Organs of the Urinary System and their Neurovasculature

4 Organs of the Urinary System and their Neurovasculature

4.1 Overview of the Urinary Organs; the Kidneys in situ


B Projection of the urinary organs onto the organs of the abdomen and pelvis

Anterior view. Owing to its large size, the liver displaces the right kidney slightly inferiorly. The bladder is shown in a fully distended state. It is anterior to the rectum in the male and anterior to the uterus (not shown here) in the female. Because of this relationship, marked distention of the rectal ampulla or enlargement of the uterus due to pregnancy exerts greater pressure on the bladder, creating an urge to urinate even when the bladder is not full. Urinary incontinence may develop due to pathological processes of longer duration, such as muscular tumors of the uterus (fibroids), or due to weakening of the bladder closure mechanism as a result of previous vaginal deliveries (descent of the muscular pelvic floor).


C Location of the kidneys, normal vs. pathological mobility

a Posterior view. The pleural cavities overlap the kidneys posteriorly owing to the convexity of the diaphragm.

Note that the right kidney is lower than the left kidney and is closer to the palpable iliac crest.

b, c Anterior view. The kidneys are located in the retroperitoneum just below the diaphragm. Hence they move passively with the diaphragm during respiratory excursions, moving inferiorly and slightly laterally during inspiration because of their oblique position (their inferior poles point away from the spine, see oblique red lines in a).

These passive movements may cause respiration-dependent pain in patients with renal disease. A pathological increase in renal mobility (“floating kidney,” see c) results from atrophy of the fat capsule that normally surrounds the kidneys and keeps them in a stable position. A wasting illness (e.g., metastatic tumors of varying origin) may cause such severe fatty atrophy that the kidneys descend to a lower level in the abdomen. As they are still tethered by the ureter and vascular stalk, this descent may kink the renal vessels or ureter and interfere with renal blood flow or urinary outflow.

4.2 Kidneys: Location, Shape, and Structure


A Position of the kidneys in the renal bed

Right renal bed. a Sagittal section at approximately the level of the renal hilum, viewed from the right side. b Transverse section through the abdomen at approximately the L 1/L 2 level, viewed from above.

The renal bed is located on each side of the spine in the retroperitoneum. It contains the kidneys, which are invested by a thin organ capsule (renal fibrous capsule), and the suprarenal glands, which are surrounded by the perirenal fat capsule that also encloses the kidneys. The fat capsule is thicker posteriorly than anteriorly.

Note: Swelling of the kidney (usually due to inflammation) may cause severe pain due to stretching of the fibrous capsule.

The fat capsule is surrounded by the renal fascia, which separates it from its surroundings by two layers:

The anterior layer behind the parietal peritoneum (to which it is fused at some sites)

The posterior layer, which is partially attached to the transversalis fascia and muscular fasciae on the posterior trunk wall

The renal fascia, and thus the renal bed, is open inferiorly and medially to allow passage of the ureter and renal vessels. It is closed laterally and superiorly by fusion of the fascial layers. Because of this arrangement, inflammatory processes that are adjacent to the kidney but within the renal fascia tend to spread to the contralateral side or inferiorly and may spread into the pelvis.

Note: The entire renal bed moves downward during inspiratory depression of the diaphragm, indirectly causing the kidney and suprarenal gland to move as well. This differs from the liver, which is attached to the diaphragm (bare area) and is directly moved by diaphragm excursions.

B Renal bed: fasciae and capsules of the kidneys

Renal fibrous capsule

Thin, firm connective-tissue capsule that closely invests each kidney

Perirenal fat capsule

Mass of fat that surrounds the kidneys and suprarenal glands and completely occupies the renal bed; it is thickest lateral and posterior to the kidneys

Renal fascia

Connective-tissue fascial sac that encloses the perirenal fat, portions of the abdominal aorta and inferior vena cava close to the kidney (see Ab), and the proximal ureter; subdivided into a thin anterior layer and a thick posterior layer (see Aa)


C Structure and shape of the kidney

Anterior view (a), posterior view (b), and medial view (c) of the right kidney. The suprarenal gland is left intact in a and b, and the ureter has been cut at the level of the inferior renal pole. The fibrous capsule that directly invests the kidney is intact in a and c and has been partially opened in b to display the underlying renal parenchyma. The renal sinus (the deep space into which the hilum opens) generally contains a certain amount of structural fat, and so the vascular structures and renal pelvis are not exposed to view intraoperatively as they are in these drawings. The normal kidney measures an average of 12 x 6 x 3 cm (L x W x T) and weighs 150–180 g. It has

two poles (superior and inferior),

two surfaces (anterior and posterior), and

two borders (lateral and medial).

The medial border bears the renal hilum, where vascular structures and the ureter enter and leave the kidney. The shallow surface grooves result from the embryonic lobulation of the kidney. The hilar structures are usually arranged as follows from anterior to posterior (as shown in c): right renal vein, right renal artery, and right ureter.

Note: The renal artery is usually posterior to the renal vein because the right renal artery passes to the right kidney behind the inferior vena cava (where the renal veins terminate), while the left renal vein passes to the left kidney i n front of the abdominal aorta (which gives origin to the renal arteries). The left renal artery may also loop around the left renal vein from above to occupy an anterior position. The ureter leaves the renal pelvis (see p. 294) below the vessels and is usually somewhat posterior in relation to the blood vessels.

4.3 Kidneys: Architecture and Microstructure


B Renal corpuscle

a With the capsule opened; b In section.

The renal corpuscle is the interface between the blood vessels and the excretory portion of the urinary tract (see C). It consists of a central convoluted vascular loop, the glomerulus, and a bulbous envelope lined by squamous epithelial cells, the glomerular capsule (Bowman’s capsule). Blood enters the glomerulus at the vascular pole of the renal corpuscle by flowing through the afferent glomerular arteriole, and it leaves the glomerulus through the efferent glomerular arteriole. The primary urine is formed within the renal corpuscle and drains through a tubular system at the urinary pole of the glomerulus. The initial portion of this tubular system that is connected to the glomerular capsule is the proximal convoluted tubule (see C)

Note: Specialized cells at the vascular pole of the renal corpuscle regulate the blood pressure that is necessary for ultrafiltration.


C Architecture of the renal vessels and intrarenal collecting system

a Renal vessels: Sectional view of a medullary pyramid with adjacent cortical areas. The intrarenal vascular and collecting systems are closely interrelated spatially and functionally. An ultrafiltrate from the blood (primary urine) drains into a microscopically small system of renal tubules. Blood flow to the kidney (a) is supplied by interlobar arteries that pass along the sides of the medullary pyramids from the renal hilum. Each interlobar artery supplies two adjacent medullary pyramids and the associated cortical zones (these branches are not shown). At the base of the pyramid, the interlobar artery gives rise to the arcuate artery, from which the interlobular arteries are distributed into the cortex as far as the fibrous renal capsule. The afferent glomerular arterioles that arise from an interlobular artery each supply one glomerulus. The efferent glomerular arterioles that emerge from the glomerulus are still carrying blood at a high oxygen tension; they supply the renal cortex or medulla.

b Intrarenal collecting system: The smallest functional unit of the kidney is the nephron, which consists of the renal corpuscle and renal tubules. Each nephron drains via a short connecting tubule into a collecting duct which collects the urine from about 10–12 nephrons. There are approximately 1 million nephrons, which process approximately 1700 liters of blood daily to form approximately 170 liters of primary urine. This primary filtrate enters the tubular system at the urinary pole of the renal corpuscle and reaches the renal papilla as the final urine, which drains into the calyceal system (approximately 1.7 liters/day). The tubular system consists of the proximal and distal tubules (each with a convoluted and straight portion) and an intermediate tubule (with descending and ascending limbs). The intermediate tubule and the adjacent straight portions of the proximal and distal tubules comprise the loop of Henle. While passing through the tubular system, substances contained in the filtrate (mainly water) are reabsorbed while other substances (e.g., ions) are secreted into the filtrate. This process yields the final urine, which passes through a collecting tubule into a collecting duct and drains through the renal papilla into the caliceal system. It is conveyed from the calyces and renal pelvis to the ureter by peristalsis.

4.4 Renal Pelvis and Urinary Transport


B Variations in the shape of the renal pelvis

Anterior view of the left renal pelvis. The renal pelvis and ureter develop from an outgrowth of the mesonephric duct. This “ureteric bud” grows from the bony pelvis toward the renal primordium and unites with it. Branching extensions of the renal pelvis form the major and minor calyces. The major calyces in particular vary in number and shape: neighboring major calyces may fuse and “become incorporated” into the renal pelvis. The renal pelvis is found in basic forms along with transitional forms:

Dendritic type (with extensive branching, also called linear) (a): very fine major calyces, narrow renal pelvis;

Transitional form (b);

Ampullary type (c): indistinct major calyces with wide renal pelvis; minor calyces arise “directly” from the renal pelvis.


D Wall structure of the ureter

Transverse section through a ureter. A characteristic feature is the stellate lumen that appears in cross-section due to the longitudinal mucosal folds. As in the urethra and bladder, the ureteral mucosa consists of a transitional epithelium of varying height (see p. 305). The smooth muscle consists basically of a longitudinal and a circular layer. It is powerfully developed and shows a functionally spiral architecture (see E). When a renal stone enters the ureter, the smooth muscle in the ureteral wall undergoes powerful contractions in an effort to expel the stone, causing very severe pain (renal or ureteral colic). The colic may be relieved by drugs that suppress the activity of the parasympathetic nervous system, though this will also inhibit normal urinary transport to the bladder. The renal pelvis is structurally analogous to the ureter, including the stellate shape of its lumen.


E Arrangement of the ureteral musculature (after Graumann, von Keyserlingk, and Sasse)

Schematic cross-sections at various levels of the ureter. The longitudinal and circular muscle layers of the ureter wall have a slightly oblique arrangement, forming a kind of spiral that propels urine toward the bladder by peristaltic contractions. Although the ureters are richly innervated, the peristaltic contractions are instigated by spontaneously depolarizing smooth-muscle cells in the walls of the renal pelvis. Peristaltic waves of contraction (with a speed of 2–3 cm/s) are propagated through direct electrical connections (gap junctions) between adjacent smooth-muscle cells. Autonomic motor innervation and local sensory reflexes serve to modulate this intrinsic activity. This mechanism may thus have some superficial similarities to the system that controls heartbeat.

4.5 Suprarenal Glands


A Location and shape

a Location of the suprarenal gland on the right kidney. b Isolated left suprarenal gland, anterior view.

The renal surface of each suprarenal gland lies upon the superior pole of the associated kidney. A thin layer of fat separates the suprarenal gland from the renal fibrous capsule (making it easy to dissect the gland from the kidney). The perirenal fat capsule, however, encompasses both the kidney and the suprarenal gland.

Note: The entire suprarenal gland cannot be seen while in situ, and its true size is not appreciated until it has been detached from the kidney. Portions descend on the posterior surface of the kidney and are not visible in situ.


B Structure of the suprarenal gland

a Right suprarenal gland, cut open. b Histological section from a suprarenal gland.

The suprarenal gland consists of an outer cortex and an inner medulla (see a). The cortex is covered by a thin fibrous capsule and consists of three morphologically distinct zones (see b) in which adrenocortical hormones are produced and secreted into the bloodstream. These zones are, from outside to inside:

Zona glomerulosa: mainly secretes mineralocorticoids (aldosterone)

Zona fasciculata: mainly secretes glucocorticoids (hydrocortisone)

Zona reticularis: sex hormones (estrogens and androgens)

Note: Loss or deficiency of both suprarenal cortices leads to Addison disease, while hyperfunction of the suprarenal cortex (or adrenocortical tumors) leads to Cushing syndrome.

The suprarenal medulla is essentially a completely different endocrine gland, of different origin, that happens to be anatomically (but also functionally) associated with the suprarenal cortex. The cortex is derived embryonically from mesoderm lining the posterior abdominal wall. The suprarenal medulla is, by contrast, a neural crest derivative, and thus has an ectodermal origin. The catecholamines epinephrine and norepinephrine are produced in the suprarenal medulla and are released into the bloodstream. From a (neuro)functional standpoint, the suprarenal medulla is less a gland than a sympathetic ganglion: presynaptic sympathetic neurons pass from the greater and lesser splanchnic nerves into the suprarenal medulla. Because the suprarenal glands are endocrine glands and sympathetic ganglia in one, they can secrete both epinephrine and glucocorticoids (cortisone) in response to stress.

Aug 4, 2021 | Posted by in GENERAL SURGERY | Comments Off on Organs of the Urinary System and their Neurovasculature
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