Renal System

Kidney

- Two bean-shaped organs located on either side of the spine, behind the abdominal cavity.

Primary Functions

Filters blood to remove waste products
Produces urine
Regulates water and electrolyte balance
Maintains blood pressure
Helps regulate acid-base (pH) balance
Produces hormones involved in red blood cell production and blood pressure regulation

Kidney Anatomy

Renal Cortex

Outer layer of the kidney
Contains the renal corpuscles and portions of the nephron where blood filtration begins

Renal Medulla

Inner region of the kidney
Contains loops of Henle and collecting ducts; establishes the concentration gradient needed to concentrate urine

Nephron

The nephron is the functional unit of the kidney responsible for filtering blood and forming urine.

Approximately 1 million nephrons are found in each kidney
Extends through both the renal cortex and renal medulla

Nephron Structures

Glomerulus

Renal cortex
Network of capillaries that filters blood under high pressure

Bowman's Capsule

Surrounds the glomerulus in the cortex
Collects the filtrate produced during filtration

Proximal Convoluted Tubule (PCT)

Renal cortex
Reabsorbs most water, sodium, glucose, amino acids, bicarbonate, and other essential nutrients

Loop of Henle

Extends from the cortex into the medulla and back toward the cortex
Helps establish the medullary osmotic gradient necessary for urine concentration

Descending Limb

Renal medulla
Permeable to water; primarily reabsorbs water — water leaves by osmosis

Ascending Limb

Renal medulla
Impermeable to water; the thick ascending limb actively reabsorbs Na⁺, K⁺, and Cl⁻

Distal Convoluted Tubule (DCT)

Located in the renal cortex
Fine-tunes electrolyte balance
Reabsorbs NaCl and helps regulate calcium reabsorption
The late distal nephron and collecting duct participate in regulated Na⁺ reabsorption and K⁺/H⁺ secretion

Collecting Duct

Passes from the cortex through the medulla
Final regulation of water reabsorption is strongly influenced by ADH
Participates in acid-base and electrolyte regulation
Carries tubular fluid toward the renal papilla and ultimately the renal pelvis

Blood Flow Through the Nephron

Renal artery → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries → Renal vein

Glomerular Filtration Forces

Filtration at the glomerulus is governed by opposing Starling forces:

Glomerular hydrostatic pressure — pushes fluid out of the capillary into Bowman's capsule (favors filtration).
Oncotic (colloid osmotic) pressure — created by plasma proteins; pulls fluid back into the capillary and opposes filtration.
Capsular (Bowman's) hydrostatic pressure — pressure of filtrate already in the capsule; also opposes filtration.

Net filtration pressure = hydrostatic − (oncotic + capsular).

(Urine) Flow

Glomerulus (blood is filtered) → Bowman's Capsule → Proximal Convoluted Tubule (PCT) → Descending Limb of Loop of Henle → Ascending Limb of Loop of Henle → Distal Convoluted Tubule (DCT) → Collecting Duct → Renal pelvis → Ureter → Urinary Bladder → Urethra

Important Transport Mechanisms

Active Transport

Uses ATP to move substances against their concentration gradient
Example: Sodium-potassium (Na⁺/K⁺) ATPase pump

Secondary Active Transport

Uses the sodium concentration gradient created by the Na⁺/K⁺ pump
Allows reabsorption of glucose, amino acids, and other solutes without directly using ATP

Osmosis

Passive movement of water across a selectively permeable membrane toward an area of higher effective solute concentration
Primary mechanism for water reabsorption in the descending limb and collecting duct

Anatomy & Physiology