Genitourinary system

18 Genitourinary system




INTRODUCTION


The urinary system functions physiologically as a complex homeostatic mechanism for the regulation of acid-base balance, the excretion of waste products, and the control of water and electrolyte balance. Approximately one-fifth of the cardiac output passes through the kidneys each minute, producing a renal blood flow of up to 400 mL/min per 100 g of kidney, which equates to approximately 650 mL/min per kidney. Under normal circumstances a total of 170–180 L of plasma per day are filtered by the glomeruli at an overall rate of 125 mL/min. Based on a normal fluid intake of approximately 2 L per day, between 1 and 1.5 L of urine is produced, which passes down the ureters to the bladder. Urine is produced within the kidneys at low pressure – not exceeding 15 cmH2O, and is stored in the bladder at low pressure. The bladder is able to store urine at low pressure because of the particular property of smooth muscle known as tonus or receptive relaxation which permits the bladder to stretch and accommodate increasing volumes without any intrinsic rise in pressure until the functional capacity of the bladder is reached. In some situations structural limitation to distension of the bladder is imposed by an increased stiffness of the bladder wall as a consequence of fibrosis secondary to collagenous infiltration. This is most commonly associated with bladder outflow obstruction, although it can occur following radiotherapy and can also be due to failure of the bladder muscle to relax in association with neurological disorders. In these conditions filling of the bladder beyond ‘anatomical’ capacity will result in a linear rise in pressure and so-called ‘low compliance’.


The bladder spends 99% of its time in a storage phase. The other function of the bladder, for which it is best recognised, is as a voiding organ – during which it contracts, expelling the contained urine within it, out via the urethra (with synchronous relaxation of the urethral sphincter mechanisms) and empties itself to completion.


The urinary tract is a complex system which subserves vital physiological functions resulting in the production of urine, its transfer from the kidneys via the ureters to the bladder, its low pressure storage and complete expulsion at a socially appropriate time. In order to understand the clinicopathological conditions affecting the urinary tract it is, therefore, important first to appreciate the anatomy (including structure, innervation and function), physiology and the techniques for structural and functional evaluation.



ANATOMY



KIDNEYS




Macroscopic anatomy


The kidneys have a characteristic shape and may retain a degree of fetal lobulation. Each kidney is approximately 11 cm long, 6 cm broad and 3 cm in thickness. Each kidney has the following features:






The renal vein, renal artery and renal pelvis enter and leave the kidney at the level of the hilum and are situated anatomically in relation to each other in the order mentioned above, moving in an anteroposterior direction. The anatomy of the contents of the hilum can be variable: for instance the renal pelvis can be bifid, and the artery and vein may split into branches or receive tributaries, respectively, to a variable extent at the hilum – which can, on occasion, lead to confusion at the time of surgical dissection in this area.


The hilum of the kidney opens into a space called the renal sinus which is surrounded by the kidney parenchyma and contains branches of the renal artery, major tributaries of the renal vein, and both the major and minor calyces (Fig. 18.1). All of the structures are surrounded and cushioned by fat.



There are two to three major calyces in each kidney, each of which comprises a number of minor calyces. An aid to visualising the anatomy is provided by the linguistic derivation of the term ‘calyx’ (latin for a wine glass). There are as many minor calyces as there are papillae, usually six to ten per kidney. The renal parenchyma comprises two zones (Fig. 18.1):




The lobulation seen in fetal life represents each medullary pyramid with its associated rim of cortex. The cortex contains about one million glomeruli and their associated proximal and distal convoluted tubules. The medulla contains the loops of Henle, which sit up in the extracellular fluid, providing a gradient of increasing osmolarity (highest near the papillae). The collecting ducts run through this area and join together to form 30–40 papillary ducts with associated papillae. The microscopic structure and anatomy of the kidney is considered in more detail below when discussing physiological functions of the kidney. The characteristic arrangement of vein, artery and pelvis (from anterior to posterior) seen at the hilum is no longer present further into the renal parenchyma at the renal sinus. The nerves supplying the kidney are associated with the walls of blood vessels and comprise sympathetic and parasympathetic postganglionic fibres from the coeliac plexus.



The renal arteries


These are direct branches from the aorta to each kidney, and in 30% of cases accessory renal arteries are present. The renal artery divides into three to five segmental arteries at the hilum, which divide within the sinus into six to ten lobar arteries – one for each each pyramid and associated cortex (Fig. 18.2). Each lobar artery divides into six interlobar arteries which are associated with papillae. The interlobar arteries give rise to arcuate arteries which run in a plane between cortex and medulla. The arterial supply to the cortex is derived from interlobular arteries which are branches of the arcuate arteries directed radially towards the kidney capsule, and each of these in turn gives off the afferent arterioles and supply glomeruli. The efferent arterioles that leave the glomeruli supply the capillary bed that surrounds the convoluted tubules. Arteries enter and leave each pyramid at its base and, as they penetrate deeper towards the papilla, they lose water by osmosis and gain ions by diffusion – thereby reaching equilibrium with surrounding extracellular fluid. They then loop back towards the base of the pyramid, lose ions by diffusion and gain water by osmosis until they reach normal osmolarity at the base of the pyramid. These medullary vessels lie in bundles named vasa recta and are fed on the arterial side by efferent arterioles with adjacent glomeruli lying near the corticomedullary junction.




The renal veins


These follow the arterial pattern closely. In the sinus of the kidney, interlobar veins unite into lobar veins then into segmental veins which join together to form the renal vein at the hilum of the kidney. The renal veins empty directly into the vena cava. Since the vena cava lies on the right side of the abdomen, the left renal vein is longer than the right and usually passes anterior to the aorta just caudal to the origin of the superior mesenteric artery. The left renal vein receives tributaries from the adrenal gland and gonad before entering the IVC. On the right side the adrenal vein usually drains directly into the IVC.


Each kidney lies within a cushioning bed of fat and tissue. The kidney itself is surrounded by renal fascia. There is a layer of perirenal fat which lies between the renal fascia and the true capsule of the kidney, which is continuous at the hilum with the fat and the renal sinus. In addition there is a layer of perirenal fat lying outside of the perirenal fascia (Gerota’s fascia) which is particularly obvious posterior to the kidney (Fig. 18.3).



The adrenal glands lie within a separate compartment of the renal fascia. The surface markings of the kidney are shown in Fig. 18.4. The hilum of both kidneys lie roughly at the level of L1 (the transpyloric plane).



In surgical practice it is very important to be aware of the anterior and posterior relationships of the kidneys – particularly in situations where normal tissue planes between the kidney and its adjacent structures are disordered due to either malignant or inflammatory disease processes, where it is important to be aware of which structures need to be mobilised (Figs. 18.5 and 18.6). The relationship of the kidneys to surface markings is also important in planning the surgical approach to the patient during any procedure.





URETER


The ureters convey urine from the kidneys to the bladder. Each is 25–30 cm long and approximately 3 mm in diameter.


The ureter is a hollow muscular tube which commences at the renal pelvis and terminates at its entry into the bladder.


The wall of the ureter is composed of smooth muscle with a rich innervation comprising both sympathetic and parasympathetic fibres. The sympathetic fibres arise from spinal segments T11–L1 and the parasympathetic fibres from sacral segments S2–S4. Most of the nerves to the ureters are sensory. Stretching the wall, for instance with the passage of a ureteric calculus, produces acute pain. In view of the segmental innervation of the ureter this pain is usually referred to T11–L1. In addition the pain may radiate down the front of the thigh to the area supplied by L2. The ureter is lined by transitional epithelium continuous with that of the renal pelvis and calyces. The transitional cell lining of the urinary tract deserves some comment because this is a highly specialised epithelial layer designed to prevent diffusion of urine and other solids out of the urine and conversely passage of water into the urine by osmosis. Despite this it must be borne in mind that the urothelium is not purely an inert barrier, and there are a number of active transport mechanisms which are present, particularly within the urothelium of the bladder, which facilitate the passage of many substances, including drugs, through the urothelial lining and hence into surrounding tissues and ultimately the blood stream.


The entire ureter acts as a functional complex responsible for the transport of urine from the renal pelvis to the bladder. Whilst the mechanism is still a matter of some debate, it is very clear that there are one or more pacemakers situated in the renal pelvis which produce antegrade impulses which result in the propagation of a peristaltic wave down the ureter to the bladder. It has been estimated that a few drops of urine are transported down each ureter in peristaltic boluses approximately three to five times per minute. Each of these boluses of urine represents a high pressure localised segment which propagates rapidly down the ureter and which relies upon coaptation of the ureteric wall both proximal and distal to the ‘bolus’. Any disruption of the normal ability of the ureter to form such ‘boluses’ results in severe pain and ureteric dysfunction as seen in the context of a diuretic challenge to a patient with pelviureteric junction obstruction and also commonly in a patient with a ureteric calculus undergoing an intra-venous urogram (IVU), which may precipitate a further bout of renal colic.


In order to achieve its functional role the ureter must have a thick muscular coat, and indeed in its upper two-thirds it has an inner longitudinal and outer circular smooth muscle coat. In the distal third of the ureter, as it passes across the bony pelvis, the ureter acquires a third coat of longitudinal muscle which surrounds the other two. In its intramural portion, as it passes through into the bladder, the circular coat is lost but the remaining longitudinal muscle has an embryological relationship to the trigone of the bladder. The exact mechanism of action of the smooth muscle surrounding the ureter in its intramural portion remains the subject of some debate; it has been suggested that it has a sphincteric function to occlude the ureter at the time of detrusor muscle contraction to prevent vesico-ureteric reflux. A more important component of the sphincteric role of the lower ureter is likely to be its oblique position as it passes through the wall of the bladder, which results in the intramural portion of the ureter being closed off at the time of any rise in intravesical pressure to help prevent reflux – this remains the mainstay of most surgical procedures designed to correct vesico-ureteric reflux.


The ureters on either side run down the posterior abdominal wall overlying the transverse processes of L2–L5 before crossing the pelvic brim overlying the bifurcation of the iliac vessels and running along the pelvic wall to the level of the ischial spine before turning medially and slightly upwards to enter the bladder at the level of the trigone.


The ureter can be considered to comprise three parts: (a) the pelvic ureter, (b) the abdominal part and (c) the part within the bony pelvis. The ureter receives a segmental innervation from both parasympathetic and sympathetic nerves. It is likely that the majority of the innervation is sensory in nature in view of the intrinsic peristaltic properties of the ureter. Each ureter receives a segmental blood supply from the following arteries:







The venous drainage is by veins following similar lines to these arteries, and there is lymphatic drainage to periaortic and internal iliac groups of lymph nodes. There is a narrow area in each of the three segments of the ureter namely:





It is at these levels that a renal calculus is likely to become arrested on its descent into the bladder. Congenital abnormalities relating to the function of the muscle at the pelviureteric junction result in the condition known as pelviureteric junction obstruction, producing a functional outflow obstruction, which may require surgical resolution.


Surgical injuries to the ureter are most common in its lower third, owing to the close proximity of the ureter to the blood supply of the uterus, where the ureter is easily damaged during hysterectomy. It is important to appreciate the relations of the ureters and in particular the close proximity of the ureter to the gonadal vessels (Fig. 18.7), particularly the gonadal vein and its lower abdominal course through the bony pelvis, since in this area it is not unknown for the gonadal vein to be mistaken for the ureter and indeed mobilised instead of the ureter! The anterior relations of the ureter are easily dealt with in the majority of circumstances, providing its retroperitoneal position is borne in mind (Fig. 18.8).





BLADDER




Macroscopic anatomy and innervation


The bladder is a distensible reservoir with muscular walls. It lies in the true pelvis posterior to the symphysis pubis. The bladder does not rise above the pubis until it is very full, and when fully distended the adult bladder projects upwards from the pelvic cavity into the abdomen, lifting the peritoneum upwards from the abdominal wall as it distends.


The relations of the bladder are as follows:






The relationship of the bladder to adjacent structures in both the male and female is best appreciated on a sagittal view (Fig. 18.9).



The bladder is a remarkable organ. As with the rest of the urinary tract it is lined by urothelium which acts as a watertight layer which nevertheless retains the ability to allow the active transport of a number of substances across its wall. The bladder should be considered to comprise two distinct functional and anatomical components. One is the trigone, which is triangular in structure and receives the two ureters at its uppermost lateral angles (Waldeyer’s sheath) and extends down to its apex at the internal urethral meatus, where the smooth muscle of the trigone is contiguous with a ridge of smooth muscle extending down the urethra to the urethral sphincter mechanism. The innervation of the trigone is distinct from that of the remainder of the bladder muscle (detrusor muscle) in that it is predominantly adrenergic (sympathetic nervous system), relying upon the release of the neurotransmitter norepinephrine. The other component is the detrusor muscle, which constitutes the majority of the bladder and forms the cap on the base provided by the trigone. The word detrusor is derived from the term detrudare (= to drive out) and represents a complex admixture of muscle fibres, passing in different directions, which are predominantly under parasympathetic neural control acting via the release of the neurotransmitter acetylcholine acting on muscarinic receptors (the M3 subtype is functionally predominant).


Despite the great deal of work that has been carried out looking at the innervation of the lower urinary tract, a number of aspects of the innervation of the bladder remain unclear. The extrinsic nerves innervating the bladder are demonstrated in Fig. 18.10. The intrinsic nerves are derived from a perivesical plexus which lies on the connective tissue at the base of the bladder and which receives autonomic fibres from two sources: (a) parasympathetic fibres from segments S2–S4, (b) sympathetic fibres from segments T11–L2. It must be borne in mind that the contemporary textbook view of the innervation of the bladder and of the disposition of the autonomic nervous system is oversimplistic, particularly considering the fact that there are ganglia on both the sympathetic and parasympathetic nerves along their course from the spinal cord to the target organ with other ganglia both around and within the target organ, e.g. bladder, prostate, and that it is likely that there are interconnections between both the parasympathetic and sympathetic nervous systems at all of these levels. Furthermore it is now well recognised that there are a number of other sensory/motor neurotransmitters, additional to the classical neurotransmitters norepinephrine and acetylcholine, which may well be implicated in neural control pathways. There is considerable debate as to the sensory innervation of the bladder although recently the importance of purinergic and nitric oxide pathways has been clearly demonstrated. No sensory end organs have yet been identified in the human bladder, and it is thought that sensory nerves are represented by non-myelinated fibres lying within the submucosa and proprioceptive receptors associated with the peritoneum and the adventitia overlying the detrusor muscle at its dome. There has been suggestion of an intramural plexus of sensorimotor neurones, similar to that seen in the intestine, but this is as yet not fully clarified.






URETHRA AND URETHRAL SPHINCTER MECHANISMS


The urethra develops from the caudal portion of the urogenital sinus and associated Mullerian and Wolffian ducts. Whilst the urethra acts as a conduit for urine from the bladder to the outside world, it must be remembered that the urethra and its associated sphincter mechanisms play a vital role in terms of continence. It must be remembered that the urethra itself is no more than a layer of urothelium lying in a blood-filled arteriovenous sinus, the corpus spongiosum.


The urethra and its sphincter mechanisms act in concert with the bladder for satisfactory voiding to occur; in other words when the bladder contracts the outlet must relax and vice versa during the storage phase. Whilst our level of knowledge relating to the cerebral control of micturition remains rudimentary, it is now recognised that there are important local spinal reflexes involved in micturition, with longer reflex tracts projecting to the pons acting under the influence of higher centres which impose voluntary control on the pons. In addition there is an important motor nucleus in the lower sacral region – the nucleus of Onufrowicz (Onuf’s nucleus) – which is important in the control of urethral sphincter muscle tone and is integral to the synchronisation of detrusor and sphincter function. It is very appropriate to consider the urethral sphincter mechanisms of the male and female separately and to bear in mind the similarities and differences which are present. In addition to the autonomic nervous system the striated urethral sphincter mechanism receives a somatic nerve supply which is both motor and sensory from the pudendal nerve.




Male urethra


The male urethra (Fig. 18.11) is approximately 20 cm inlength and comprises an anterior and posterior part. The posterior urethra, approximately 6 cm in length, is composed of that area which traverses the prostate, which is approximately 3–4 cm in length, and that which lies within the confines of the distal sphincter mechanism, which is 2 cm in length. At the external border of the distal sphincter mechanism is the junction of the posterior urethra with the anterior urethra. The anterior urethra can be further subdivided into two areas which are divided on the basis of the areas anterior and posterior to the penoscrotal junction.



In the male there are two principal sphincteric mechanisms. The bladder neck mechanism lies at the outlet of the bladder around the internal urethral meatus and is composed principally of circularly oriented fibres, although there is a longitudinal component. This sphincter is sufficiently strong to maintain continence even if the distal sphincter mechanism is destroyed. Its principal role, however, is as a genital sphincter causing rapid closure of the bladder neck at the time of emission of semen into the prostatic urethra. The principal motor control of the bladder neck mechanism appears to be adrenergic via the release of norepinephrine from the sympathetic nerves.


Just distal to the bladder neck mechanism is the prostatic urethra, and it must be remembered that the human prostate comprises a significant smooth musclecomponent. At the apex of the prostate lies the distalsphincter mechanism, which is analogous to the urethralsphincter mechanism of the female and comprises botha lissosphincter and a rhabdosphincter as in the female urethra. Just as for the female sphincter mechanism there is a triple innervation from parasympathetic, sympathetic and somatic nerves (pudendal nerve).


In both the male and female urethra a number of glands open into the posterior urethra and can be the site of infection and source of confusion on occasion at the time of urethrography. There is slight dilatation of the urethra in the bulbar area where the urethra itself is surrounded by the bulbospongiosus muscle. There is a relative constriction of the urethra within the glans penis which helps focus the stream of urine as it comes through the dilatation present at the site of the navicular fossa; this is the narrowest part of the whole urethra.


The lining of the urethra varies in different portions. In the prostatic part it is transitional epithelium; in the membraneous and penile parts, pseudostratified and stratified columnar epithelium occur proximally, but, progressing more distally, islands of stratified squamous epithelium appear until in the distal part and the navicular fossa there is a complete sheet of stratified squamous epithelium.



REPRODUCTIVE SYSTEMS


Female reproductive organs consist of paired ovaries, paired fallopian tubes, a single midline uterus and a single midline vagina. Equivalent to the penis in the female is the clitoris, which is also composed of erectile tissue.


In the male there are paired testes where spermatozoa are produced. A pair of tubes, the vasa deferentia, carry sperm back from the testes into the pelvic cavity and into the urethra. There are paired seminal vesicles which produce materials, including sugars, needed for sperm to mature and which drain into the common termination of the vasa deferentia to form the paired common ejaculatory ducts opening into the prostatic urethra. The prostate gland produces much of the bulk of the semen. The penis is traversed by the urethra. In addition to the corpus spongiosum which surrounds the urethra, it comprises paired corpora cavernosa which represent the erectile tissue. It must be remembered that at the tip of the penis, the glans penis is contiguous with the corpus spongiosum and abuts against the corpus cavernosum (Fig. 18.12). Penile erection is essential to successful intercourse and is mediated by the nervi erigentes arising from the S2–S4 nerve roots. Disorders of both penile and clitoral erection have been increasingly recognised in recent years to be a cause of significant concern within the population, and management of these disorders is an important mainstay of the subspecialty of andrology.




MALE AND FEMALE GERM CELLS


Both male and female germ cells present within either the testis or ovary respectively result from meiosis and contain 23 chromosomes. There are significant differences between male and female germ cells in terms of timing of production, the number of germ cells produced and their size and shape.


In the female, cell division (mitosis) in the stem cells that result in germ cells ceases during embryonic life, and all the oogonia start their first meiotic division before birth. They remain in a resting phase until released from the ovary at ovulation, when the second meiotic division occurs rapidly after the ovum is penetrated by sperm. Meiosis can last up to 50 years. In contrast, mitosis in the male spermatogonia continues from puberty to old age and death. Cells are always entering meiosis, passing through the two divisions and maturing into sperm during a process that takes approximately 30 days. In the female, mitosis between oogonia in embryonic life produces a peak population of about six million cells two-thirds of the way through intrauterinal life. There are approximately two million left at birth, which is followed by a dramatic loss of germ cells: by puberty there are only 150 000, and 1,000 are left at the age of 50. In contrast, in the male, large numbers of germ cells persist through life, and in a healthy young man a single ejaculate contains 300 million sperm. The oocyte in the female is one of the largest cells in the body, measuring about 120 μm in diameter. It is spherical with a large active nucleus. In contrast the sperm comprises a head, which is the nucleus, containing tightly packed, condensed genetic material with a small cap. The acrosome and the body contain many mitochondria packed around a central cilium. The sperm relies upon energy reserves in the surrounding semen. It represents a motile cell packed with genetic material.



FEMALE REPRODUCTIVE TRACT



Ovary


The ovary is the size and shape of an almond and is attached to the posterior aspect of the broad ligament by the mesovarium (Fig. 18.13). At the superior (tubal) pole of the ovary is attached a prominent fold of peritoneum, the suspensory ligament of the ovary, which passes upwards over the pelvic brim and external iliac vessels to merge with the peritoneum over psoas major muscle. The ovarian artery gains access to the ovary through the mesovarium and suspensory ligament. A further ligament, the ovarian ligament, runs within the broad ligament to the cornu of the uterus.







Uterus


The uterus is a pear-shaped organ which is approximately 7 cm long, 5 cm from side to side at its widest point, and 3 cm anteroposteriorly. It is composed of a fundus, body and cervix. The fallopian tubes enter into each supralateral angle, above which lies the fundus. The features of the uterus, uterine tubes and vagina are shown in Fig. 18.14.







Fallopian tubes


The fallopian or uterine tubes are 10–12 cm long and run from the lateral side of the body of the uterus to the pelvic wall, where they end by opening near the ovary. The opening of the fallopian tube is called the ostium. The broad ligament of peritoneum is draped over the fallopian tube like a sheet over a washing line. Each tube comprises the following parts:






The fallopian tube is covered by peritoneum except for the intramural part. It contains a muscular coat of outer longitudinal and inner circular fibres, and the mucosa is formed of columnar-ciliated cells and lies in longitudinal ridges, each of which is thrown into numerous folds. The function of the tube is to propel ova along the lumen to the uterus. This is accomplished by muscular contraction, ciliary action and by the production of a lubricating fluid. A fertilised ovum may occasionally implant ectopically in the tube. This gives rise to an ectopic pregnancy which may cause rupture of the tube with consequent intraperitoneal haemorrhage. The distal end of the tube is open into the peritoneal cavity, providing direct communication between the peritoneum and the outside and, therefore, a potential pathway for infection.




Vagina


The vagina surrounds the cervix of the uterus and then passes downwards and forwards through the pelvic floor to open into the vestibule, which is the area enclosed by the labia minora and also contains the urethral orifice lying immediately behind the clitoris. The vagina is a muscular tube approximately 7 cm in length. The cervix opens into the anterior wall of the vagina superiorly bulging into the vaginal lumen. The vagina forms a ring around the cervix, and although this ring is continuous it is divided into anterior, posterior and lateral fornices.






MALE REPRODUCTIVE TRACT



Testis and epididymis


Each testis is ovoid, measuring 4 cm from upper to lower pole, 3 cm anterior to posterior and 2.5 cm from medial to lateral surface. The left testis lies at a lower level than the right within the scrotum. Each testis is contained by a white fibrous capsule, the tunica albuginea, and is covered by a double serous membrane into which it became invaginated in fetal life, the tunica vaginalis. Irregular septa arise from the tunica albuginea, dividing the testis into some 250 lobules, each lobule contains one to three tightly coiled tubules, seminiferous tubules, within which the sperm are produced.


The testes lie outside the body because spermatogenesis requires a temperature below that of the body, and if testes fail to descend properly this invariably leads to malfunction in spermatogenesis. At the hilum of the testis the seminiferous tubules drain into an irregular series of ducts called the rete testis from which efferent tubules arise, transporting the sperm into the head of the epididymis and subsequently down the epididymis through the vasa, joining with the seminal vesicles prior to forming the common ejaculatory ducts. The epididymis lies along the posterior border of the testis, somewhat to its lateral side. The epididymis is covered by the tunica vaginalis except at its posterior margin. The testis and epididymis each may bear at their upper extremities a small stalked body named respectively the appendix testis (also known as the hydatid of Morgagni) and the appendix epididymis. These may undergo torsion.







Prostate


The human prostate surrounds the prostatic urethra. There are two principal components to the prostate:the glandular component and a smooth muscle component. Approximately 25% of normal prostate is composed of smooth muscle, and with development ofbenign enlargement of the prostate, contrary to popular perception, although this is a glandular hyperplasia there is also a relative increase in the amount of smooth muscle such that smooth muscle in the hyperplastic prostate constitutes 40% of the gland. The majority of the prostate lies on the lateral and posterior aspects of the urethra, with little anterior prostatic tissue.






PHYSIOLOGY



FLUID AND ELECTROLYTE BALANCE


The main physiological function of the urinary tract is the maintenance of fluid, acid-base and electrolyte balance and the excretion of waste products. A subsidiary but extremely important role is that of the production of certain hormones.


Approximately one-fifth of the cardiac output passes through the kidneys each minute, resulting in a renal blood flow of up to 400 mL per 100 g of kidney per min (650 mL/min per kidney). The renal blood pressure remains extremely constant despite profound changes in systemic blood pressure, and the survival advantage of this mechanism is apparent on reflection. This phenomenon is described as autoregulation and is principally mediated via effects on preglomerular vascular resistance. Whilst the underlying mechanism is the subject of intensive study, it is thought to be related to intrinsic myogenic tone within blood vessels, independent of neural factors.


A total of 170–180 L of plasma per day are filtered through the glomeruli at an approximate rate of 125 mL/min. The glomerular membrane acts as a main filtration mechanism and is impermeable to molecules larger than 4 nm diameter, which relates to an average molecular weight of 70 000 Da. The ultrafiltrate of plasma then passes down to the tubules.






MAINTENANCE OF WATER BALANCE


The osmolality of urine varies between 50 mosm/L and 1200 mosm/L and depends upon the amount of water in the collecting tubule, which also is related to an appropriate corticomedullary osmotic gradient and the permeability of the collecting ducts under the control of ADH. Sodium and chloride are transported out of the ascending limb of the loop of Henle, and the sodium concentration falls progressively as the distal tubule is reached. The remainder of the loop of Henle is in osmotic equilibrium with the substance of the kidney. As the iso-osmolar filtrate reaches the bottom of the loop of Henle the contents of the descending limb become more concentrated as a result of being pushed towards the ascending limb. Further concentration occurs due to active sodium resorbtion in the ascending limb, resulting in an osmolar gradient in the renal medulla. Any increase in medullary blood flow results in dissipation of medullary osmolality, decreased water resorbtion and the production of large quantities of dilute urine. Dehydration results in release of ADH, increasing permeability in the distal nephron and results in increased water resorbtion. ADH is released from the posterior lobe of the pituitary gland. The endogenous control of ADH release is under the regulation of osmoreceptors adjacent to the supraoptic nucleus, which is under the influence of sodium and chloride concentration in the plasma. There are also volume receptors in the atria and great veins which seem to be under the control of the vagus nerve.




HORMONE PRODUCTION BY THE KIDNEY


A number of important hormones are produced with-in the kidney. The renin-angiotensin system is important, with renin being released from juxtaglomerular cells in response to sympathetic nerve stimulation via a decrease in afferent arteriolar pressure and hyponatraemia. Renin acts on circulating angiotensinogen to produce angiotensin I, which is converted by a circulating enzyme to angiotensin II. Angiotensin II stimulates the zona glomerulosa of the adrenal gland to produce aldosterone, which increases the sodium resorbtion by the kidneys and also produces vasoconstriction. These effects feed back in a negative fashion and switch off renin secretion and, therefore, maintain homeostasis. This is a gross oversimplification of an extremely complex system, but nevertheless it is evident that this homeostatic mechanism is essential to maintain a smooth blood pressure and compensate for changes in extracellular fluid volume and sodium excretion.


Other important hormones which are the subject of contemporary study include kallikrein (produced in the distal nephron) and other related agents. These substances are important vasodilators and also have been shown to have motor effects within the lower urinary tract and may be involved in sensorimotor mechanisms within the bladder.


The kidney is also involved in calcium metabolism and produces 1 α-hydroxylase in response to low circulating levels of calcium, which acts to convert 25-hydroxycholecalciferol into the active metabolite 1,25-dihydroxycholecalciferol, which then promotes calcium reabsorbtion and decreases urine excretion to maintain homeostasis.


Erythropoietin is produced by the kidney in response to hypoxia (either due to anaemia or respiratory causes), high circulating levels of the products of red cell destruction, and vasoconstriction. It is also produced in smaller amounts by the liver and spleen. Erythropoietin stimulates an increase in the number of nucleated red cells in the haemopoietic tissue, thereby raising red cell and reticulocyte counts in peripheral blood. Indeed synthesised erythropoietin is used in contemporary haematological practice, for these very purposes, especially in intractable anaemia associated with chronic renal failure.


The renal cortex and medulla synthesise a number of prostaglandins whose exact function remains obscure at present.

Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Genitourinary system
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