URINARY SYSTEM components:

kidneys, ureters, urinary bladder and urethra

function

excretion of waste products

elimination of foreign substances

regulation of the amount of water in the body

control of the concentration of most compounds in the extracellular fluid

functional steps (related to anatomy)

filtration - glomeruli of the kidney

selective resorption and excretion - tubular system of the kidney

also functions as an endocrine organ secreting erythropoietin (cortical fibrocytes) and prostaglandin (modified medullary fibrocytes)

The Kidney

Nephron: basic functional unit Glomeruli and

the tubular system

The Glomerulus (or renal corpuscle)

the round(~0.2 mm in diameter) blind beginning of the nephron.

invaginated by a tuft of capillaries at the vascular pole of the glomerulus

Substances which leave the capillaries enter the renal tubule at the urinary pole of the glomerulus

the glomerulus

Bowman's capsule encloses the

glomerulus

two layers of epithelium

outer or parietal layer: form a simple squamous epithelium.

inner layer or visceral layer: podocytes; are extremely complex in shape

the glomerulus pedicles

Small foot-like processes of the cytoplasm of podocytes form a fenestrated epithelium around the fenestrated capillaries of the glomerulus

filtration slits

openings between the pedicles

filtration slit membrane

spans filtration slits

basal lamina

thick connective tissue between the podocytes and endothelium

glomerular filtration barrier

The basal lamina and the slit membranes

which prevents some large molecules from entering the capsular space between the outer and inner epithelial layers of Bowman's capsule

Mesangial cells

form the connective tissue that gives structural support to podocytes and vessels

Blood pressure

is the driving force in the formation of about 125 ml of glomerular filtrate per minute.

About 124 ml of the glomerular filtrate is reabsorbed in the tubules of the nephron.

Tubules of the Nephron

can be divided into proximal and distal tubules, which in turn have convoluted and straight portions.

Intermediate tubules: connect the proximal and distal tubules. loop of Henle: Runs from the cortex of the kidney

towards the medulla (descending), then turns and runs back towards the cortex (ascending)

The proximal tubule is the longest section of the nephron (about 14 mm)

The convoluted part coils close to the glomerulus in the cortex.

diameter is ~65 µm.

Their walls are formed by a low columnar epithelium.

The eosinophilic cells of the epithelium have a wide brush border and are active in endocytosis.

They almost completely resorb substances of nutritional value from the glomerular filtrate

In the proximal tubules the volume of the glomerular filtrate is reduced by about 75

straight portion descends towards the medulla.

The Thin segment Of the Loop of Henle

intermediate tubule

flattened, only ~1-2 µm high epithelium; only ~15 µm wide

permeable to water but not to solutes (together with straight portion of Proximal tubule)

leads into the straight part of the distal tubule

Distal Tubule

straight part: formed by low cuboidal cells without a brush border (few short microvilli present but difficult to see)

diameter of the tubule expands to ~35 µm

Epithelial cells transport Cl (active) and Na ions (passive) out of the tubular lumen into the surrounding peritubular space (together with ascending loop of henle)

epithelium is impermeable to water.

macula densa: structure formed by the contact of distal tubule with glomerulus

Convoluted distal tubule: after the straight part

about 5 mm long

Cells are sensitive to the hormone aldosterone, which is produced in the zona glomerulosa of the adrenal glands. Aldosterone stimulates the active resorption of sodium ions and the excretion of potassium ions.

Collecting ducts

formed from the merging of convoluted tubule via connecting ducts

epithelia are permeable to water but not to sodium ions in the presence of ADH

merge to form papillary ducts (of Bellini), which terminate on the tips of the renal papilla and empty into a distended, funnel shaped part (minor calyx) of the ureter.

The Juxtaglomerular Apparatus

includes the macula densa, extraglomerular mesangial cells and the juxtaglomerular cells surrounding the afferent arteriole (modified smooth muscle cells), which produce and secrete renin.

Excretory Passages renal pelvis: found

within the kidney; formed from the merging of the major calyces which in turn are formed by the merging of minor calyces.

urine flows through these structures to the ureter and is channelled to the bladder.

Excretory passages mucosa: transitional epithelium; is virtually

impenetrable to any components of the urine

lamina propria: dense connective tissue, with many bundles of coarse collagenous fibres.

muscularis: 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.

urethra

Initially lined by a transitional epithelium in males and females.

In males, it is replaced by a pseudostratified or stratified columnar epithelium below the openings of the ejaculatory ducts into the urethra.

The distal parts of the female urethra and the distal end of the male urethra are lined by a stratified squamous epithelium.

The lamina propria contains loose connective tissue.

Smooth muscle cells in the muscularis are mainly oriented longitudinally. They are surrounded, in the middle part of the urethra (below the prostate in males), by striated muscle cells of the sphincter urethrae