You have two kidneys, each about the size of your fist, located on either side of your spine just below your rib cage. Despite their small size (about 150g each), they receive about 20-25% of your cardiac output — that's how important they are!

Kidney Anatomy:

• Cortex: Outer region containing the glomeruli and convoluted tubules
• Medulla: Inner region with renal pyramids containing loops of Henle
• Renal pelvis: Funnel-shaped collecting area that leads to the ureter

The Nephron: Each kidney contains about 1 million nephrons — the functional units that filter blood and produce urine.

Nephron Components:

• Renal Corpuscle: Glomerulus (tuft of capillaries) + Bowman's capsule (cup-like structure that collects filtrate)
• Proximal Convoluted Tubule (PCT): Reabsorbs 65% of filtrate including all glucose and amino acids
• Loop of Henle: Creates concentration gradient in medulla; critical for water reabsorption
• Distal Convoluted Tubule (DCT): Fine-tunes electrolyte balance; regulated by hormones
• Collecting Duct: Final concentration of urine; regulated by ADH

Glomerular filtration is where it all begins. Blood enters the glomerulus through the afferent arteriole under high pressure, forcing fluid and small molecules through the filtration barrier into Bowman's capsule.

The Filtration Barrier (Three Layers):

• Fenestrated endothelium: Capillary cells with pores that allow most substances through
• Basement membrane: Prevents large proteins from passing
• Podocytes: Specialized cells with foot processes and filtration slits — the final size barrier

What gets filtered:

• Water, electrolytes, glucose, amino acids, urea, creatinine
• NOT filtered: Large proteins, blood cells (these stay in the blood)

Glomerular Filtration Rate (GFR): About 125 mL/min or 180 L/day. This means your kidneys filter your entire blood volume about 60 times per day! From this 180 L of filtrate, only 1-2 L becomes urine — the rest is reabsorbed.

Net Filtration Pressure: About 10 mmHg. This is the balance between blood pressure pushing fluid out and two opposing forces (capsular pressure and blood colloid osmotic pressure).

After filtration, the kidney must decide what to keep and what to throw away. This happens through reabsorption (returning useful substances to blood) and secretion (adding additional waste to urine).

Major Reabsorption Sites:

• Proximal tubule: Reabsorbs 65% of water, ALL glucose and amino acids, most Na+, K+, Cl-, and bicarbonate
• Loop of Henle: Descending limb reabsorbs water; ascending limb reabsorbs NaCl (but not water)
• Distal tubule: Reabsorbs Na+ (regulated by aldosterone) and Ca2+ (regulated by PTH)
• Collecting duct: Reabsorbs water (regulated by ADH) and Na+ (regulated by aldosterone)

Secretion: Some substances are actively secreted FROM blood INTO the tubule:

• Hydrogen ions (H+) — important for acid-base balance
• Potassium (K+) — regulated by aldosterone
• Ammonia (NH3)
• Creatinine
• Certain drugs (penicillin, some diuretics)

Transport Maximum: There's a limit to how much of certain substances (like glucose) can be reabsorbed. When blood glucose exceeds about 180 mg/dL (as in uncontrolled diabetes), glucose appears in urine (glycosuria).

Kidneys do far more than make urine. They are master regulators of your internal environment:

1. Excretion: Remove metabolic wastes (urea, creatinine, uric acid, drug metabolites)

2. Fluid Balance: Regulate blood volume and osmolality by adjusting water excretion. When you're dehydrated, ADH causes more water reabsorption, concentrating urine.

3. Electrolyte Balance: Precisely regulate Na+, K+, Ca2+, Mg2+, PO4 3-, and Cl- levels. The kidneys are the main route of excretion for most electrolytes.

4. Acid-Base Balance: Excrete H+ and reabsorb/generate HCO3- to maintain blood pH between 7.35-7.45.

5. Blood Pressure Regulation:

• Renin-Angiotensin-Aldosterone System (RAAS): Kidneys release renin when blood pressure drops, initiating a cascade that increases blood volume and vessel constriction
• Control of sodium and water directly affects blood volume

6. Hormone Production:

• Erythropoietin (EPO): Stimulates red blood cell production in bone marrow
• Renin: Initiates RAAS for blood pressure regulation
• Calcitriol: Active vitamin D, essential for calcium absorption

Clinical Note: Kidney disease often causes anemia (low EPO), bone problems (low calcitriol), and high blood pressure (disrupted RAAS).