehydration
- hypohydration
- rehydration
- Good hydration levels - euhydration(norm) deviations from this norm producing compensatory responses that act mainly through altering renal water concentration and thirst sensation. An oscillating line because it is a constant loss and gain of water - always happening
- Hyperhydration - increase in body water(consumption)
- Dehydration - urination, also from a euhydration state into a hypohydration state –> Typically, hypohydration experienced by athletes is equivalent to ~ 1 to 5% body mass, with consequences on performance
- Rehydration - from hypohydration to euhydration
- Dehydration is a process of losing water, when water molecule is liberated from another compound(usually short-lived)
What is meant by osmolality?
- Osmolality - is the number of osmotically active compounds in the water, urine is easier to access than plasma, but very slow response to water content
Describe the regulation of plasma osmolality:
What are the 2 types of receptors involved?
How closely do they regulate the system?
- Plasma osmolality is regulated in the range 280-290 mosmol·l-1
- Regulation of water and solute excretion by the kidneys and intake by the thirst mechanism - very rarely loss or gain pure water
- Control centres in hypothalamus/ forebrain and respond to changes in blood osmolality, volume and pressure
- Osmoreceptors: 2-3% change in plasma osmolality in blood is registered by hypothalamus creating knock on effects
- Baroreceptors: ~10% change in blood volume/ pressure - less sensitive receptor response, because BP can change drastically from activity
- Renal excretion can only reduce losses, cannot correct deficit. Elevated pre-exercise urine concentration, indicative of hypohydration, or at least an attempt by the kidneys to conserve water
- Thirst, most fluid intake is habitual, rather than due to physiological needs. e.g.: buying a coffee
What are the properties of sweat?
- Sweat is hypotonic relative to plasma - lower electrolyte concentration to plasma. To make sure that appropriate water content can be held in and out of cells –> hypohydration in this situation is called: intracellular hypohydration
- Sweat secreted during exercise is hypotonic relative to serum, except from genetic conditions e.g.: CF
- Sweat has a low sodium concentration which when lost causes an increases electrolyte concentration in blood, which increases osmolality, causing us to be thirsty and drink more
- This is an import part of the fluid regulatory response to exercise-induced dehydration
- Sweat losses are high in warm/hot environments, creating significant competition for blood flow demands between working muscle and skin
What is AVP?
What is its role?
- AVP- arginine vasopressin
An increased extracellular osmolality of~2% (i.e.~6 mosmol/kg) = stimulates arginine vasopressin (AVP) secretion, = which decreases water loss by promoting renal water reabsorption + stimulating thirst = prompt behavioural responses that facilitate water intake
What are the cascade responses caused by hyperosmolality?
- Reductions in muscle and cerebral blood flow
- Increased body temperature
- Increased HR/CV strain
- Increased muscle glycogenolysis
- Limit to peak O2 uptake
Describe the rehydration process:
- mouth- Palatability and flavour is important for rehydration, to encourage drinking
- stomach - Rate of movement from stomach to intestine to blood dictates how much rehydration is occurring
- small intestine for absorption
- circulation
Why is the rate of drinking in rehydration important?
Drinking slowly improves rehydration
To rehydrate a person needs to drink more than they’ve lost
Guidelines for fluid intake during exercise:
- ‘planned drinking’
- ‘thirst-driven drinking’
- Some argue that thirst, rather than hypohydration, limits performance - could be due to studies measuring hypohydration at an uncomfortable level
Why is sodium content important in rehydration?
- Rehydrated with 50, 100, 150, 200% volume of fluid loss
- The lower the sodium content the more water is produced
- Double the amount of salt to improve rehydration significantly
Why is potassium content important in rehydration?
- Potassium is the main cation in the intracellular fluid and is lost in relatively small amounts in sweat (typically 4 – 8 mmol/l)
- potassium loss is relatively low even at high sweat rates and unlikely to cause significant health consequences such as hypokalaemia.
