CLINICAL ANATOMY OF ANTERIOR ABDOMINAL WALL & RECTUS SHEATH

Abdomen

Structure of Abdominal Cavity

• Superiorly it is formed by diaphragm which separates the abdominal cavity from the thoracic cavity
• Inferiorly the abdominal cavity is continuous with the pelvic cavity through the pelvic inlet

Structure of Abdominal Wall
   Anteriorly:

• The abdominal wall is formed above by lower part of the thoracic cage
• Below by the rectus abdominis, external oblique, internal oblique, and transversus abdominis muscles and fasciae

Structure of Ant. Abdominal Wall

• It is made up of skin, superficial fascia, deep fascia, muscles, extraperitoneal fascia and parietal peritoneum
• The abdominal walls are lined by a fascial envelope and the parietal peritoneum

Skin

• Natural lines of cleavage in the skin are constant and run almost horizontally around the trunk
• An incision along a cleavage line will heal as a narrow scar, while one that crosses the lines will heal as a wide scar

Cutaneous Nerve Supply

• Is derived from the anterior rami of the lower six thoracic and first lumbar nerves
• Thoracic nerves are the lower five intercostal and the subcostal nerves
• First lumbar nerve is represented by the iliohypogastric and ilioinguinal nerves 

Blood Supply

• Skin near the midline is supplied by branches of the superior epigastric artery (br. of int. thoracic artery) and the inferior epigastric artery ( br. of external iliac artery)
• Skin of the flanks is supplied by branches from the intercostal, lumbar, and deep circumflex arteries

Superficial Fascia

• Fatty layer or fascia of camper is continuous with the superficial fat over the rest of the body and may be extremely thick in obese patients
• The membranous layer or scarpa’s fascia is thin and fades out laterally and above
• Becomes continuous with the superficial fascia of the back and the thorax

Superficial Fascia

• Inferiorly the membranous layer passes onto the front of the thigh, where it fuses with the deep fascia
• In the midline inferiorly forms a tubular sheath for the penis or clitoris
• Below in the perineum, enters the wall of the scrotum or labia majora
• From there it passes to be attached on each side to the margins of pubic arch, here it is called Colle’s fascia

Superficial Fascia

• Posteriorly it fuses with the perineal body and the margin of the perineal membrane
• The fatty layer is represented as a smooth muscle in the scrotum, the dartos muscle
• The membranous layer persists as a separate layer

Deep Fascia

• Deep fascia in the anterior abdominal wall is merely a thin layer of connective tissue covering the muscles
• It lies immediately deep to the membranous layer of the superficial fascia

Muscles

• Consists of Three broad thin sheets that are aponeurotic in front
• From exterior to interior they are:
• External oblique, internal oblique, and transverse
• A wide vertical muscle, the rectus abdominis
• They lie on either side of the midline anteriorly
• As the aponeurosis of three sheets pass forward, they enclose the rectus abdominis to form the rectus sheath
• The cremaster muscle which is derived from the lower fibers of internal oblique, passes inferiorly as a covering of the spermatic cord and enters scrotum

External Oblique Muscle

• Is a broad, thin, muscular sheet
• Origin: Lower 8 ribs
• Insertion: Xiphoid process, linea alba, pubic tubercle, iliac crest
• Nerve Supply: Lower 6 thoracic nerves, iliohypogastric & ilioinguinal nerves
• Action: Supports abdominal contents, assist in forced expiration, micturition, defecation, parturition, vomiting 
• A triangular shaped defect in the external oblique aponeurosis lies immediately above and medial to the pubic tubercle, known as superficial inguinal ring
• Between the anterosuperior iliac spine and the pubic tubercle, the lower border of the aponeurosis is folded backward on itself, forming the inguinal ligament

Internal Oblique Muscle

• Origin: Lumbar fascia, iliac crest, lateral two-thirds of inguinal ligament
• Insertion: Lower three ribs and costal cartilages, xiphoid process, linea alba, symphysis pubis
• Nerve Supply: Lower six thoracic nerves, iliohypogastric & ilioinguinal nerves
• Action: Supports abdominal contents, assist in forced expiration, micturition, defecation, parturition, vomiting 

Transversus Abdominis

• Origin: Lower six costal cartilages, lumbar fascia, iliac crest, lateral third of inguinal ligament
• Insertion: Xiphoid process, linea alba, symphysis pubis
• Nerve Supply: Lower six thoracic nerves, iliohypogastric & ilioinguinal nerves
• Action: Compresses abdominal contents

Rectus Abdominis

• Origin: Symphysis pubis and pubic crest
• Insertion: 5th, 6th and 7th costal cartilages and xiphoid process
• Nerve Supply: Lower six thoracic nerves
• Action: Compresses abdominal contents, flexes vertebral column, accessory muscle of expiration

Lymph Drainage

• Lymph drainage of the skin of the anterior abdominal wall above the umbilicus is upward to the anterior axillary (pectoral group of nodes)
• Below the level of umbilicus drains downward and laterally to the superficial inguinal nodes
• Swelling in the groin is may be due to enlarged superficial inguinal node

Venous Drainage

• Venous blood is collected into a network of veins that radiate from the umbilicus
• The network is drained above into the axillary vein via the lateral thoracic vein
• Below into the femoral vein via the superficial epigastric and the great saphenous veins
• Few small veins, the paraumbilical veins form a clinically important portal-system venous anastomosis

Caput Medusae

• The superficial veins around the umbilicus and the paraumbilical veins connecting them to the portal vein may become grossly distended in case of portal vein obstruction
• The distended subcutaneous veins radiate out from the umbilicus, producing in severe cases the clinical picture called Caput Medusae 

Nerves

• Nerves of the anterior abdominal wall supply the skin, muscles and the parietal peritoneum
• They are derived from the anterior rami of lower six thoracic and the first lumbar nerves
• Inflammation of parietal peritoneum causes pain in the overlying skin and also a reflex increase in tone of the abdominal musculature in the same area 

Rectus Sheath

• Is a long fibrous sheath
• Encloses the rectus abdominis and pyramidalis muscle (if present)
• Contains the anterior rami of lower six thoracic nerves and the superior and inferior epigastric vessels and lymph vessels
• Formed mainly by aponeurosis of three lateral abdominal muscles
•    For description it is considered at three levels:
• Above the costal margin the anterior wall is formed by the aponeurosis of the external oblique and posterior wall is formed by the thoracic wall
• That is the 5th , 6th and 7th costal cartilages and the intercostal spaces
• Between the costal margin and the level of the anterosuperior iliac spine, the aponeurosis of the internal oblique splits to enclose the rectus muscle
• The external oblique aponeurosis is directed in front of the muscle
• Transversus aponeurosis is directed behind the muscle

• Between the level of the anterosuperior iliac spine and the pubis, the aponeurosis of all three muscles form the anterior wall
• The posterior wall is absent
• The rectus muscle lies in contact with the fascia transversalis
• The posterior wall of the rectus sheath is not attached to the rectus abdominis muscle
• The anterior wall is firmly attached to it by the muscle’s tendinous intersections 

Linea Alba

• The rectus sheath is separated from its fellow on the opposite side by a fibrous band called the linea alba
• Extends from the xiphoid process to the symphysis pubis. 

Lungs And Pleura

Pleura 

• It is the outer covering of the lungs.
• It develops from the primitive coelom.
• It has two layers:
• Parietal layer
• Visceral layer
• The parietal layer attached to the chest wall and it is pain sensitive.
• The visceral layer is attached to the lungs, and it is pain insensitive.

• There is a potential space between the pleura, that is known as the pleural space/ cavity.
• This space contains nearly a few ml of fluid, called pleural fluid.
• This fluid helps in the lubrication between the pleura, during the respiration.
• Collection of excessive pleural fluid is called as the pleural effusion.
• The pressure is negative inside the pleural cavity.
• There are two most dependent parts of the pleura, they are called pleural recess,
• Costodiaphragmatic recess
• Costomediastinal recess.

• The parietal pleura is not attached with the lungs, but there is space between the lungs and the pleura.
• Pleural reflections:
• Pleura are attached to the chest wall and are reflected to them.

Nerve supply: 

• Intercoastal nerves
• Phrenic nerves 

Trachea

• 10-12 cm long
• 1.5-2.5 cm wide
• Extends to second rib anteriorly and T4-T5 posteriorly
• 15-20 C shaped rings, and the posterior surface has a muscle called as trachealis.
• The first generation is zero (0), the trachea itself, then it divides upto 23 generations  to become the alveoli.
• Bifurcates at the carina.

Main Stem Bronchi

Right bronchus

• Wider
• More vertical
• 5 cm shorter
• Supported by C shaped cartilages
• 20-30 degree angle
• First generation

Left bronchus

• Narrower
• More angular
• Longer
• Supported by C shaped cartilages
• 40-60 degree angle
• First generation

Lobar Bronchi

R main stem divides into:

• Upper lobar bronchus
• Middle lobar bronchus
• Lower lobar bronchus

L main stem divides into:

• Upper lobar bronchus
• Lower lobar bronchus

Segmental Bronchi,3rd generation

R lobar divides into

• Segmental bronchi
• 10 segments on right

L lobar divides into

• Segmental bronchi
• 8 segments on left

Subsegmental Bronchi

• 4th to 9th generations
• Progressively smaller airways
• 1-4 mm diameter
• At 1 mm diameter connective tissue sheath disappears

Noncartilagenous Airways

Bronchioles

• 10th to 15th generation
• Cartilage is absent
• Lamina propria is directly connected with lung parenchyma
• Surrounded by spiral muscle fibers
• Epithelial cells are cuboidal
• Less goblet cells and cilia
• With no cartilage, airway remains open due to pressure gradients

Terminal Bronchioles

• 16th to 19th generation
• Average diameter is 0.5 mm
• Cilia and mucous glands begin to disappear totally
• End of the conducting airway
• Canals of Lambert-interconnect this generation, provide collateral ventilation

Gas exchange zone

• Respiratory bronchioles
• Acinus (aka primary acinus; aka primary lobule)—respiratory bronchioles to the alveoli
• Ducts, sacs, alveolar
• Squamous epithelium

Lungs 

• Human lungs are located in two cavities on either side of the heart.
• Each lung is divided into the different lobes with the help of the fissures:
• Right- three lobes, upper, middle and lower lobes
• Left- two lobes, upper and lower lobes.

The right lung has two fissures:

• Oblique fissure
• Transverse fissure

The left lung has only one fissure.

• Oblique fissure.

• Paired, cone-shaped organs in thoracic cavity
• Separated by heart and other mediastinal structures
• Covered by pleura 
• Extend from diaphragm inferiorly to just above clavicles superiorly
• Lies against thoracic cage (pleura, muscles, ribs) anteriorly, laterally and posteriorly

Parts-

• Apex
• Base
• Costal surface
• Mediastinal surface and hilum.

Each lung is further divided into the small independent respiratory units called as the bronchopulmonary segments.

There are 10 segments in the right lung and 8 in the left one.
Right:

• Upper lobe- apical, anterior, posterior
• Middle lobe- medial and lateral
• Lower lobe-  superior, anterior basal, posterior basal, lateral basal and medial basal

Left lung

• Superior lobe- apical, anterior, posterior
• Lower lobe-
• Superior and inferior lingular
• Anteromedial basal, lateral basal and posterior basal.

They are the segment of the lung, that are supplied by the segmental bronchi, and a single branch of the pulmonary artery.

Hilum 

• It is on the medial surface of the lung.
• It is also called as the root of the lung. 

It has the following structures:

• Pulmonary artery
• Pulmonary veins
• Bronchus
• Lymph nodes
• Bronchial vessels
• Pleural folds

Blood supply
Dual Supply

• Bronchial Supply: arises from superior thoracic aorta or the aortic arch. 2 % supply.
• Supply bronchi, airway airway walls and pleura
• Pulmonary Supply (98% supply)
• Pulmonary arteries enter at hila and branch with airways
• Venous drainage accompany the arteries.

Lymphatic drainage follows vessels

• Parabronchial (peribronchial) lymphatics and nodes  
• hilar nodes  (bronchopulmonary nodes)
• bronchomediastinal nodes   
• Thoracic and right lymphatic duct and to the venous system.
• Occasionally some lymphatics may reach up to pre- and para-tracheal nodes  and  supraclavicular nodes

Functional Units of Gas Exchange

• Three generations of respiratory bronchioles
• Three generations of alveolar ducts
• 15-20 clusters--sacs

Gas exchange terminology

All of the structures arising from a single terminal bronchiole are called

• Primary lobule
• Acinus
• Terminal respiratory unit
• Lung parenchyma
• Functional units

Acinus/Primary lobule

• Respiratory bronchioles with some alveoli arising from their walls
• Alveolar ducts arise from respiratory bronchioles--alveoli whose septal wall contain smooth muscle

Alveoli

• contains 300 million alveoli
• Between 75 µ  to 300 µ in diameter
• Most gas exchange takes place at alveolar-capillary membrane

Anatomic Arrangement of Alveoli

• 85-95% of alveoli covered by small pulmonary capillaires
• The cross-sectional area or surface area is approximately 70m2

Acinus or Lobule

• Each acinus (unit) is approximately 3.5 mm in diameter
• Each contains about 2000 aveloli
• Approximately 130,000 primary lobules in the lung

Alveolar epithelium
Two principle cell types:

• Type I cell, squamous pneumocyte
• Type II cell, granular pneumocyte

Type I Cell

• 95% of the alveolar surface is made up of squamous pneumocyte cells
• Between 0.1 µ and 0.5µ thick
• Major site of gas exchange

Type II Cell

• 5% of the surface of alveoli composed of granular pneumocyte cells
• Cuboidal in shape with microvilli
• Primary source of pulmonary surfactant
• Involved with reabsorption of fluids in the dry, alveolar spaces

Pore of Khon

• Small holes in the walls of adjoining alveoli (alveaolar septa)
• Between 3 to 13 µ in diameter

Formation of pores may be due to:

• Desquamation due to disease
• Normal degeneration due to aging
• Movement of macrophages leaving holes

Canals of Lambert/Pores of Kohn

• Provide for collateral ventilation of difference acinii or primary lobules
• Additional ventilation of blocked units
• May explain why diseases spread so quickly at the lung tissue (paremchymal) level

Alveolar macrophages

• So-called Type III cell
• Remove bacteria and foreign particles
• May originate as
• Stem cells precursors in bone marro
• Migrate as monocytes through the blood and into the lungs

Intersitium/interstial space

• Surround, supports, and shapes the alveoli and capillaries
• Composed of a gel like substance and collagen fibers
• Contains tight space and loose space areas

Functions of lungs:

• Respiration
• Angiotensin converting enzyme production
• Protection
• Metabolism of certain chemicals and drugs
• Excretion of volatile chemicals like ethanol