Lecture: Water Balance, Filtration & Edema

Vocabulary flashcards covering aquaporins, Starling forces, net filtration, edema mechanisms, thirst, and ADH regulation from the UCDavis Water lecture.

Aquaporin

Selective water channel protein that permits rapid, bidirectional water movement across cell membranes.

ECF (Extracellular Fluid)

Fluid outside cells; exchanges water freely with ICF through aquaporins and paracellular routes.

ICF (Intracellular Fluid)

Fluid inside cells that reaches osmotic equilibrium with ECF via aquaporin-mediated water movement.

Hydrostatic Pressure (HP)

Force exerted by a fluid pushing outward from its compartment; drives filtration out of capillaries.

Osmotic (Oncotic) Pressure (OP)

Pulling force exerted by solutes, especially plasma proteins like albumin, drawing water toward them.

Starling Forces

Combined hydrostatic and oncotic pressures that determine net filtration or reabsorption across capillary walls.

Capillary Hydrostatic Pressure (HPc)

Blood pressure within capillaries that pushes fluid into the interstitium; normally ~35 mmHg arterially, 18 mmHg venously.

Interstitial Hydrostatic Pressure (HPi)

Pressure of interstitial fluid that pushes fluid back into capillaries; usually low (≈1–2 mmHg).

Capillary Oncotic Pressure (OPc)

Pulling force of plasma proteins drawing fluid into the capillary; ~24–25 mmHg.

Interstitial Oncotic Pressure (OPi)

Osmotic pull of proteins in interstitial fluid that attracts water out of capillaries; normally ~0 mmHg.

Net Filtration

[HPc + OPi] – [OPc + HPi]; positive values indicate filtration, negative values indicate reabsorption.

Filtration

Movement of fluid out of capillaries into interstitial space.

Reabsorption

Movement of fluid from interstitial space back into blood or lymphatic vessels.

Venous Obstruction

Blockage that elevates venous HP, reduces reabsorption, and promotes edema formation.

Increased Capillary Permeability

Inflammation-induced loss of endothelial integrity causing protein leakage, ↓OPc, ↑OPi, and edema.

Edema

Swelling produced by excess interstitial fluid accumulation.

Localized Edema

Edema confined to a specific area due to local injury or inflammation.

Generalized Edema

Widespread interstitial fluid accumulation throughout the body.

Dependent Edema

Fluid pooling in gravity-dependent areas (feet when standing, sacrum when supine).

Lymphatic Obstruction

Blockage of lymph flow that prevents interstitial fluid return and causes edema.

Thirst Mechanism

Physiologic drive to seek water triggered by ↑serum osmolarity (osmoreceptors), ↓blood pressure (baroreceptors), or hormones (e.g., angiotensin II).

Polydipsia

Excessive thirst, commonly seen in diabetes mellitus or certain brain/psychogenic disorders.

Anti-Diuretic Hormone (ADH) / Arginine-Vasopressin (AVP)

Hormone from hypothalamus/posterior pituitary that increases renal water reabsorption via aquaporin insertion in distal tubules.

ADH Stimulus

Elevated plasma osmolarity detected by hypothalamic osmoreceptors (also low BP or angiotensin II).

ADH Effect on Kidney

↑Aquaporins in distal nephron → ↑water reabsorption → ↓urine output.

Plasma/Interstitial Fluid Exchange

Dynamic movement of fluid between blood and tissues at capillary beds governed by Starling forces.

Increased Capillary Hydrostatic Pressure

Common edema mechanism caused by heart failure, venous thrombosis, hepatic obstruction, or prolonged standing.

Decreased Plasma Oncotic Pressure

Edema mechanism resulting from hypoalbuminemia (e.g., liver failure, malnutrition, nephrotic syndrome).