Lung–kidney system ↗Intake and losses set mass balance
Water arrives through drinks, food, and metabolism and leaves through urine, skin, lungs, stool, bleeding, or abnormal fluid sequestration.
KIDNEYS / 06
Concentration and amount
are different questions.
Water balance strongly influences sodium concentration, while total-body sodium helps shape extracellular volume. Potassium regulation combines cell shifts with renal excretion.
Osmoreceptors sense concentration
Hypothalamic sensing responds to small osmolality changes while volume and pressure signals modify the response.
Thirst drives behavior
Neural perception, angiotensin signaling, habits, access, temperature, activity, and illness influence drinking.
ADH changes collecting-duct water permeability
Vasopressin promotes aquaporin-2 trafficking, allowing water recovery when a medullary gradient is available.
Sodium balance shapes extracellular volume
Kidney sodium excretion eventually matches intake; hormones, pressure, nerves, filtration, and segment transport control the path.
Sodium concentration tracks water context
A high or low serum sodium concentration does not directly state whether total-body sodium or extracellular volume is high or low.
Potassium shifts across cell membranes
Insulin, catecholamines, acid–base state, cell injury, and osmolality can redistribute potassium rapidly without changing total amount.
Distal nephron controls potassium excretion
Aldosterone, distal sodium delivery, flow, intake, kidney function, and medicines influence secretion.
Heart and muscle require narrow extracellular ranges
Large potassium disturbances can impair conduction and contraction; symptoms cannot reliably grade the concentration.
Edema has multiple causes
Cardiac, renal, liver, venous, lymphatic, medication, protein, and local tissue factors can all alter fluid distribution.