Abdomen

Part 4 · The trunk

The abdominal wall, the peritoneum and the viscera

Two reasons this chapter matters to you

The first is mechanical. The abdominal wall is not a container; it is a force-transmitting cylinder. It generates intra-abdominal pressure, provides the diaphragm’s fulcrum, powers the cough, transmits load between thorax and pelvis through the thoracolumbar fascia, and is the muscle group most often damaged by pregnancy, surgery and deconditioning.

The second is diagnostic. Abdominal viscera refer pain to the back, the flank, the shoulder and the groin, in patterns that mimic musculoskeletal problems precisely. A physiotherapist who cannot recognise those patterns will eventually treat a kidney stone as a facet joint, an abdominal aortic aneurysm as lumbar pain, or a cholecystitis as a scapular strain. Those are not hypothetical errors; they are documented ones.

Learning outcomes

• Describe the regions and surface landmarks of the abdomen.
• Describe the layers and muscles of the anterolateral abdominal wall and the rectus sheath.
• Explain intra-abdominal pressure, the abdominal canister, and the mechanics of lifting and the Valsalva manoeuvre.
• Describe the inguinal canal and distinguish inguinal from femoral hernia.
• Describe the peritoneum, its reflections, the greater and lesser sacs, and the classification of viscera.
• Describe the abdominal viscera in outline, with their surface projections.
• Describe the abdominal aorta, its branches, the portal system and portosystemic anastomoses.
• Describe the posterior abdominal wall, psoas and quadratus lumborum, and the lumbar plexus.
• Explain visceral referred pain patterns and apply them to musculoskeletal screening.
• Explain diastasis recti, post-surgical rehabilitation, and abdominal red flags.

Surface anatomy and regions

Figure 1 · The nine regions and the four quadrants

The nine regions and four quadrants

Four quadrants (by the median and transumbilical planes): right upper, left upper, right lower, left lower — the scheme used in emergency documentation.

Nine regions (by two vertical mid-clavicular planes and two horizontal planes):

Horizontal planes worth knowing

Dermatome landmarks: T7 epigastrium, T10 umbilicus, L1 groin and inguinal region.

The anterolateral abdominal wall

Figure 2 · The layers of the wall and the rectus sheath

Figure 3 · Three muscles, three fibre directions

Layers, superficial to deep

• Skin
• Superficial fascia — below the umbilicus it splits into a superficial fatty layer (Camper’s fascia) and a deep membranous layer (Scarpa’s fascia), which continues into the perineum as Colles’ fascia and into the penis and scrotum. This continuity determines the direction in which extravasated urine can spread — a classic anatomical point.
• Three flat muscles with their aponeuroses, and the rectus abdominis
• Transversalis fascia
• Extraperitoneal fat
• Parietal peritoneum

The muscles

Trunk rotation is produced by a force couple: the external oblique of one side with the internal oblique of the other (Chapter 6).

Note the parallel with the intercostals: external oblique continues the direction of external intercostal, internal oblique that of internal intercostal, and transversus abdominis that of innermost intercostal. They are the same three layers, continued into the abdomen — which also explains the segmental thoracoabdominal innervation.

The rectus sheath

Formed by the aponeuroses of the three flat muscles, and its composition changes at the arcuate line, roughly midway between the umbilicus and the pubis:

Consequences:

• the inferior epigastric vessels enter the sheath below the arcuate line and lie directly on transversalis fascia
• a rectus sheath haematoma below the arcuate line can spread widely because there is no posterior containment, presenting as an acute abdomen
• and this is where surgical port placement must be considered carefully

Contents of the sheath: rectus abdominis, pyramidalis, the superior epigastric (from the internal thoracic) and inferior epigastric (from the external iliac) vessels — which anastomose within it, providing a collateral route between the subclavian and external iliac systems in aortic coarctation — plus the terminal parts of the lower six thoracic nerves.

The linea alba is the midline raphe of interlacing aponeurotic fibres from xiphoid to pubis — relatively avascular, hence the surgical midline incision, and the site of divarication (diastasis) recti.

The linea semilunaris is the lateral border of rectus abdominis, curving from the 9th costal cartilage to the pubic tubercle — the site of the rare Spigelian hernia.

Intra-abdominal pressure and the abdominal canister

The abdominal cavity behaves as a pressurised cylinder:

Co-contraction of these four raises intra-abdominal pressure (IAP), which:

• Stiffens the trunk and increases spinal stability, by creating an anterior “pressurised cylinder” that resists flexion moments and by tensioning the thoracolumbar fascia.
• Provides the diaphragm’s fulcrum, allowing its costal fibres to lift the ribs (Chapter 18).
• Powers the cough, sneeze, vomiting, defecation and childbirth.

Coordination matters more than strength. The four walls must act together: if the pelvic floor does not co-contract while IAP rises, the pressure is transmitted downwards, which is the mechanism linking heavy lifting, chronic cough and constipation to pelvic organ prolapse and stress urinary incontinence. Conversely, an over-braced abdominal wall with a poorly coordinated pelvic floor is a recognised pattern in pelvic pain.

The Valsalva manoeuvre — useful and dangerous

Forced expiration against a closed glottis raises IAP and intrathoracic pressure markedly, increasing trunk stiffness — which is why powerlifters use it deliberately.

But it also reduces venous return, drops cardiac output and then causes a sharp rebound rise in blood pressure. It is therefore contraindicated in cardiac rehabilitation, uncontrolled hypertension, and after abdominal or ophthalmic surgery, and it must be actively taught against in patients with pelvic floor dysfunction, hernia or recent abdominal repair. “Breathe out on effort” is not folklore; it is this physiology.

Diastasis recti abdominis (DRA) — separation of the recti at the linea alba, present in the great majority of women in late pregnancy and persisting in around a third at 12 months.

The contemporary understanding has shifted in an important way: the inter-recti distance matters less than the ability of the linea alba to generate tension. A narrow but slack linea alba may function worse than a wider taut one. Consequently:

• Measure function (doming, ability to generate tension, load transfer) as well as width
• Blanket prohibition of all flexion exercise is not supported; graded, symptom-guided loading including curl-up work is appropriate for many women, and exercise reduces DRA prevalence
• Train the whole canister — including the pelvic floor and breathing coordination — rather than transversus abdominis in isolation

The inguinal canal

Figure 4 · The inguinal canal and the two inguinal hernias

An oblique passage, ~4 cm long, above the medial half of the inguinal ligament — the anatomical price paid for the descent of the testis.

Contents: in the male, the spermatic cord (vas deferens, testicular artery, pampiniform plexus, genital branch of the genitofemoral nerve, and the three fascial coverings); in the female, the round ligament of the uterus. In both, the ilioinguinal nerve enters through the side wall and exits through the superficial ring.

The two protective mechanisms are worth knowing because they explain why the canal usually does not herniate: the obliquity of the canal (so raised IAP presses the anterior and posterior walls together) and the shutter mechanism (contraction of internal oblique and transversus lowers the arching roof onto the floor).

Hernia — the distinction that matters

The pubic tubercle relationship is the single most useful bedside discriminator, and it is one of the anatomical facts most likely to be tested.

The peritoneum

Figure 5 · The peritoneum in section

A serous membrane with parietal (lining the wall) and visceral (covering organs) layers, enclosing the peritoneal cavity — the largest serous cavity in the body.

Innervation, and why it matters clinically:

This is the anatomy of appendicitis. Early inflammation irritates the visceral peritoneum of a midgut structure, so the pain is felt vaguely in the periumbilical (T10) region. When inflammation reaches the parietal peritoneum of the right iliac fossa, the pain becomes sharp, localised and associated with guarding at McBurney’s point. The migration of the pain is the diagnosis.

Classification of viscera

Mnemonic for retroperitoneal structures: SAD PUCKER — Suprarenal, Aorta and IVC, Duodenum (2nd–4th), Pancreas (except tail), Ureters, Colon (ascending and descending), Kidneys, Esophagus (lower), Rectum.

Peritoneal folds and spaces

• Greater omentum — a four-layered apron from the greater curvature of the stomach, hanging over the intestines. “The abdominal policeman”: it migrates to sites of inflammation and walls them off, limiting peritonitis.
• Lesser omentum — from the lesser curvature to the liver, with its free right edge (hepatoduodenal ligament) containing the portal triad: the bile duct (right), hepatic artery (left) and portal vein (posterior).
• Omental (epiploic) foramen of Winslow — the entrance to the lesser sac (omental bursa), bounded anteriorly by the hepatoduodenal ligament, posteriorly by the IVC, superiorly by the caudate lobe and inferiorly by the duodenum. The Pringle manoeuvre compresses the portal triad here to control liver bleeding.
• The mesentery of the small intestine, ~15 cm along its root from the duodenojejunal flexure (L2, left of midline) to the ileocaecal junction (right iliac fossa).
• Recesses: the hepatorenal recess (Morison’s pouch) — the lowest part of the peritoneal cavity in the supine position, and therefore where fluid, blood or pus collects; the rectouterine pouch (of Douglas) in females and the rectovesical pouch in males — the lowest points when erect, and the sites accessible for drainage.

The viscera, in outline

Vessels and the portal system

The abdominal aorta

The gut is supplied by three arteries corresponding to three embryological divisions, and this determines both the anastomotic watershed areas (the splenic flexure and rectosigmoid junction — the classic sites of ischaemic colitis) and the pattern of referred pain:

The portal venous system

The hepatic portal vein is formed behind the neck of the pancreas by the union of the superior mesenteric and splenic veins, carrying nutrient-rich blood from the gut to the liver.

Portosystemic anastomoses become clinically important in portal hypertension, when portal blood is forced through them:

The posterior abdominal wall

Psoas is worth pausing on. Its direct segmental innervation from the anterior rami, its attachment to every lumbar vertebra and disc, and its line of pull mean it is simultaneously the strongest hip flexor and a significant lumbar compressive and shear force generator. A psoas abscess (tuberculous or pyogenic — still seen in India) tracks down within the psoas sheath to present as a groin or thigh swelling with a flexion deformity of the hip and pain on passive extension — a presentation that reaches physiotherapy clinics misdiagnosed as a hip flexor strain.

The lumbar plexus (L1–L4, within psoas)

Iliohypogastric and ilioinguinal nerve entrapment after appendicectomy, hernia repair, Pfannenstiel incision or Caesarean section is a common and under-recognised cause of chronic groin and lower abdominal pain — and it is a genuinely treatable one.

Visceral referred pain — the screening table

Figure 6 · Referred pain from the three parts of the gut

This is the practical heart of the chapter for a physiotherapist.

Abdominal red flags a physiotherapist must recognise

Abdominal aortic aneurysm. Deep, boring, non-mechanical low back or abdominal pain in a patient over 60, usually male, with a smoking history, hypertension or known vascular disease. A pulsatile, expansile abdominal mass (expansile is the key word — a transmitted pulsation is not the same). Pain unrelated to position or movement, often worse at night. A ruptured AAA presents as sudden severe back or abdominal pain with collapse — and it is a not-uncommon cause of a patient dying after presenting to a physiotherapist with “back pain”.

Other red flags:

• unexplained weight loss
• night pain that is unremitting and non-positional
• fever
• a history of cancer
• new bowel or bladder change
• blood in urine or stool
• jaundice
• a palpable abdominal mass
• pain that is unaffected by any position, movement or rest
• and pain accompanied by systemic illness

The general rule again: musculoskeletal pain has a mechanical behaviour. Pain that cannot be provoked or eased by any position, movement or palpation is not musculoskeletal until proved otherwise.

Clinical applications

Post-surgical abdominal rehabilitation

Pregnancy and the postnatal abdomen

The abdominal wall lengthens, the linea alba widens, the diaphragm is displaced upwards ~4 cm (with a compensatory increase in the transverse thoracic diameter), lumbar lordosis increases and the centre of mass shifts forwards. Postnatal rehabilitation addresses the entire canister — diaphragm, abdominal wall, pelvic floor and lumbar musculature — with graded load and breathing coordination, rather than isolated exercises or blanket restrictions.

Hernia

Physiotherapists should recognise the presentations, understand the mechanics (raised IAP, weak posterior wall, chronic cough or constipation as drivers), and be able to teach a safe breathing and loading strategy post-repair. A tender, irreducible, painful hernia with vomiting is a strangulated hernia — a surgical emergency.

Where students consistently go wrong

• Forgetting the rectus sheath changes at the arcuate line.
• Confusing the mid-inguinal point (femoral pulse) with the midpoint of the inguinal ligament (deep ring).
• Getting the hernia–pubic tubercle relationship backwards. Inguinal is above and medial; femoral is below and lateral.
• Assuming visceral peritoneum is pain-sensitive in a localised way. It gives dull, referred midline pain; the parietal layer localises.
• Forgetting the appendix pain migration — periumbilical to right iliac fossa — is the visceral-to-parietal transition.
• Not knowing the foregut/midgut/hindgut referral levels. T6–T9, T10, T11–L1.
• Calling pancreatic pain “thoracic spine pain”. It bores through to the back and is eased by leaning forward.
• Missing an abdominal aortic aneurysm in an older patient with non-mechanical back pain.
• Prohibiting all abdominal flexion work in diastasis recti. Function matters more than width.
• Teaching Valsalva to a cardiac, hypertensive, hernia or pelvic floor patient.

Check yourself

15 questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Chapter 20 of 24 · Human Anatomy · Physiotherapist India Next: Chapter 21 — Pelvis and Endocrine Glands: the pelvic floor, and the glands that regulate the body.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents