Urinary System - Structure and Function

Pete TakizawaDept of Cell Biology

What we’ll talk about…

• Structure and function of nephrons

• Filtration barrier in renal corpuscles

• Segments of nephron that reabsorb solutes and water

• Feedback to regulate the filtration rate in individual nephrons

Components of the urinary system.

• Kidney

• Ureter

• Urinary bladder

• Urethra

The kidney performs several critical functions.

• Excretion of metabolic waste products

• Water and electrolyte homeostasis

• pH homeostasis

• Renin production

• Vitamin D

Nephrons filter blood and reabsorb solutes and water.

• Filtration Unit - Renal Corpuscle

• Capillaries

• Basement Membrane

• Epithelium

• Reabsorption - Long Tubule

The kidney contains two types of nephrons to filter blood and regulate electrolyte concentration and pH.

Thin Descending

Thin Ascending

Collecting Duct

Medulla Cortex

Thick Ascending

Efferent Arteriole

Proximal Convoluted Tubule

Renal Corpuscle

Afferent Arteriole

Distal Convoluted Tubule

Connecting Tubule

Arcuate arteries define the border between cortex and medulla in the kidney.

Arcuate Artery

Collecting Ducts

Renal Corpuscle

Medulla Cortex

Renal corpuscles filter blood from a tuft of capillaries into Bowman’s space.

Distal Convoluted TubuleBowman’s Capsule

Podocyte

Capillary

Bowman’s Space

Mesangial CellArteriole

Macula Densa

Starling forces determine the direction of fluid flow in the glomerular capillaries.

Afferent Arteriole

Efferent Arteriole

Capillary Hydrostatic

Bowman’s Oncotic Bowman’s Hydrostatic

Capillary Oncotic

Renal corpuscles filter solutes based on size and charge.

1.8 2.2 2.6 3.43.0 3.8 4.2 4.60

0.2

0.4

0.6

0.8

1.0

Effective Molecular Radius (nm)

Cle

aran

ce R

atio

(Cx/

Cin)

Cationic Molecules

Neutral Molecules

Anionic Molecules

The filtration barrier consists of fenestrated endothelium, basement membrane and podocytes.

Foot Process

Slit Diaphragm

Basement MembraneEndothelial Cell

Fenestrae

Blood

Rara Interna

Rara Externa

Lamina Densa

Basement Membrane

Glycocalyx

Foot Process

Endothelium

Podocytes form foot processes that attach to the basement membrane.

Podocyte

Capillary Lumen

Bowman’s Space

Foot ProcessBasement Membrane

Endothelial Cell

Slit diaphagms span the distance between foot processes and contribute to the filtration barrier.

Basement Membrane

Endothelial Cell

Slit Diaphragm

Rara Externa

Lamina Densa

Rara Interna

Plasma

Urinary Space

Foot ProcessFoot Process

Foot processes from adjacent podocytes interdigitate and surround capillaries.

Foot Processes - Cell 1

Foot Processes - Cell 2

Podocyte

Podocyte Process

Different stains and microscopy methods are used to visualize features of renal corpuscles.

H&E PAS

Silver Stain Electron Micrograph

Nephron Tubule

The proximal convoluted tubule absorbs most of the water and important biological solutes.

Proximal Convoluted Tubule

Microvilli on the apical surface and basal striations increase the surface area of epithelial cells.

Microvilli

Basal Striations

Mitochondrion

The loop of Henle in juxtamedullary nephrons increases the osmolality of the interstitium.

Thick Ascending Limb

Proximal Straight Tubule

Thin Limb

The distal convoluted tubule reabsorbs sodium and resides near the renal corpuscle.

Renal Corpuscle

Proximal Convoluted Tubule

Distal Convoluted Tubule

Cells of the distal convoluted tubule and afferent arteriole form the juxtamedullary complex.

Afferent Arteriole

Juxtaglomerular CellMacula Densa

Distal Convoluted Tubule

Macula densa cells sense sodium to regulate the glomerular filtration rate.

Proximal Convoluted Tubule

Distal Convoluted Tubule

Afferent Arteriole

Efferent Arteriole

Macula Densa Cells

Glomerulus

Juxtaglomerular Cells

Smooth Muscle

Sodium chloride uptake triggers release of ATP and adenosine which constricts the afferent arteriole.

Afferent Arteriole

DCT Lumen

NKCC2

Na2+ K+ Cl-

ATP

Macula Densa Cell

Ca2+

ATPAdenosine

AdenosineCl-

K+

Contraction

Collecting ducts contain intercalated cells and principle cells to regulate pH and water absorption.

Collecting Duct

The ureter is a muscular tube that connects the kidney to the urinary bladder.

Transitional Epithelium

Circular Smooth Muscle

Longitudinal Smooth Muscle

The urinary bladder is lined by a transitional epithelium that stretches as urine fills the bladder.

Transitional Epithelium

Basement Membrane

Smooth Muscle

Take home messages…

• Nephrons filter blood and reabsorb biologically important molecules.

• A filtration barrier of endothelium, basement membrane and podocyte determines what passes from blood into urine.

• Segments of the nephron tube have different properties to absorb biological molecules, ions and water.

• The juxtaglomerular complex generates feedback to regulate the glomerular filtration rate.