THE KIDNEYS (NEPHROS)

Functions 1. Removal of excess water, salts and products of protein metabolism 2. Maintenance of PH 3. Production and release of erythopoietin, which controls blood cell production 4. Synthesis and release of renin to influence blood pressure 5. Production of 1, 25-hydroxycholecalciferol (activated form of vitamin D) for control of calcium metabolism.

There are 2 kidneys in the body, one on either side of the median plane. The kidneys are bean-shaped about 10cm long, 5cm wide and weigh about 150g. The kidneys are intra-abdominal extending from T12-L3. The left kidney is about 1cm higher than the right one, owing to the large right lobe of the liver. The kidneys lay retroperitoneally on the posterior abdominal wall against Psoas major muscle. Each kidney is covered by a tough fibrous renal capsule. This is surrounded by fat known as perirenal /perinephric fat. The latter is enclosed in a renal fascia which attaches it firmly to the posterior abdominal wall. However, the renal fascia is flexible enough to allow kidneys shift slightly as the diaphragm moves during respiration.

The kidney has • Anterior and posterior surfaces • Medial and lateral borders • Superior and inferior poles

The lateral border is convex and lies against psoas major muscle. The medial border is concave. The hilus/hilum is a prominent medial indentation on this border. It’s a point of entry for the renal artery, renal nerves and exit for the renal vein and renal pelvis. From anterior to posterior are the; renal vein, renal artery and renal pelvis. The posterior surface of the superior pole is related to the diaphragm while the anteromedial surface to the suprarenal gland. The inferior poles are smaller and thinner than the superior poles and extend to the iliac crests.

Relations The left kidney is related to the left suprarenal gland, stomach, spleen, pancreas, small intestine, left colic flexure and splenic vessels. The left kidney along with the left suprarenal gland, spleen and pancreas lie in the stomach bed. The right kidney is related to the right suprarenal gland, right lobe of liver, duodenum, right colic flexure and small intestine. The position of the kidneys in the abdominal cavity is maintained by; 1. Overlying peritoneum, 2. Contact with adjacent visceral organs 3. Supporting connective tissue (renal capsule, adipose tissue, renal fascia)

Internal structure The renal capsule lines the renal sinus, an internal cavity within the kidney of about 2.5cm deep. The hilum leads into the renal sinus. The renal sinus is occupied by renal pelvis, renal calices, renal vessels and nerves. When a coronal section through the kidney is made, two distinct regions are seen with the naked eye; the outer renal cortex and inner renal medulla

The renal cortex is darker extending from the capsule to the base of the renal pyramids. There is some tissue of the cortex that penetrates into the renal medulla. This cortical tissue is located between the renal pyramids and is called renal colums.

The medulla is lighter and consists of 6-18 conical/triangular structures called renal pyramids. The base of each pyramid faces the cortex while the tip (renal papilla) projects into the renal sinus. Urine production begins in microscopic tubular structures called nephrons. Papillary ducts discharge urine into a cup-shaped drain called a minor calyx/calyx. 4-5 minor calices merge to form a major calyx. 2-3 major calices combine to form a renal pelvis that is connected to the ureter that empties into the urinary bladder.

Vessels The kidney receives large amounts of blood for filtration. Each kidney is supplied by a renal artery that arises from the abdominal Aorta. The renal artery divides in the renal sinus into 5 segmental arteries. These in turn divide into lobar, interlobar, arcuate and interlobular branches. The latter give rise to afferent arterioles which supply the glomeruli.

Venous drainage is by the same routes to the renal vein that empties into the inferior venacava

Nerves are derived from the renal plexus and consist of sympathetic and parasympathetic fibres

Lymph vessels drain into lumbar/lateral aortic lymph nodes The Ureters

• These are thick-walled, expandable muscular ducts with a narrow lumen. • There are 2 ureters that transport urine from the kidneys to the urinary bladder. Each ureter is continuous above with the funnel-shaped renal pelvis

• As urine passes along the ureters, peristaltic waves occur in their walls. • The ureter is about 25-30cm long in the adult. It traverses abdominal and pelvic cavities hence the two parts; Abdominal and pelvic part.

The Abdominal Ureter

• The abdominal part of the ureter is about 12cm long • It is retroperitoneal throughout its entire course and descends almost vertically.

• As the right ureter descends, it is closely related to the IVC, the lumbar lymph nodes and the sympathetic trunk. The left ureter is crossed by left colic vessels.

The Pelvic Ureter

As the ureters leave the abdomen and enter pelvis minor, they pass over the pelvic brim and course posteroinferiorly on the lateral wall of the pelvis. The ureters enter the posterosuperior angle of the urinary bladder.

• The ureters pass obliquely through the bladder wall. • This oblique course allows contraction of the bladder wall muscles to act as a valve of the ureter. This prevents regurgitation of urine and the chance of ascending infections.

The narrowest parts of the ureter are;

• Pelviureteric region, where it begins, • At the pelvic inlet/brim • Its entrance to the bladder • At these sites, there is the likelihood of impaction of ureteric stones.

Blood Supply of the Ureters

• External and Internal iliac arteries and veins • Branches of vesical arteries and veins in males • Branches of uterine arteries and veins in females

NB. Excessive distension of the ureter leads to severe pain, a condition known as renal colic. Distension usually results from obstruction of the ureter due to presence of kidneys stones (caliculi) in the lumen of the ureter. The Urinary Bladder (vesica) -This is a hollow, muscular organ that serves as a reservoir for urine and expels it via the urethra. -In adults, the empty bladder lies in the pelvis minor between pubic bones. It is related to the rectum posteriorly. -As the bladder fills, it ascends into the pelvis major and a very full bladder may ascend to the level of the umbilicus. In infants, the bladder is entirely in the abdomen, enters pelvis major at about 6 years and pelvis minor at puberty. -When filled it contains about 500 ml, micturition occurring at about 250 ml. -The bladder has strong muscular walls, and characterized by its distensibility. -Its shape, size, position, and relations are dependent with the amount of urine it contains and with the age of the person.

SHAPE The bladder is rounded when it contains urine. The shape of the empty bladder is like a tetrahedron. It has 4 sides, and 4 angles The sides include: a) The base/fundus. This is separated from the rectum by the cervix and vagina in females and by the deferent duct and seminal vesicles in males. b) The superior surface c) And 2 inferolateral surfaces The angles include: 1. The apex which points towards the pubic symphysis. The median umbilical ligament passes from the apex superiorly to the umbilicus 2. The neck, which is the most inferior part and is related to the prostate (males) or pelvic fascia (females); 3. And 2 lateral angles where the ureters enter the bladder.

The Ligaments of the Bladder In both sexes, there are ligaments that extend from the pubis to the neck of the bladder. These are the pubovesicle ligaments. The bladder apex is also connected to the umbilicus by the urachus/ median umbilical ligament. NB. The bladder is free within the loose extra peritoneal fat except the neck where it’s held firmly by ligaments.

The interior of the bladder The mucous membrane is loosely connected to its muscular wall, except in a triangular region in its base (fundus). This region is known as the trigone of the bladder.

The mucous membrane in the empty bladder is in numerous folds or rugae except in the trigone area where it is always smooth (due it its firm attachment to the muscular wall).

The trigone is bounded by the two ureteric orifices and the internal urethral orifice. It acts as a funnel that channels urine into the urethra when bladder contracts. The neck of the bladder is surrounded by the internal urethral sphincter (smooth muscle). Blood vessels of the urinary bladder The main arteries supplying the bladder are branches of the internal iliac arteries.

Veins drain into the vesical venous plexus that empties into the internal iliac vein. In the males, this plexus communicates with the prostatic venous plexus while in the female with the vaginal venous plexus.

Lymph drains into the iliac nodes

Nerve supply is both motor and sensory. Sensory innervation is responsible for the conscious sensation of a full bladder and desire to pass urine. They also carry pain sensation. Motor fibers are both parasympathetic and sympathetic.

The Urethra

The urethra is a muscular tube that extends from the neck of the urinary bladder to the external urethral orifice. The course of the urethra differs in the two sexes.

The Male Urethra In the male, the urethra is about 15-20 cm long and it extends from the neck of the bladder to the tip of the penis. The male urethra is a passage way for both urine and semen. The urethra passes through the prostate, the floor of the pelvis/urogenital diaphragm, the perineal membrane and the penis. Hence it is divided into 3 parts: the prostatic, membranous and spongy/penile parts.

The Prostatic Part This passes through the center of the prostate gland and is 3-4 cm long. Orifices of the prostatic ducts and of the ejaculatory ducts open into the prostatic urethra.

The Membranous Part This is the shortest part (2 cm) and the least dilatable part. It lies between the apex of the prostate gland and the bulb of the penis. It traverses the urogenital diaphragm and is surrounded by the sphincter urethrae muscle and the perineal membrane hence the name membranous. On either side of this urethra are the bulbourethral glands.

The Spongy/penile Part This is the longest segment of about 15-16cm. This part is contained within the corpus spongiosum of the penis. Its lumen is dilated in the bulb of the penis to form the intrabulbar fossa, and again within the glans penis as the navicular fossa. The ducts of the bulbourethral gland open into the proximal part of the spongy urethra. Mucous glands known as urethral glands open into this part of the urethra. The external urethral orifice is the narrowest part of the urethra

The Female Urethra The female urethra is a short muscular tube about 4 cm long. It runs from the neck of the bladder through the urogenital diaphragm and perineal membrane. The urethra lies anterior to the vagina and throughout its course; it’s intimately bound to the anterior wall of the vagina. The superior part has urethral glands and it s inferior end is surrounded by the sphincter urethrae muscle. The inferior half is in the perineum. The urethra opens at the external urethral orifice which is about 2.5 cm posterior to the clitoris and immediately anterior to the vaginal opening. On its either side are openings of the ducts of Para urethral glands.

NB. In both sexes, as the urethra passes through the urogenital diaphragm, a circular band of the sphincter urethrae muscle forms the external urethral sphincter (skeletal muscle). This acts as a valve and is under voluntary control.