16th February 2024

Abdomen and Pelvis

Abdominal wall and peritoneum

Abdominal wall

The abdominal wall can be divided into the anterior, posterior, and inguinal canals. 

Anterior abdominal wall

The abdominal wall is made up of 4 muscles: external and internal obliques, transverses abdominis and rectus abdominis. The aponeurosis of the former three muscles comes together to form a rectus sheath. The anterior abdominal wall is supplied by the anterior rami of the lower six thoracic and the first lumbar nerves. Some paraumbilical veins connect through the ligamentum teres to the portal vein, which is one of the sites of portosystemic anastomosis.

Posterior abdominal wall

The posterior abdominal walls are made up of the lumbar vertebrae with their intervertebral discs, twelfth ribs, upper part of the iliac bones, psoas major, quadratus lumborum, iliacus and origin of transversus abdominis.

Inguinal canal

There are two oblique passages, one on each side, in the lower abdomen, known as the inguinal canal. These canals transmit the spermatic cord in males and the round ligament in females, along with the ilioinguinal nerve and genital branch of the genitofemoral nerve in both sexes. 

The inguinal canal is bordered by the aponeurosis of the external oblique and internal oblique muscle anteriorly; transversal fascia posteriorly; transversal fascia, internal oblique and transversus abdominis superiorly; and the inguinal ligament and lacunar ligament inferiorly. 

Peritoneum

The peritoneum is a double membrane covering the abdomen, pelvis, and the organs within. It is divided into parietal layer and visceral layers. The parietal layer lines the cavity of the abdomen and pelvis, while the visceral layer covers the intraabdominal and intrapelvic organs. 

The space between the two layers is called the peritoneal cavity and is further divided into the greater sac and a lesser sac. These two parts of the peritoneal cavity interconnect via epiploic foramen. The greater sac is the main compartment, while the lesser sac is small and sits behind the stomach.

The parietal layer is sensitive to touch, temperature, pressure and pain, while the visceral layer is sensitive to stretch.

References

Wineski, L. E., & Snell, R. S. (2019). Snell’s clinical anatomy by regions. 

Chaurasia, B. D. (2019). Human anatomy: Regional and applied, dissection and clinical. New Delhi, India: CBS Publishers & Distributors.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Moore’s Clinically Orientated Anatomy (7th ed.). China: Lippincott.

Gastrointestinal tract

Oesophagus

The oesophagus starts from the hypopharynx at the lower border of the cricoid cartilage, passing in through the superior mediastinum, then the posterior mediastinum just to the left of the midline and through the diaphragm at the level of T10 to enter the stomach at the abdomen.

There are important port-systemic anastomoses at the gastro-oesophageal junction and lower oesophagus level.

Stomach

The stomach has five parts: cardia, fundus, body, antrum and pylorus. The oesophagus enters the stomach at the cardia, the site of the physiologic sphincter, preventing food from regurgitating. The fundus is a rounded part above the cardia and typically contains gas. 

The body is the largest section of the stomach. The body leads to the antrum, which leads to the pylorus. The pylorus is the distalmost part of the stomach connecting with the duodenum and containing the pylorus sphincter. The stomach has two curvatures, the greater and lesser curvatures.

Small intestines

The duodenum is a 20-30 cm C-shaped small bowel region. The pancreatic head usually lies inside the C-shape of the duodenum. The initial 2-3cm of the 1st part of the duodenum is intraperitoneal. The second part of the duodenum receives the pancreatic and common bile duct at the ampulla of Vater. Just inferior to the ampulla is the junction of the foregut and midgut. The duodenum ends at the ligament of Treitz aka duodenojejunal (DJ) flexure continuing as jejunum.

The remainder of the small bowel is divided into the jejunum, the proximal 2/5th, and the ileum, the latter 3/5th. Small bowel loops contain mucosal folds called valvulae conniventes, which start at the second part of the duodenum, becoming the thickest at the jejunum and then fading away distally towards the terminal ileum. 

These folds make complete circles in the bowel wall and give a characteristic appearance on plain radiographs or barium studies. The ileum enters the caecum at the ileo-caecal valve.

Large intestine

The caecum is an intraperitoneal part of the large bowel and has three longitudinal muscular bands (Tinea coli), which converge inferiorly towards the base of the appendix. The caecum continues as the retroperitoneal ascending colon. 

The transverse colon starts at the hepatic flexure from the ascending colon and ends at the splenic flexure to continue as the retroperitoneal descending colon. The sigmoid colon connects the descending colon to the rectum and has a variable length. It is intraperitoneal and is also suspended by its mesentery. 

Rectum and anus

The rectum, which is retroperitoneal, starts at the level of S3, passing anterior to the sacrum. The tinea coli converges into a single muscular layer at the rectum. The rectum leads to the anus, the distal end of the GI tract.

References

Wineski, L. E., & Snell, R. S. (2019). Snell’s clinical anatomy by regions. 

Chaurasia, B. D. (2019). Human anatomy: Regional and applied, dissection and clinical. New Delhi, India: CBS Publishers & Distributors.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Moore’s Clinically Orientated Anatomy (7th ed.). China: Lippincott.

Other intra-abdominal organs

Liver

The liver is divided into four lobes: right, left, quadrate and caudate. Three major recesses around the liver serve as potential spaces for abscesses to develop around the liver, including the subphrenic recess, subhepatic recess and Morison’s pouch. 

All the major structures, except hepatic veins, enter and leave the liver at the porta hepatis. The portal vein, formed by the superior mesenteric and splenic veins, supplies about 75% of the blood to the liver. This contains deoxygenated, nutrient-rich blood from the GI tract and spleen. The remaining 25% of the blood is oxygenated and supplied by the hepatic artery. Hepatic veins drain the blood from the liver to the inferior vena cava.

Gallbladder

The liver secretes bile, which initially passes through the canaliculi, which join together to form intrahepatic bile ducts. The intrahepatic ducts will converge to form the right and left hepatic ducts. The right and left hepatic ducts combine to form a common hepatic duct. The common hepatic duct will converge with the cystic duct to form the common bile duct. 

The pancreatic duct will converge with the common bile duct at the ampulla of Vater before it drains into the duodenum. The gallbladder lies at the inferior surface of the liver and stores bile, which is excreted via the cystic duct. The gallbladder is divided into fundus, body and neck. The fundus of the gallbladder lies at the tip of the right ninth rib.

Pancreas

The pancreas is divided into head, neck, body and tail. The head lies in the cavity created by the C-shape of the duodenum. The pancreas extends across the midline and ends at the hilum of the spleen. The main pancreatic duct drains the exocrine part of the pancreas.

Spleen

The spleen is a part of the lymphoid system. It is connected through peritoneal ligaments to the stomach and left kidney. The splenic artery and splenic vein supply and drain the spleen, respectively.

Adrenal glands

Two adrenal glands lie adjacent to the superior pole of each kidney. These glands are supplied by the superior suprarenal artery, middle suprarenal artery, and inferior suprarenal artery, which are branches of the inferior phrenic, aorta and renal artery, respectively. The right adrenal is drained by a single vein directly into the IVC, while the left one drains into the renal vein.

References

Wineski, L. E., & Snell, R. S. (2019). Snell’s clinical anatomy by regions. 

Chaurasia, B. D. (2019). Human anatomy: Regional and applied, dissection and clinical. New Delhi, India: CBS Publishers & Distributors.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Moore’s Clinically Orientated Anatomy (7th ed.). China: Lippincott.

Renal system

Kidneys

The kidneys are retroperitoneal and are located at the level of T12-L3, with the right kidney slightly lower than the left. The renal parenchyma is divided into the renal cortex and medulla. The renal cortex is the outer aspect of renal parenchyma, which contains renal corpuscles (glomeruli, vessels), proximal portions of collecting tubules, and parts of the loop of Henle. The inner portion of renal parenchyma contains renal pyramids, distal parts of collecting tubules, and parts of the loop of Henle. 

The collecting tubules will eventually drain into the minor and major calyx before emptying into the ureter. The blood supply to the kidneys originates from the renal artery, and venous drainage occurs via the renal vein.

Ureter

The renal pelvis becomes the ureter at the pelvic ureteric junction (PUJ). The ureters are narrow fibro-muscular tubes connecting the kidneys to the urinary bladder. There are three narrowings of the ureter- at the PUJ, as it crosses the common iliac artery bifurcation, and at the vesicoureteric junction (VUJ).

Urinary bladder

The ureters drain into the urinary bladder, acting as a urine reservoir. It is a triangular structure with an apex, base and neck. The lining of the urinary bladder consists of trabeculated transitional cells except at trigone. The trigone is a small triangular area with smooth mucosa at the base of the bladder. 

Both ureteric orifices form the superolateral angles, and the internal urethral orifice forms the inferior angle. The urinary bladder is only partially covered by the peritoneum, which passes on the superior surface and continues back to cover the anterior surface of the rectum. The space created between the urinary bladder and rectum is called the rectovesical pouch and is the most dependent part in males when erect.

Urethra

The male urethra commences at the internal urethral orifice in the trigone of the bladder and opens at the external urethral meatus in the glans penis. It is divided into penile, bulbar, membranous and prostatic urethra. 

The female urethra is much shorter compared to the male (Approximately 4cm vs. 18-20cm). It starts at the internal urethral meatus at the bladder neck and opens in the vestibule of the vagina.

References

Wineski, L. E., & Snell, R. S. (2019). Snell’s clinical anatomy by regions. 

Chaurasia, B. D. (2019). Human anatomy: Regional and applied, dissection and clinical. New Delhi, India: CBS Publishers & Distributors.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Moore’s Clinically Orientated Anatomy (7th ed.). China: Lippincott.

Reproductive system

Pelvic cavity

The inferior part of the abdominopelvic cavity is the pelvic cavity. The bony pelvis and pelvic muscles bound it. The lower spine (sacrum and coccyx) and hip bones form the bony pelvis. The hip bone combines three separate bones that fuse at puberty: ilium, ischium and pubis.

The pelvic cavity contains male and female reproductive organs, rectum, sigmoid colon, terminal coils of ileum, urinary bladder, distal ureters and urethra.

Male reproductive system

The male internal genital organs consist of vas deferens, ejaculatory ducts, seminal vesicles, prostate and urethra. The external parts of the male reproductive system include the penis, the scrotum and the testicles.

The testes are mobile ovoid organs (measuring about 12.5-19ml in volume) in the scrotum. Each test is covered by a thick fibrous layer known as tunica albuginea, which is covered by a thin visceral layer of tunica vaginalis. The testes produce male sex hormones and sperm. Seminiferous tubules produce the sperms within the testes. 

Epididymis is a long-convoluted tube closely related to the testes. It has three parts:  Head, body and tail. The head is the superior part which stores the sperm. The body connects the head to the tail. The tail of the epididymis then continues as vas deferens.

The vas deferens arise from the tail of the epididymis, travel within the spermatic cord, and cross the ureter and bladder. Each vas deferens converges with the duct from the seminal vesicle, forming ejaculatory ducts, which go through the prostate and empties into the prostatic urethra.

The prostate is located inferior to the neck of the urinary bladder. There are five lobes: prostate-anterior, median, right lateral, left lateral and posterior lobes. The prostatic urethra passes through the prostate and becomes the spongy urethra.

The penis is the male organ for sexual intercourse. It has three parts: The root, the shaft and the glans. The urethral opening is at the tip of the glans. The penis contains three erectile structures: corpora cavernosa (right and left) and corpus spongiosum. During an erection, these erectile structures become engorged with blood. The corpus spongiosum expands distally to become glans penis. The penis houses the spongy urethra.

The scrotum is the loose pouch-like sac of skin which houses the testes. It helps with the temperature regulation of the testes.

Female reproductive system

Female internal genital organs consist of ovaries, fallopian tubes, uterus and vagina. The ovary lies typically against the lateral wall of the pelvis. Each ovary is closely related to the distal part of the fallopian tube. The fallopian tube has five segments. From lateral to medial, these are Fimbriae-finger-like projections draping over the ovary, infundibulum, ampulla (usual site of fertilisation), isthmus and intramural parts. It extends from the uterus’s upper side to the ovary’s region. The uterus is a pear-shaped organ. This is divided into the fundus, body, and cervix. The cervix opens into the superior end of the vagina. The long axis of the uterus is typically bent forward at about a 90-degree angle to the long axis of the vagina, resulting in the anteversion of the uterus. 

The vagina extends superiorly and posteriorly from the vulva to the uterus. Its superior half lies above the pelvic floor, whereas the lower half lies within the perineum. The upper vaginal wall surrounds the cervix and forms the vaginal fornices. Two spaces are created by the peritoneal reflection over the urinary bladder, vagina and rectum, called the uterovesical and rectovaginal pouch (pouch of Douglas). The rectovaginal pouch is the most dependent part in females in an erect position. One peritoneal fold on each side extends from the uterus to the lateral pelvic wall, known as the broad ligament.

References

Wineski, L. E., & Snell, R. S. (2019). Snell’s clinical anatomy by regions. 

Chaurasia, B. D. (2019). Human anatomy: Regional and applied, dissection and clinical. New Delhi, India: CBS Publishers & Distributors.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Moore’s Clinically Orientated Anatomy (7th ed.). China: Lippincott.