URINARY SYSTEM

Assoc. Prof Dr. Karim Al-Jshamy

IMS/MSU 2010

• The kidneys, ureters, urinary bladder and urethra are the main components of the urinary system.

• A function of the urinary system that immediately comes to mind is the excretion of waste products from the body. This is only one of many functions of the system. Others are

• elimination of foreign substances • regulation of the amount of water in the body • control of the concentration of most compounds in the extracellular fluid • Most of these tasks are performed in the kidneys. Functionally the processes

can be divided into two steps, each of which have their anatomical correlate:• filtration - glomeruli of the kidney • selective resorption and excretion - tubular system of the kidney • In addition, the kidney also functions as an endocrine organ. Fibrocytes in the

cortex release the hormone erythropoietin, which stimulates the formation of red blood cells.

• Modified fibrocytes of the medulla secrete prostaglandins which are able to decrease blood pressure.

StromaCapsuleDense FECTMyofibroblast layerInterstitial stroma (loose FECT)

ParenchymaNephronsCollecting ductsVascular components

Organized into cortex and medulla

Overall Organization of the Kidney

Kidney Glomeruli and the tubular system

are both part of the basic functionalunit of the kidney, the nephron.

The Glomerulus (or renal corpuscle) The glomerulus is the round (~0.2

mm in diameter) blind beginning ofthe nephron. It is invaginated by atuft of capillaries at the vascularpole of the glomerulus.

The tuft of capillaries and othercells in contact with them form theanatomical glomerulus. Glomerulus.

The anatomical glomerulus is enclosed by two layers of epithelium, Bowman'scapsule. Cells of the outer or parietal layer of Bowman's capsule form a simplesquamous epithelium.Cells of the inner layer, podocytes in the visceral layer, are extremely complex inshape. Small foot-like processes, pedicles, of their cytoplasm form a fenestratedepithelium around the fenestrated capillaries of the glomerulus.

• The openings between the pedicles arecalled filtration slits. They are spannedby a thin membrane, the filtration slitmembrane.

• Between the podocytes and theendothelial cells of the capillaries wefind a comparatively thick basal lamina,which can be subdivided into an outerlamina rara externa, a middle laminadensa and an inner lamina rara interna.The basal lamina and the slitmembranes form the glomerularfiltration barrier,

• Mesangial cells in the glomerulus formthe connective tissue that givesstructural support to podocytes andvessels.

Blood pressure is the driving force in the formation of about 125 ml ofglomerular filtrate per minute.About 124 ml of the glomerular filtrate is reabsorbed in the tubules of thenephron.

KidneyCapsule as thin membrane ofconnective tissue, cortex of the kidneyand scan over the tissue, presence ofglomeruli (convoluted parts ofproximal and distal tubuli).

• to identify the vascular pole of a goodglomerulus by the attachment of thecapillary tuft to the wall of theglomerulus.

• The nuclei are located side by side ormay even overlap.

• Proximal tubules are characterised bytheir eosinophilic (pink) low columnarcells and by large amounts of fuzzymaterial, which may fill the entirelumen of the tubulus.

The anatomical glomerulus, the parietal blade of Bowman's capsule (squamous cells),

podocytes (fairly large and light nuclei ),

endothelial cells (smaller and darker nuclei), vascular pole.

Tubules of the Nephron• The tubular system can be divided into

proximal and distal tubules, which in turnhave convoluted and straight portions.

• Intermediate tubules connect the proximaland distal tubules. Running from the cortexof the kidney towards the medulla(descending), then turning and running backtowards the cortex (ascending), the tubulesform the loop of Henle.

• The proximal tubule is the longest section ofthe nephron (about 14 mm).

• The convoluted part of the proximal tubulescoils close to the glomerulus in the cortex.

• The proximal tubules are formed by a lowcolumnar epithelium. The eosinophilic cellsof the epithelium have a wide brush border(long microvilli) and are active inendocytosis.

• They almost completely resorb substancesof nutritional value from the glomerularfiltrate (glucose, amino acids, protein,vitamins etc.)

• In the proximal tubules the volume of theglomerular filtrate is reduced by about 75%.Sodium ions are actively resorbed from theglomerular filtrate.

• hey are followed by passively diffusingchloride ions and the osmotic absorption ofwater. The straight portion of the proximaltubule descends towards the medulla.

• The straight portion of the proximal tubulemerges with the intermediate tubule (thinsegment of the loop of Henle).

• A flattened, only ~1-2 µm high epitheliumforms the intermediate tubule, which is only~15 µm wide. Descending parts of thestraight proximal and intermediate tubulesare permeable to water but not to solutes.

Kidney The medulla of the kidney, there

is a collecting ducts (cuboidal tocolumnar cells, well-definedboundaries between cells,cytoplasm only weakly stained orunstained, large ducts)

An intermediate (very flatepithelium, nuclei bulge into thelumen of the tubulus, diameter ofthe duct is small) and distal tubule(cuboidal epithelium, cells stainweakly pink).

The Juxtaglomerular Apparatus

• The distal tubule contacts theglomerulus forming aspecialized section of tubularepithelium, the macula densa.

• At the point of contact with theglomerulus, the distal tubule isalways in close contact with theefferent and afferent arteriolesof the glomerulus.

• The juxtaglomerular (JG)apparatus are extraglomerularmesangial cells and thejuxtaglomerular cellssurrounding the afferentarteriole (modified smoothmuscle cells), which produceand secrete renin.

• URETER• The urine flows through these

structures to the ureter and is channelled to the bladder.

• The mucosa is lined with a transitional epithelium , which occurs exclusively in the urinary system.

• The lamina propria consists mainly of dense connective tissue, with many bundles of coarse collagenous fibres.

• The muscularis usually consists of an inner longitudinal and outer circular layer of smooth muscle cells .

• In lower parts of the ureter and the bladder an additional outer longitudinal layer of muscles is added to the first two.

Cross Section of a Ureter Lumen“Artifact”

Circular smoothmuscle

Longitudinal smooth muscle

Dome cells

Transitional epithelia changes depending on how full the urinary bladder is

These are retroperitoneal structures. They enter the bladder at an oblique angle whichhelps to prevent backflow of urine. Smooth muscle in the wall of the ureters rhythmicallycontracts (peristalsis) to move urine into the bladder

Histology of Bladder: mucosa of transitional epithelium, Submucosa, and thick muscular layer know as the detrusor muscle

BLADDER

Urinary Bladder Mucosa (transitional epithelium)

Transitional Epithelium of the U. Bladder –note the different cell shapes

The Urethra • Initially, the urethra is lined by a

transitional epithelium in males andfemales.

• In males, it is replaced by apseudostratified or stratified columnarepithelium below the openings of theejaculatory ducts into the urethra.

• The distal parts of the female urethraand the distal end of the male urethraare lined by a stratified squamousepithelium.

• The lamina propria contains looseconnective tissue. Smooth muscle cellsin the muscularis are mainly orientedlongitudinally.

• They are surrounded, in the middlepart of the urethra (below the prostatein males), by striated muscle cells ofthe sphincter urethrae.