Water and Electrolyte Balance

1. Water and electrolyte balance refers to the regulated distribution of water and essential ions in the body to maintain homeostasis.
2. This ensures that cellular processes function efficiently, fluid levels remain stable, and pH, temperature, and blood pressure are maintained within an optimal range.

Roles of Water in the Body

Water carries out four jobs the body cannot do without:

1. Solvent: transports nutrients, waste products, hormones, and other metabolites between cells.
2. Heat redistribution: absorbs and moves heat around the body, and cools the body through evaporation of sweat from the skin.
3. Lubricant: water resists compression, so it cushions sliding surfaces such as joint spaces and the tissue around tendons and muscles.
4. Medium for metabolism: provides the fluid environment in which the biochemical reactions of metabolism take place, both inside and outside cells.

Importance of Homeostasis in Hydration

The human body is composed of 50 to 70% water, and maintaining homeostasis (internal balance) is critical for survival and athletic performance. Fat-free tissue is about 60 to 80% water, while fat tissue holds very little water, so a leaner athlete carries a greater percentage of their body mass as water. Maintaining homeostasis in hydration is essential for:

1. Regulating body temperature (via sweating and cooling).
2. Maintaining blood pressure (by ensuring adequate plasma volume).
3. Ensuring efficient metabolic reactions (enzymes require optimal fluid levels).
4. Supporting cognitive function and endurance (dehydration can cause dizziness and fatigue).
5. Preventing muscle cramps and weakness (caused by electrolyte imbalances).

Intracellular and Extracellular Fluid

1. Intracellular fluid (ICF): makes up around 2/3 of total body water and sits inside the cells.
2. Extracellular fluid (ECF): makes up around 1/3 of total body water and sits outside cells. About 80% of ECF is interstitial fluid between cells, and around 20% is blood plasma.
3. ICF composition: rich in potassium (K⁺) salts.
4. ECF composition: rich in sodium (Na⁺) salts.
5. Maintaining the gradient: the difference between ICF and ECF is held in place by active transport, which uses ATP and costs the body continuous energy.
6. Osmolarity: despite the different compositions, ICF and ECF have the same overall solute concentration.

States of Fluid Balance

1. Dehydration: the dynamic process of losing body water. Dehydration is a process, not a state.
2. Euhydration: the state of normal, appropriate body water content.
3. Hyperhydration: the state of excess body water content.
4. Hypohydration: the state of insufficient body water content.

Key Electrolytes and Their Functions

Electrolytes are charged minerals that dissolve in bodily fluids and help regulate essential physiological processes.

Intake of Water and Electrolytes

How Water and Electrolytes Enter the Body

Water and electrolytes enter the body primarily through:

Ingestion (Drinking and Eating)

1. Water is consumed through fluids like water, juices, milk, and sports drinks.
2. Electrolytes come from foods (fruits, vegetables, dairy, processed foods) and beverages.

Metabolic Water Production

1. Water is produced as a byproduct of cellular respiration in mitochondria.
2. Metabolic water makes a small contribution to total hydration but becomes more relevant when fluid intake is very low.

Intravenous (IV) Fluids

In cases of severe dehydration, medical professionals administer IV fluids containing water and electrolytes to restore balance quickly.

Daily Intake Values

1. Daily turnover: healthy adults turn over around 2.5 litres of body water per day, with intake balancing loss.
2. Fluids drunk: around 1.5 litres per day comes from drinks such as water, sports drinks, juices, and milk.
3. Water inside food: around 0.7 litres per day comes from the water content of food, particularly fruits and vegetables.
4. Metabolic water: around 0.3 litres per day is produced when the body oxidises nutrients during cellular respiration.

Absorption of Water and Electrolytes in the Large Intestine

Water Absorption

1. Around 80-90% of the water that enters the digestive system is reabsorbed before elimination.
2. This prevents excessive fluid loss, maintaining hydration.

Electrolyte Absorption

1. Sodium (Na⁺) and chloride (Cl⁻) are actively absorbed to help retain water in the body.
2. Potassium (K⁺) and bicarbonate (HCO₃⁻) are also regulated for pH balance and nerve function.

Loss of Water and Electrolytes

Water and electrolytes leave the body through four primary routes:

1. Sweating (Evaporation via Skin)

Sweating is a thermoregulatory process where water and electrolytes are lost to cool the body.

Sweat composition

1. Primarily water but also contains sodium (Na⁺), chloride (Cl⁻), potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺).
2. Sodium: the biggest electrolyte loss in sweat, and must be replenished for muscle and nerve function.

Factors Increasing Sweat Loss

1. Exercise intensity: Higher workloads lead to more sweat production.
2. Environmental conditions: Hot/humid environments increase evaporation rate.
3. Individual variation: Some people sweat more and lose more sodium per litre of sweat.

2. Respiratory Water Loss (Breath Vapor)

When breathing, moisture is lost through water vapor in exhaled air. Respiratory water loss depends on:

1. Breathing rate: Higher rates (e.g., during exercise) increase moisture loss.
2. Cold, dry air conditions: More water is lost to humidify inhaled air.
3. Altitude: Increased respiration at higher altitudes raises fluid loss.

3. Urine Excretion (Kidney Filtration)

1. The kidneys regulate water and sodium loss based on hydration status.
2. Increased urine production occurs when hydration is sufficient, whereas dehydration triggers water conservation via ADH release.

4. Fecal Water Loss (Osmotic Excretion via the Intestines)

1. The large intestine absorbs most water, but some water is lost in feces.
2. Diarrhea leads to excessive electrolyte loss, causing potential dehydration and electrolyte imbalances.

Daily Loss Values

In a healthy person at rest, daily water loss adds up to roughly 2.5 litres, matching daily intake.

1. Sweat (evaporation from skin): around 0.45 litres per day at rest, rising sharply during exercise.
2. Respiratory water loss: around 0.35 litres per day as water vapour leaves the body in exhaled breath.
3. Urine: around 1.5 litres per day under normal conditions, regulated by the kidneys.
4. Faeces: around 0.2 litres per day, after most water is reabsorbed in the large intestine.

Regulation of Water and Electrolyte Balance

1. Water and electrolyte balance is regulated through a complex interplay between the hypothalamus, the pituitary gland, and the kidneys.
2. This system works through negative feedback to keep body fluid concentration within a narrow range around the set point.
   1. Osmoreceptors in the hypothalamus: continuously monitor blood osmolarity (the concentration of solutes in body fluid).
   2. When body water falls: solute concentration rises, the hypothalamus detects the change, thirst is triggered, and the hypothalamus signals the posterior pituitary to release ADH.
   3. When body water rises: solute concentration falls, the hypothalamus reduces ADH secretion, and the kidneys produce a larger volume of more dilute urine to restore balance.
   4. Negative feedback: a small change in body fluid concentration is detected by receptors, which trigger a response that corrects the change and brings the variable back inside its normal range.
   5. Common error: the hypothalamus does not release ADH itself; it signals the posterior pituitary gland, which releases ADH into the bloodstream.

Hormones Involved in Regulation

1. Antidiuretic Hormone (ADH): released by the posterior pituitary in response to signals from the hypothalamus, ADH acts on the kidneys to increase water reabsorption and prevent dehydration.
2. Aldosterone: released by the adrenal cortex, aldosterone increases sodium reabsorption in the kidneys, which indirectly retains water and supports blood volume and blood pressure.

Measuring Hydration Status

1. Thirst on its own is not a precise measure of hydration.
2. For athletes and patients, three more objective methods are used: body weight, urine colour, and urine osmolarity.

Body Weight

1. Method: weigh the athlete before and after a session. The difference in body mass, corrected for any fluid drunk during the session, gives an estimate of sweat loss and the remaining water deficit.
2. Threshold: a loss of more than 2% of body mass during a session is the point at which performance starts to fall measurably.
3. Rehydration: for every 1 kg of body mass lost, an athlete should aim to drink around 1.5 litres of fluid to fully rehydrate.

Urine Colour

1. How it works: when the body loses water, ADH causes the kidneys to produce smaller volumes of more concentrated urine. The darker the urine, the more dehydrated the athlete.
2. Pale straw colour: indicates good hydration.
3. Dark yellow or amber: indicates dehydration and the need to drink more fluids.
4. Field tool: coaches use an 8-point urine colour chart to give athletes a quick visual check before and after training.

Urine Osmolarity

Osmolarity is a precise laboratory measure of the concentration of dissolved particles in a fluid.

1. Hydrometer: measures the specific gravity of urine, which rises as solute concentration rises.
2. Osmometer: measures the freezing point of urine, which drops as solute concentration rises.
3. Use: both tools allow physiologists to put a numerical value on hydration status.

When Balance Fails

1. When water and electrolyte balance is not maintained, three states can develop: dehydration, hyponatremia (low blood sodium), and hypernatremia (high blood sodium).
2. All three affect health, and all three reduce exercise performance.