Файл: Kaplan USMLE-1 (2013) - Anatomy.pdf

Clinical Correlate

The most common site for an abdominal aneurysm is in the area between the renal arteries and the bifurcation of the abdominal aorta. Signs include decreased circulation to the lower limbs and pain radiating down the back ofthe lower limbs. The most common site of atherosclerotic plaques is at the bifurcation of the abdominal aorta.

Clinical Correlate

•The splenic artery may be subject to erosion by a penetrating ulcer of the posteriorwall of the stomach into the lesser sac.

•The left gastric artery may be subject to erosion by a penetrating ulcer ofthe lesser curvature ofthe stomach.

•The gastroduodenal artery may be subject to erosion by a penetrating ulcer ofthe posterior wall ofthe first part of the duodenum.

Three Unpaired Visceral Arteries

CeliacArtery(Trunk)

The celiac artery (Figure IIl-3- 15) is the blood supply to the structures derived from the foregut. The artery arises from the anterior surface of the aorta just in­ ferior to the aortic hiatus at the level ofT12-Ll vertebra. The celiac artery passes above the superior border of the pancreas and then divides into 3 retroperitoneal branches.

The left gastric artery courses superiorly and upward to the left to reach the lesser curvature of the stomach. The artery enters the lesser omentum and follows the lesser curvature distally to the pylorus. The distribution of the left gastric includes the following:

•Esophageal branch to the distal one inch of the esophagus in the abdo­ men

•Most ofthe lesser curvature

The splenic artery is the longest branch of the celiac trunk and runs a very tortu­ ous course along the superior border of the pancreas. The artery is retroperitone­ al until it reaches the tail of the pancreas, where it enters the splenorenal ligament to enter the hilum of the spleen. The distributions of the splenic artery include:

•Direct branches to the spleen

•Direct branches to the neck, body, and tail ofpancreas

•Left gastroepiploic artery that supplies the left side of the greater cur­ vature of the stomach

•Short gastric branches that supply to the fundus of the stomach

The common hepatic artery passes to the right to reach the superior surface of the first part of the duodenwn, where it divides into its 2 terminal branches:

•Properhepatic artery ascends within the hepatoduodenal ligament of the lesser omentum to reach the porta hepatis, where it divides into the right and left hepatic arteries. The right and left arteries enter the

2 lobes of the liver, with the right hepatic artery first giving rise to the cystic artery to the gallbladder.

•Gastroduodenal artery descends posterior to the first part of the duo­ denum and divides into the right gastroepiploic artery (supplies the pyloric end of the greater curvature of the stomach) and the superior pancreaticoduodenal arteries (supplies the head of the pancreas, where it anastomoses the inferior pancreaticoduodenal branches of the supe­ rior mesenteric artery) .

Section Ill • Gross Anatomy

Clinical Correlate

Blockage by Renal Calculi

The most common sites of ureteral constriction susceptible to blockage by renal calculi are:

•Where the renal pelvis joins the ureter

•Where the ureter crosses the pelvic inlet

•Where the ureter enters the wall of the urinary bladder

Congenital Abnormalities ofthe Renal System

•Renal agenesis

Renal agenesis results from failure of one or both kidneys to develop because of early degeneration of the ureteric bud. Unilateral genesis is fairly common; bilateral agenesis is fatal (associated with oligohydram­ nios, and the fetus may have Pottersequence: clubbed feet, pulmonary hypoplasia, and craniofacial anomalies) .

•Pelvic and horseshoekidney

Pelvic kidney is caused by a failure of one kidney to ascend. Horseshoe kidney (usually normalrenalfunction, predisposition to calculi) is a fusion of both kidneys at their ends and failure of the fused kidney to ascend. The horseshoe kidney hooks under the origin of the inferior mesenteric artery.

•Double ureter

Caused by the early splitting of the ureteric bud or the development of 2 separate buds

•Patent urachus

Failure ofthe allantois to be obliterated results in urachal fistulas or sinuses. In male children with congenital valvular obstruction of the pros­ tatic urethra or in older men with enlarged prostates, a patent urachus maycause drainage ofurine through the umbilicus.

Posterior Abdominal Wall and Pelvic Viscera

Kidneys

The kidneys are a pair ofbean-shaped organs approximately 1 2 cm long. They ex­ tend from vertebral level Tl2 to L3 when the body is in the erect position (Figure III-3-24A and -24B). The right kidney is positioned slightly lower than the left because of the mass of the liver.

•Kidney's Relation to the PosteriorAbdominalWall

Both kidneys are in contact with the diaphragm, psoas major, and qua­ dratus lumborum (Figure III-3-24) .

-Right kidney-contacts the above structures and the 1 2th rib

-Left kidney-contacts the above structures and the 1 1th and 12th ribs

Ureters

Ureters are fibromuscular tubes that connect the kidneys to the urinary bladder in the pelvis. They run posterior to the ductus deferens in males and posterior to the uterine artery in females. They begin as continuations of the renal pelves and run retroperitoneally, crossing the external iliac arteries as they pass over the pelvic brim.

Ureter's Relation to the PosteriorAbdominalWall

The ureter lies on the anterior surface of the psoas major muscle.

Section Ill • Gross Anatomy

Clinical Correlate

Spastic bladder results from lesions of the spinal cord above the sacral spinal cord levels. There is a loss of inhibition ofthe parasympathetic nerve fibers that innervate the detrusor muscle during the filling stage. Thus, the detrusor muscle responds to a minimum amount of stretch, causing urge incontinence.

Clinical Correlate

Atonic bladder results from lesions to the sacral spinal cord segments or the sacral spinal nerve roots. Loss of pelvic splanchnic motor innervation

with loss of contraction of the detrusor muscle results in a full bladder with a continuous dribble of urine from the bladder.

Clinical Correlate

Weakness of the puborectalis part of the levator ani muscle may result in rectal incontinence.

Weakness of the sphincter urethrae part of the urogenital diaphragm may result in urinary incontinence.

268 MEDICAL

• Muscles

The detrusor muscle forms most of the smooth-muscle walls of the bladder and contracts during emptying of the bladder (micturition). The contraction of these muscles is under control of the parasympa­ thetic fibers of the pelvic splanchnics (52, 3, 4)

The internal urethral sphincter (sphincter vesicae) is smooth-muscle fibers that enclose the origin of the urethra at the neck ofthe bladder. These muscles are under control of the sympathetic fibers of the lower thoracic and lumbar splanchnics (Tl l -L2) and are activated during the filling phase of the bladder to prevent urinary leakage.

-The external urethralsphincter (sphincter urethrae) is the voluntary skeletal muscle component of the urogenital diaphragm that encloses the urethra and is relaxed during micturition (voluntary muscle

of micturition). The external sphincter is innervated by perineal branches of the pudendal nerve.

Urethra

The male urethra is a muscular tube approximately 20 cm in length. The urethra in men extends from the neck of the bladder through the prostate gland (prostat­ ic urethra) to the urogenital diaphragm ofthe perineum (membranous urethra), and then to the external opening of the glans (penile or spongy urethra) (Figure

III-3-26).

The male urethra is anatomically divided into 3 portions: prostatic, membra­ nous, and spongy (penile).

•The distal spongyurethra of the male is derived from the ectodermal cells of the glans penis.

The female urethra is approximately 4 cm in length and extends from the neck of the bladder to the external urethral orifice ofthe vulva (Figure III-3-27).

PELVIS

Pelvic and Urogenital Diaphragms

The pelvic andurogenital diaphragms (Figure III-3-25) are 2 important skeletal muscle diaphragms that provide support of the pelvic and perinea! structures. They are each innervated by branches of the pudenda! nerve.

•The pelvic diaphragm forms the muscular floor of the pelvis and separates the pelvic cavity from the perineum. The pelvic diaphragm is a strong support for the pelvic organs and transmits the distal parts of the genitourinary system and GI tract from the pelvis to the perineum.

-The diaphragm is formed by 2 layers of fascia and the 2 muscles: the levator ani and coccygeus.

-The puborectalis component of the levator ani muscle forms a muscu­

lar sling around the anorectal junction, marks the boundary between the rectum and anal canal and is important in fecal continence.