(17) Abdomen V: Posterior Abdominal Wall & Urinary System Practice Flashcards

POSTERIOR ABDOMINAL WALL COMPONENTS AND THE RESPIRATORY DIAPHRAGM

The posterior abdominal wall is a complex anatomical region that serves as the boundary between the abdominal cavity and the posterior structures of the trunk. It is fundamentally composed of the lumbar vertebral column, specific posterior wall musculature, the respiratory diaphragm, and the neural network known as the lumbar plexus.

The Thoracic (Respiratory) Diaphragm

The thoracic diaphragm is the primary muscle of respiration and acts as a massive musculofibrous partition separating the thoracic and abdominal cavities. Its structural configuration includes:

• Muscle Composition: It consists of peripheral skeletal muscle which attaches anteriorly to the sternum and the costal arches.

• The Central Tendon: The muscle fibers converge into a central fibrous region known as the central tendon.

• The Crura: Posteriorly, two muscular arches called crura extend along the lateral aspects of the lumbar vertebrae. These structures contribute to the formation of specific openings.

Major Diaphragmatic Apertures (Hiatuses)

There are three primary hiatuses through which vital structures pass between the thorax and abdomen, situated at specific vertebral levels:

1. Aortic Hiatus ($T12$): This posterior opening allows for the passage of the aorta, the thoracic duct, and the azygos vein.

2. Esophageal Hiatus ($T10$): The esophagus traverses the diaphragm at this level, accompanied by the vagus nerves ($CN\,X$).

3. Caval Opening ($T8$): This aperture is located within the central tendon itself. It facilitates the passage of the inferior vena cava (IVC) and the phrenic nerve.

Diaphragmatic Innervation and Clinical Conditions

• Innervation: The diaphragm is innervated by the phrenic nerve, which originates from the spinal cord levels $C3$, $C4$, and $C5$.

• Clinical Conditions: Pathologies associated with the diaphragm include diaphragmatic hernia (protrusion of abdominal contents into the thorax) and hiatal hernia (protrusion of the stomach through the esophageal hiatus).

PRIMARY MUSCULATURE OF THE POSTERIOR ABDOMINAL WALL

Three major muscles define the posterior wall, serving roles in trunk movement, stabilization, and rib fixation.

• Psoas Major: This muscle arises from the transverse processes and the vertebral bodies of levels $T12$ through $L5$. It descends to insert onto the lesser trochanter of the femur. It is innervated by the lumbar plexus, specifically the ventral rami of the $L1$, $L2$, and usually $L3$ spinal nerves.

• Quadratus Lumborum: This muscle arises from the iliac crest and inserts onto the $12^{th}$ rib as well as the transverse processes of the upper four lumbar vertebrae ($L1-L4$). It is innervated by the $T12$ to $L4$ spinal nerves. Its primary functions are to extend and laterally flex the vertebral column and to fix the $12^{th}$ rib during inspiration.

• Iliacus: Arising from the iliac fossa, this muscle attaches to the lesser trochanter of the femur. It is innervated by the femoral nerve.

The Iliopsoas Muscle: The psoas major and the iliacus muscles are collectively referred to as the iliopsoas. This muscle group serves as the chief flexor of the thigh.

THE LUMBAR PLEXUS: DISTRIBUTION AND INNERVATION

The lumbar plexus is a network of nerves formed within the substance of the psoas major muscle by the ventral rami of spinal nerves $L1$ to $L4$. It provides motor and sensory innervation to the abdominal wall, parts of the pelvis, and the perineum.

1. Iliohypogastric Nerve ($L1$): Provides motor supply to the internal oblique and transversus abdominis muscles and sensory innervation to the pubic region.

2. Ilioinguinal Nerve ($L1$): Provides motor supply to the abdominal wall and sensory innervation to the scrotum or labia and the base of the penis or clitoris. Its terminal branch passes through the superficial inguinal ring.

3. Genitofemoral Nerve ($L1-2$): Formed within the psoas major. It divides into a genital branch (motor to the cremaster muscle) and a femoral branch (sensory to the skin of the medial thigh).

4. Lateral Femoral Cutaneous Nerve ($L2-3$): Travels along the lateral aspect of the iliacus muscle and passes beneath the inguinal ligament to provide sensation to the lateral thigh.

5. Femoral Nerve ($L2-4$): This is the major nerve supplying the muscles of the anterior thigh. It passes beneath the inguinal ligament just lateral to the psoas major muscle.

6. Obturator Nerve ($L2-4$): This is the major nerve for the adductor muscles in the medial thigh. It passes medial to the psoas major and exits the pelvis through the obturator foramen.

ANATOMY AND MORPHOLOGY OF THE KIDNEYS

Location and External Features

The kidneys are retroperitoneal organs, meaning they are located posterior to the peritoneum. They are encased in a mass of perirenal fat and are generally situated between the $L1$ and $L4$ vertebral levels. Due to the presence of the liver, the right kidney is often positioned lower than the left.

The Renal Hilum, Sinus, and Pelvis

• Hilum: The hilum is the concave medial surface of the kidney. It serves as the entry and exit point for the renal artery, renal vein, and the ureter.

• Renal Pelvis: As the ureter approaches the hilum, it dilates to form the renal pelvis. The renal pelvis and the renal vessels sit within a space called the renal sinus.

• Calyces: Within the hilum, the renal pelvis divides into several major calyces. These further divide into $7-14$ minor calyces. The minor calyces receive urine from the largest collecting ducts. The pelvis and calyces constitute the conducting system for urine.

INTERNAL KIDNEY STRUCTURE, PHYSIOLOGY, AND CLINICAL ASPECTS

Internal Anatomy (Coronal Perspective)

A coronal section reveals two distinct functional regions:

• Cortex: The outer layer, approximately $1-2\,cm$ thick, which parallels the convex surface.

• Medulla: Located between the cortex and the hilum, the medulla is divided into lobes consisting of pyramids and columns. The renal columns are histologically similar to the cortex.

• Renal Papilla: This is the apex of the renal pyramid. It protrudes into a minor calyx and is the exact site where urine is secreted to begin its transit to the bladder.

Vasculature and Function

• Vessels: The renal artery and renal vein are positioned anterior to the renal pelvis.

• Primary Functions:     - Maintenance of the ionic balance of the blood.     - Excretion of metabolic waste products as urine.     - Regulation of blood pressure.

• Renal Transplantation: This is a standard procedure for chronic renal failure. The typical transplant site for a donor kidney is the lower abdomen.

THE URETERS: ANATOMY, CLINICAL PATHOLOGY, AND VASCULAR SUPPLY

Course and Structure

The ureters are retroperitoneal tubes that conduct urine from the renal pelvis to the urinary bladder. They descend anterior to the psoas major muscle, cross anterior to the external iliac vessels to enter the pelvis, and reach the posterosuperior aspect of the bladder. They penetrate the bladder wall obliquely, which creates a functional valve that prevents the reflux of urine.

Blood Supply and Innervation

• Arterial Supply: The ureters receive blood from the renal artery (superiorly), the gonadal artery (during descent), and the superior vesical artery (near the bladder).

• Innervation: Autonomic nerves originate from the renal plexus.

Clinical Conditions of the Ureters

• Renal Stones (Calculi): Stones passing from the kidney can obstruct the ureter, causing flank pain that radiates to the groin (ureteric colic) and often resulting in hematuria (blood in the urine).

• Obstruction and Hydronephrosis: Obstruction can lead to hydronephrosis (dilation of the kidney). There are three common sites of obstruction:     1. The ureteropelvic junction (UPJ).     2. Where the ureter crosses the pelvic inlet.     3. The intramural portion (within the bladder wall).

THE SUPRARENAL (ADRENAL) GLANDS: MORPHOLOGY AND ENDOCRINE FUNCTION

Position and Structure

The suprarenal glands are located at the superior pole of each kidney, enclosed within the renal fascia. Each gland consists of an outer capsule and two distinct functional layers:

1. Cortex: This layer secretes essential steroid hormones, including mineralocorticoids (e.g., aldosterone), glucocorticoids (e.g., cortisol), and sex hormones (e.g., progesterone, estradiol, and androgens).

2. Medulla: This layer secretes catecholamines, specifically epinephrine and norepinephrine.

Vasculature and Innervation

• Blood Supply: The glands receive blood from the suprarenal arteries, which branch directly from the aorta.

• Innervation: Medullary cells are developmentally homologous with sympathetic ganglion cells. Consequently, they are innervated by preganglionic sympathetic fibers derived from splanchnic nerves. These fibers regulate the secretion of epinephrine.

Note: The anatomy of the urinary bladder is covered in the separate lecture regarding the pelvis.