Renal Physiology: Filtration, Reabsorption, and Secretion

This set of vocabulary flashcards covers the fundamental mechanisms of renal physiology, including the roles of specific nephron segments, transport proteins, and pharmacological interventions.

Filtration

The first step in urine formation where fluid and solutes move from the glomerular capillaries into Bowman's capsule.

Reabsorption

The movement of substances from the tubule filtrate back into the peritubular capillary blood.

Secretion

The process of moving substances from the peritubular capillary blood into the tubule filtrate.

Excretion

The final removal of substances from the body in the form of urine, calculated as $\text{Excretion} = ext{Filtration} - ext{Reabsorption} + ext{Secretion}$.

Proximal Convoluted Tubule (PCT)

Known as the "mass absorber," it is responsible for the reabsorption of solutes at a rate of approximately $125,ml/min$.

Brush Border

The extensively amplified apical (luminal) membrane of the PCT that contains microvilli to increase surface area for reabsorption.

Transport Maximum ($T_m$)

The maximum speed of solute transport reached when all carrier proteins are saturated.

Thin Descending Limb of the LOH

A segment highly permeable to water via $AQP-1$ water channels ($20\%$ of filtered water reabsorbed) but impermeable to solutes, with no active sodium transport.

Thick Ascending Limb of the LOH

Called the "Diluting Segment," it is impermeable to water and actively pumps sodium out of the tubule using the $Na^+/2Cl^-/K^+$ co-transporter.

Loop Diuretics

A class of drugs including Furosemide, Ethacrynic acid, and Bumetanide that inhibit transport in the thick ascending loop of Henle.

Counter Current Multiplier

The process by which the Loop of Henle creates a salt gradient in the medullary interstitium, allowing the kidney to excrete hyper-osmotic or hypo-osmotic urine.

Urine Concentration Range

The possible osmolality of urine depending on body needs, ranging from $50,mOsmolal$ to $1400,mOsmolal$.

Sluggish Blood

Low blood flow in the renal medulla/vasa recta that helps minimize solute loss from the medullary interstitium, maintaining the osmotic gradient.

Early Distal Tubule

A segment that reabsorbs $Na^+$, $Cl^-$, $Ca^{2+}$, and $Mg^{2+}$ but is NOT permeable to water or urea.

Principal Cells

Cells located in the late DCT and collecting tubule that reabsorb sodium and water and secrete potassium, regulated by aldosterone.

Intercalated Cells

Cells in the late DCT and collecting tubule responsible for long-term plasma $pH$ control by secreting or reabsorbing hydrogen and bicarbonate ions.

Potassium-Sparing Diuretics

Diuretics that work in the late DCT and collecting tubules, classified as either aldosterone antagonists (e.g., Spironolactone, Eplerenone) or sodium channel blockers (e.g., Amiloride, Triamterene).

Medullary Collecting Ducts

The final part of the nephron, which is permeable to urea via facilitated diffusion and is the site for final water reabsorption regulated by $ADH$.