The kidney is a retroperitoneal structure lies lateral to the vertebral column at the level of T12-L3. The inferior pole of the right kidney contacts the right colic flexure while the medial aspect of the right kidney lies posterior to the descending portion of the duodenum. Approximately 3/4 of the anterior surface contacts the visceral surface of the liver. The right kidney lies mostly inferior to the 12th rib with only the superior pole contacting it. The superior pole of the left kidney contacts the spleen and stomach while its medial surface contacts the tail of the pancreas and its inferior pole contacts the left colic flexure. The mid-portion of the left kidney contacts the 12th rib; therefore, the left kidney lies more cranial in the retroperitoneal space compared to the right kidney.
KIDNEY, URETER, AND RENAL VESSELS. The left kidney receives the renal artery and vein which enter the substance of the kidney through its hilum. The renal artery is a tributary of the abdominal aorta while the renal vein drains into the inferior vena cava. These structures enter the hilus of the kidney. The renal pelvis lies posterior to these renal vessels and the ureter is the inferior continuation of the renal pelvis. The ureter runs lateral to the aorta on the left side and lateral to the IVC on the right. On hemisection, numerous structures can be seen in the kidney. A fibrous capsule envelopes the kidney. A renal cortex forms the outer 1/3 of the kidney which will provide as renal columns into the inner kidney core or renal medulla. Renal pyramids consist of collecting ducts and lie between the renal columns. Filtrate from renal pyramids travels through a renal papilla and into a minor calyx. Approximately 3-4 minor calyces empty into a major calyx. The waste eventually exits the kidney through the renal pelvis and ureter.
SUPRARENAL (ADRENAL) GLANDS. These glands lie along the superior pole of the kidney and are triangular in shape. This gland is richly supplied by numerous arteries. Most notably, arterial branches arise from the inferior phrenic, aorta and renal arteries. Venous drainage is through the renal vein or directly into the inferior vena cava. The adrenals consist of two separate portions which have very different functions. The outer cortex is an endocrine gland which secretes aldosterone, involved in regulating electrolyte balance, glucocorticoids such as cortisol and cortisone which are involved in carbohydrate metabolism, and sex hormones. The cortex receives no autonomic innervation, but rather it is influenced by circulating hormones (e.g., ACTH). The inner medulla contains modified nerve cells, which secrete hormones epinephrine and norepinephrine) following stimulation by the sympathetic nervous system.
ABDOMINAL AORTA AND INFERIOR VENA CAVA. The inferior vena cava lies along the right aspect of the posterior abdominal wall, receives the renal veins and right gonadal vein (testicular or ovarian; the left gonadal vein drains into the left renal vein). The inferior vena cava originates at the junction of the common iliac veins (about L4), runs through the right vertical fissure of the liver, and eventually pierces the diaphragm to enter the right atrium. The abdominal aorta lies to the left of the IVC ad provides three unpaired arterial pedicles (celiac, superior mesenteric and inferior mesenteric) as well as bilateral branches such as the lumbar arteries, gonadal arteries and renal arteries.
MUSCLES OF THE POSTERIOR ABDOMINAL WALL. The psoas major muscle arises from the lumbar vertebrae and joins with the iliacus muscle, which occupies the iliac fossa. Both muscles form one functional unit called the iliopsoas muscle which attaches to the lesser trochanter of the femur and functions as the most powerful flexor of the thigh. Quadratus lumborum is a thick rhomboidal shaped muscle running from the iliac crest to the 12th rib. It flexes the vertebral column. Technically speaking, transversus abdominis also contributes to the muscles of the posterior abdominal wall and its fibers run posterior to the oblique borders of the quadratus lumborum.
LUMBAR NERVE PLEXUS. The ventral rami of T12-L5 form the lumbar plexus. Variations exist, but generally eight major nerves arise from this plexus. Progressing form most superior to most inferior the following nerves form the lumbar plexus. The subcostal nerve (T12) runs into the intercostal groove of the 12th rib. Iliohypogastric (L1) and ilioinguinal (L1) are large segmental nerves that provide cutaneous innervation to the lower abdomen. The genitofemoral nerve (L2) runs along the anterior surface of the psoas and supplies the cremasteric muscle as well as cutaneous cremasteric reflex. The lateral femoral cutaneous nerve (L2, 3) passes deep to the inguinal ligament and provides sensory innervation to the lateral surface of the thigh. The femoral nerve (L2, 3, 4) is a large nerve lying between the psoas and iliacus. It provides motor and sensory contributions to the anterior and medial thigh. The obturator nerve (L3, 4) runs along the medial border of the psoas, descends through the obturator canal and supplies the adductor muscles of the thigh. The lumbosacral trunk is a large never comprising part of L4 and all of L5. This nerve closely adheres to the sacrum and contributes to the sacral plexus.
SYMPATHETIC TRUNK. The sympathetic lumbar chain represent the continuation of the sympathetic thoracic chain into the abdomen. Since preganglionic sympathetic cell bodies only occur between T1-L2, preganglionic (white) rami communicantes do not exist below L2 and you should only see one rami communicantes (postganglionic or grey) which convey postganglionic sympathetic fibers. The bilateral sympathetic chain ganglia descend along the sacrum and eventually fuse at the base of the coccyx forming a small ganglion called the tuberculum impar.
DIAPHRAGM. The diaphragm forms the border between the thorax and abdomen. The diaphragm can be differentiated into various components based on its embryology including the sternal part, costal part, central tendon and vertebral part (includes the right crus and left crus). Large openings occur in the diaphragm including the vena caval foramen, esophageal hiatus and the aortic hiatus. Recall that the phrenic nerve provides motor innervation to the diaphragm, but also transmits sensory fibers from the peritoneum covering the inferior surface of the diaphragm.
Quiet respiration requires only the diaphragm (diaphragmatic breathing). During inspiration, the diaphragm contracts (lowers) to expand the chest cavity. This causes the thorax to increase in size, so the thoracic pressure drops. Air flows in to equalize the pressure, which expands the lungs. During expiration, the diaphragm relaxes, moving up to its resting position. Thoracic volume is reduced, so thoracic pressure increases, forcing air out
Exertion requires that the ribs be moved during breathing, as well as the diaphragm. This is called costal breathing. The abdominal muscles are involved in forced expiration, as well as some neck muscles (scalenes). In addition, the intercostal muscles assist in respiration by keeping the ribs together as the rib cage is elevated or lowered. The intercostals are innervated by thoracic spinal nerves.
PREVERTEBRAL GANGLIA AND THE SPLANCHNIC NERVES. As stated in the Thorax Laboratory Session, some preganglionic sympathetic fibers pass through the sympathetic chain ganglia without synapsing. These are the splanchnic nerves, of which there are three including the greater, lesser and least splanchnic nerves. The splanchnic nerves must synapse on their postganglionic cell bodies whose axons enter the target organ. The bilateral prevertebral ganglia, including the celiac ganglia, superior mesenteric ganglia and the aorticorenal ganglia are the aggregations of these postganglionic cell bodies. The celiac ganglia lies on either side of the celiac trunk, the superior mesenteric ganglic form along either side of the superior mesenteric artery, but may coalesce anteriorly to give the appearance of an unpaired ganglion. The aorticorenal ganglia are small aggregations of postganglionic nerve cell bodies lying along the superior and anterior surface of the renal arteries as they branch from the aorta. Postganglionic fibers branch from the prevertebral ganglia and innervate the abdominal viscera.
The digestive organs also receive parasympathetic innervation. Those associated with the foregut and midgut receive parasympathetic innervation from the vagus nerve. The vagus also contains some general sensory fibers which convey diffuse sensations such as nausea from the abdomen. Pelvic splanchnics (see next Lab Session) provide parasympathetic innervation for the hindgut (distal 1/3 of transverse colon to rectum). Pain fibers from the gut return to the CNS with these pelvic splanchnic nerves. Remember, the spinal nerves do not supply the abdominal organs! This is a good time to review the autonomic nervous system.