Anatomy of Kidneys/Urinary Tract

200 μm diameter, formed by invagination of a tuft of capillaries into Bowman's capsule (the dilated, blind end of the nephron)
Capillaries are supplied by an afferent arteriole and drained by the efferent arteriole
The diameter of the afferent arteriole is greater than the efferent arteriole
Filtrate is formed from the glomerulus, crossing the capillary endothelium of the capillary and the specialized epithelium of the capsule
The glomerular endothelium is fenestrated, with pores 70-90nm in diameter, completely surrounded by glomerular basement membrane with specialised cells called podocytes
Podocytes have pseudopodia which interdigitate to form filtration slits along the capillary wall, each 25nm wide and closed by a thin membrane
The glomerular basement membrane (the basal lamina) does not contain visible gaps or pores
Stellate cells called mesangial cells are located between the basal lamina and the endothelium (similar to pericytes found elsewhere in the body).

These are common between two adjacent capillaries, where the basement membrane forms a sheath shared by both capillaries
The mesangial cells are contractile and play a role in glomerular filtration regulation
Mesangial cells secrete ECM, take up immune complexes and are involved in glomerular disease progression

The glomerular membrane permits free passage of neutral substances up to 4 nm in diameter, and excludes anything with a diameter greater than 8 nm
Charge on molecules and their diameter affects passage into bowman's capsule
Total area of glomerular capillary endothelium is about 0.8m²

The Tubule:

15mm long, 55μm diameter, made up of a single layer of cells that interdigitate with one another and are united by apical tight junctions
Between the cells are extensions of extracellular space the lateral intercellular spaces
The luminal edges of the cells have a striated brush border of microvilli

Descending and proximal ascending limbs made up of thin, permeable cells
Thick portion of the ascending limb made up of thick cells with many mitochondria
Nephrons with peripheral glomeruli have short loops of Henle (cortical nephrons), while those in the cortex (juxtamedullary nephrons) have long loops extending into medullary pyramids

5mm long, starting at the macula densa
Contains a few microvilli but no distinct brush border
Distal tubules coalesce to form collecting ducts - 20mm long, pass through the renal cortex and medulla to empty into the renal pelvis at the apexes of the medullary pyramids
Collecting duct epithelium is made up of principal cells (P cells - small, few organelles, involved in Na+ and vasopressin stimulated water reabsorption) and intercalated cells (I cells - more microvilli, vesicles and mitochondria, involved in acid secretion and HCO3- transport)
Total nephron length 45-65mm

Renal medullary interstitial cells - fibroblast-like cells which contain lipid droplets and synthesise COX-2 and Prostaglandin synthase PGES which produces PGE₂ - important in regulation of salt and water homeostasis
Prostacyclin PGI2 and other prostaglandins are secreted by the arterioles and glomeruli

Divide into multiple capillary brancehs to form the vessel tuft of the glomerulus

These capillaries then coalesce to form the efferent arteriole, which then breaks up into capillaries which supply the tubules (peritubular capillaries)
Arterial segments between glomeruli and tubules are therefore a portal system, and the glomerular capillaries are the only capillaries which drain into arterioles
Capillaries draining tubules of the cortical nephrons form a peritubular network
Efferent arterioles from juxtamedullary glomeruli drain into the peritubular network, but also into vasa recta - vessels that form hairpin loops into the medullary pyramids alongside the loops of Henle

Descending vasa recta have non-fenestrated endothelium containing a fenestrated endothelium, allowing solutes to be conserved

Each efferent arteriole from each glomerulus breaks up into capillaries supplying a number of different nephrons, therefore supplying different tubules
Total surface area of renal capillaries = total surface area of tubules = 12m²
Total volume of blood in renal capillaries = 30-40ml
Innervation

Renal nerves travel with renal blood vessels as they enter the kidney
Contain many post-ganglionic sympathetic efferent fibres and a few afferent fibres arising from lower thoracic and upper lumbar segments of the spinal cord with ganglions in the sympathetic ganglion chain, superior mesenteric ganglion and along the renal artery
Sympathetic fibres are distributed to afferent and efferent arterioles, proximal and distal tubules, and the JGA
Nociceptive afferents mediating pain in kidney disease parallel the sympathetic efferents, entering the spinal cord in the thoracic and upper lumbar dorsal roots
Other renal afferents mediate the reno-renal reflex - increase in ureteral pressure in one kidney decreases efferent nerve activity to the other kidney, increasing excretion of Na⁺ and water
Also contains vagus nerve innervation of unknown function

The Capsule

Renal capsule is thin but tough
Limits swelling in oedematous kidneys causing an increase in renal interstitial pressure. This decreases the GFR and enhances and prolongs anuria in ARF.

The Ureters contain smooth muscle arranged in spiral, longitudinal and circular bundles

Regular peristaltic contractions occuring one to five times per minute moving the urine from the renal pelvis
The ureters pass obliquely through the bladder wall, and this angulation keeps the ureters closed except during peristaltic waves, preventing urine from refluxing from the bladder

The bladder has a similar arrangement of smooth muscle arranged in spiral, longitudinal and circular bundles

Contraction of the circular detrusor muscle causes bladder emptying during micturition
Muscle bundles pass on either side of the urethra, and are sometimes called the internal urethral sphincter (although these do not encircle the urethra)
Further along the urethra is a sphincter of skeletal muscle - the external urethral sphincter
The bladder epithelium is made up of a superficial layer of flat cells, and a deep layer of cuboidal cells

A spinal reflex, facilitated and inhibited by higher brain centres
Urine entering the bladder does not cause much increase in intra-vesical pressure until the viscus is well filled
During micturition, perineal and external urethral sphincter are relaxed, the detrusor muscle contracts, and urine passes through the urethra
Voluntary initiation of micturition begins with relaxation of pelvic floor muscles, which tug downwards on the detrusor muscle which in turn initiates its contraction
During micturition, perineal muscles and external sphincter can be contracted voluntarily, preventing urine from passing down the urethra or interrupting flow once urination begins
The bands of smooth muscle of the internal urethral sphincter are not involved - these are for prevention of semen reflux into the bladder during ejaculation

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