Kidney Function and Transport Mechanisms: Nephrons, Filtration, and Reabsorption

Functions of the Kidney

Filters blood, regulates blood pH, and performs endocrine (hormonal) functions

Filtrate

A liquid identical to blood plasma but containing no blood cells

Renal Blood Flow Pathway

Renal artery -> smaller vessels -> Nephron

Nephron Functions

Regulates pH, filters blood, and helps regulate blood pressure (BP)

Glomerulus

The site of filtration that keeps blood cells and large proteins out of the urine

Glomerular Filtration Pressure

The Afferent arteriole is larger than the Efferent, creating high BP to produce filtrate

Reabsorption

The process where the nephron takes back water, electrolytes, glucose, and amino acids from the filtrate

Proximal Convoluted Tubule (PCT)

Site where electrolytes, glucose, and amino acids are primarily reabsorbed

Creatinine

A waste product of muscle metabolism that should stay in the filtrate

Urea (BUN)

A waste product of protein metabolism

Sodium-Potassium (SP) Pump

Uses ATP to lower intracellular Na+, creating a "sodium vacuum" to pull in glucose

Symporters

Transporters that move two different things in the same direction (e.g., SGLT2)

Antiporters

Transporters that move two different things in opposite directions

SGLT2

Sodium-Glucose Transporter 2; a symporter used to force glucose into the cell with sodium

GLUT2

A transporter on the basolateral membrane that is always open, allowing glucose to diffuse into the blood

Renal Threshold for Glucose

200 mg/dL; below this, 100% of glucose is reabsorbed by the kidneys

Transport Maximum

400 mg/dL; the point where the kidney can no longer reabsorb any additional glucose

Descending Loop of Henle

Highly permeable to water; responsible for the majority of water reabsorption

Ascending Loop of Henle

Impermeable to water; responsible for the majority of salt (Na+ and Cl-) reabsorption

Countercurrent Mechanism

Water leaves descending limb to concentrate Na+; Na+ is then reabsorbed in the ascending limb

Distal Convoluted Tubule (DCT)

Fine-tunes urine by managing waste products, water, and blood pH

pH Regulation in DCT

Maintains pH by forcing Hydrogen ($H^+$) into the tubule (secretion) via antiporters

Facilitated Diffusion

The method used by most renal transporters; notably, only the SP pump uses ATP

Polyuria

Excessive urination

Polydipsia

Excessive thirst

Osmotic Diuresis

Increased urination caused by unabsorbed solutes (like sugar) preventing water reabsorption (less osmosis)