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$60-65\%$ of body weight is water.
Intracellular Fluid (ICF): $2/3$ of body water, found within cells and essential for cellular metabolic processes like metabolism and maintaining cell volume.
Extracellular Fluid (ECF): $1/3$ of body water, includes interstitial fluid (fluid surrounding cells), intravascular fluid (plasma within blood vessels), and other transcellular fluids (e.g., cerebrospinal, synovial fluid, pleural fluid).

Electrolytes are minerals that carry an electric charge when dissolved in water, vital for various bodily functions such as nerve impulse transmission, muscle contraction, and maintaining fluid balance.
Na+, K+, and Cl- are key electrolytes in ECF and ICF.
ECF: Primarily contains Na+ (sodium) and Cl- (chloride), which are crucial for maintaining fluid volume, blood pressure, and nerve signal transmission.
ICF: Primarily contains K+ (potassium), essential for intracellular osmotic balance, nerve impulses, and muscle contraction.

Water moves passively by osmosis from an area of lower solute concentration (higher water concentration) to an area of higher solute concentration (lower water concentration) across a semi-permeable membrane, striving for osmotic equilibrium.
Hydrostatic Pressure: A pushing force exerted by fluid within a confined space, such as blood pressure pushing fluid out of capillaries into the interstitial space.
Oncotic Pressure (also known as colloid osmotic pressure): A pulling force primarily exerted by large plasma proteins (like albumin) that draws water into the circulation (e.g., back into capillaries from the interstitial space), thereby counteracting hydrostatic pressure.

Fluid accumulation in ECF; can be localized (e.g., swelling in an injured ankle) or generalized (e.g., systemic fluid retention).
Causes: Decreased plasma proteins, increased capillary permeability (allowing more fluid to leak out), increased hydrostatic pressure, lymphatic obstruction (impairing fluid drainage).

A complex hormonal system that regulates blood pressure and ECF volume by restoring renal perfusion (blood flow to kidneys) and sodium balance. When blood pressure or sodium levels are low, the kidneys release renin, initiating a cascade that leads to the production of angiotensin II, which then stimulates aldosterone release.
Aldosterone: A steroid hormone secreted by the adrenal cortex that acts on the kidneys to increase sodium and water retention and excrete potassium, ultimately increasing blood volume and blood pressure.

Also known as vasopressin, it is produced by the hypothalamus and released by the posterior pituitary gland. It regulates water balance in response to changes in plasma osmolality (concentration of solutes in the blood) or blood volume.
Increases water reabsorption in the kidneys' collecting ducts and tubules, leading to more concentrated urine and an increase in blood volume to dilute elevated plasma osmolality.

Na+ normal range: $136-145$ mEq/L. Regulates osmotic forces, fluid distribution, and neuromuscular irritability.
K+ normal range: $3.5-5.0$ mmol/L. Essential for nerve impulses, muscle contraction (especially cardiac muscle), and enzyme function.

Hypernatremia: High sodium levels, often caused by dehydration, leading to increased plasma osmolality and causing cells to shrink as water moves out of them.
Hyponatremia: Low sodium levels, often caused by excess water, leading to decreased plasma osmolality and causing cells to swell, which can result in severe neurological symptoms (e.g., confusion, seizures) due to cerebral edema.

Ca++ Normal Range: $2.1-2.6$ mmol/L. Essential for bone health, blood clotting, hormone secretion, and muscle function.
Mg++ Normal Range: $0.75-1.25$ mmol/L. Important for neuromuscular function, enzyme activity, and DNA synthesis.

Mr. Pink's case illustrates the impact of aging on fluid and electrolyte management, compounded by conditions like hypertension and dementia. He exhibits signs of fluid deficiency and electrolyte imbalances (Na+ $130$ mEq/L, K+ $3.1$ mEq/L) indicative of potential hypovolemia and risk for complications like falls, confusion, and cardiac arrhythmias.