HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 23 URINARY SYSTEM

The kidneys eliminate waste metabolic products using water, but conserve water, electrolytes and other materials to maintain the body homeostatically in fluid, pH and electrolyte balance. The urine produced is evacuated periodically via urinary passages.

A KIDNEY

This separates from the blood large quantities of ultra-filtered fluid in more than a million small, tubular units, nephrons/uriniferous tubules. Most needed materials are then recovered to the bloodstream, and some secretion of other substances occurs, to give a solution of unwanted materials -the excretion - to be collected as urine from the tubules. The kidney is a compound, tubular, excretory gland, and an endocrine gland. Powerpoint.

l Kidney's general architecture
l Outside are perirenal fat, and nearby suprarenal glands.
2 Thin, fibrous capsule.
3 Reniform (kidney-shaped!), around a hilum and sinus for the
4 renal artery, renal vein, and ureter.
5 Ureter opens from a renal pelvis, for which
6 major and minor calyces* collect the urine from
7 bluntly pointed apical papillae of pyramids.
8 Pyramid + overlying tissue constitute a lobe.
9 The human kidney is multilobar, with 8-l8 lobes.
l0 Pyramidal tissue has a pale striated appearance from many parallel tubules and blood vessels. It is the medulla.
ll The outer cortex of the kidney is darker, with many round structures - renal corpuscles/Malpighian corpuscles, and coiled tubules cut in cross and oblique section.
l2 Cortical tissue - columns of Bertin - runs inward to partly separate the pyramids.
l3 Medullary tissue extends rays up from the medulla into the cortex. A medullary ray defines the centre of a lobule, but the lateral limits of the lobule remain undefined in the cortical tissue.
(* The minor calyces act as a curved row of funnels with cut-off stems, dripping into a second row of stem-less major funnels, delivering into the single pelvis. This arrangement and the concept of the sinus cannot be made out in a single section.)

2 Form of nephron and relations with cortex and medulla
Cortex
l Renal corpuscle (round, l50-240 µm diameter) - glomerulus of epithelium-invested capillaries, and enclosed in a Bowman's capsule, opening out at the urinary pole into the
2 proximal convoluted tubule, which leads to the

Medulla
3 descending limb of the hairpin loop of Henle,
4 then the ascending limb of Henle's loop.

Cortex
5 Distal convoluted tubule follows, attached at one point to the renal corpuscle of origin; thence leading to an
6 arched collecting/junctional tubule joining a

Medulla
7 straight collecting tubule, receiving many branches and running down from a medullary ray through the medulla to a
8 papillary duct of Bellini, opening at the papilla of the pyramid. The papilla is cribriform from the many openings.

Alternative terms for these parts are:
.. 2 may be termed the pars convoluta of the proximal tubule;
.. 5 may be termed the pars convoluta of the distal tubule;
.. the loop of Henle comprises the pars recta of the proximal tubule, the thin segment, and the pars recta of the distal tubule.
Thin segments and loops vary in length dependent on the position in the cortex of their glomeruli of origin. The appearance of the kidneys is dominated by the nephrons, since the connective tissue element (reticular fibres) is slight, and the very many small blood vessels follow the pattern of the nephrons, because the two work together.

3 Functional unit of the kidney
Consists of (a) nephron, (b) blood vessels, (c) interstitium, and (d) collecting tubule.
The functions of the various parts of the unit are given briefly, so that all aspects of the finer structure can be presented together.
l Renal corpuscle, with vascular glomerulus - ultrafiltration of arterial blood.
2 Proximal convoluted tubule - from the ultra-filtrate received from the corpuscle, the prompt massive recovery (reabsorption), by active transport, cotransport, facilitated and downhill diffusion, of sodium, chloride, glucose, amino acids, etc, and of small proteins by endocytosis.
3 Loop of Henle - urine concentration by active and passive functions in a complicated counter-current osmotic multiplier interaction of loops of Henle, blood vessels, interstitium, and collecting tubules.
4 Distal tubule (partly in the loop) - continued active reabsorption of Na+ under the control of aldosterone, and the secretion of potassium.
5 Collecting tubule - passive reabsorption of water to the blood, making the urine hypertonic, under the influence of pituitary antidiuretic hormone (ADH); and a variety of fine adjustments to electrolytes and acidity.
6 The nephron is controlled by hormones from other endocrine glands, but the kidney itself produces hormones that affect non-renal tissues (Chapter 27.F).

4 Nephron cytology
1 Glomerulus

2 Proximal tubule (40-50 µm diameter)
3 Thin segment (l5 µm diameter) 4 Distal tubule (20-50 µm diameter) 5 Juxtaglomerular apparatus 6 Collecting duct (40-200 µm diameter)

5 Renal interstitium
1 lies between the kidney tubules and vessels.
2 It comprises: (a) reticular fibres, (b) a little ground substance, and (c) interstitial fibroblasts, looking after the matrix and secreting erythropoietin.
3 The interstitial elements are more prominent in the medulla than the cortex.

6 Renal blood vessels
l Renal artery branches to form
2 interlobar arteries (interpyramidal), extending to the cortico-medullary junction, where they branch and turn as arching
3 arcuate arteries, giving off outward branches called
4 interlobular arteries; from which
5 intralobular arteries provide
6 afferent arterioles to
7 glomeruli; from the capillaries of which the blood is taken via
8 efferent arterioles to serve one or both of
9 two capillary beds - around the convoluted tubules, and between the straight medullary tubules.
l0 The blood collected in stellate, deep cortical, and interlobular veins, traces back the arterial path to the renal vein.
ll The sympathetic nervous supply to the kidney goes mainly to the renal vasculature, including the juxtaglomerular cells.
l2 Vasa recta is a collective name for arteriolar, capillary, and venous straight blood vessels in the medulla. They participate in the counter-current exchange.

B URINARY PASSAGES

. The kidney's calyces and pelvis, and the passages to the urethra are lined by transitional epithelium. Powerpoint.

l Transitional epithelium/urothelium
l Multilayered, with large surface/umbrella cells, intermediate cells and basal cuboidal cells on a thin BL.
2 The surface cells have unique properties of:
.. (a) making a barrier impermeable to urine;
.. (b) changing their shape and extent during bladder distension.
3 For 2 (a), the luminal umbrella cell membrane is asymmetrically thickened (to l2 nm) and has unusual lipids and proteins, including uroplakins
4 For 2(b), the Golgi complex forms fusiform vacuoles, bounded by thick membranes. During bladder dilation, the vesicles attach to the thick luminal membrane and become part of it, thus increasing its extent and allowing the cell to flatten. No cell-over-cell sliding occurs, the cells being joined by tight and adhaerens junctions and desmosomes.
5 Large lysosomes destroy defective membrane.
6 The rate of cell turnover is very low for an epithelium.

2 Ureter
l Transitional epithelium lies on a collagenous lamina propria.
2 Mucosa has several longitudinal folds, giving the lumen a stellate shape in the cross-section.
3 Two smooth muscle coats: outer, circular; inner, longitudinal; (the terminal ureter has an extra, outer longitudinal one).
4 CT adventitia, rich in vessels and nerves.

3 Urinary bladder
l Transitional epithelium, on a wide collagenous lamina propria without glands, constitutes the mucosa.
2 Three smooth muscle tunics interweave in the muscularis, in a pattern to squeeze the bladder empty. Retention of urine invites infection.
3 A CT adventitia has blood and lymphatic vessels, nerve fibres and ganglion cells. The part of the bladder facing the pelvic cavity has a serosa.
4 The ureters enter obliquely, with mucosal flaps to prevent reflux; smooth muscle forms a sphincter at the urethral outlet.

. Urethra (male)
l Epithelium lies on a very loose, elastic, vascular, distensible lamina propria. The lumen is stellate in cross-section.
2 Epithelium is transitional changing to pseudostratified columnar, stratified columnar, and finally stratified squamous, as it traverses the three sections: prostatic, membranous (short) and penile/cavernous (long).
3 Branching out in the penile mucosa are Littré's small tubular mucous glands.
4 There is a meagre smooth muscle muscularis, except at
5 the smooth and skeletal muscle sphincters
6 Female urethra is much shorter than the male; structurally it is similar, but, ending in the pelvic floor, has a skeletal muscle sphincter at its terminus.


William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159