12 Respiratory and Gastrointestinal Systems (RGI)

Abdominal Cavity

• The alimentary canal resides within the abdominal or PERITONEAL cavity.
• The abdominal cavity, also known as the PERITONEAL cavity, is separated from the THORACIC cavity by the diaphragm.
• The peritoneal cavity is lined by a smooth endothelium or serosa, featuring an outermost layer called MESOTHELIUM.
• The outermost layer is referred to as the PARIETAL PERITONEUM.
• The layer that covers the organs is the VISCERAL PERITONEUM.

Small Intestine

• The small intestine's total length is about 3 meters.
• The small intestine is divided into three primary parts:
  • Duodenum
  • Jejunum
  • Ileum
• The DUODENUM is the first part of the small intestine, located beyond the pylorus of the stomach.
• The gut follows a basic plan:
  • A simple tube comprised of the mucosa, connective tissue submucosa, two layers of smooth muscle, and a serous covering suspended by folds of mesentery.
  • A special feature of the small intestine: circularly arranged folds of the mucosa and submucosa called the PLICAE CIRCULARES.

Duodenum

• Curved tube, 21 cm long x 4 cm diameter, with VILLI developed in the mucosa.
• In the submucosa, there are compound branched tubular glands called Brunner’s glands.
• Brunner’s glands secrete mucin, protecting the duodenum from the acidic chyme of the stomach.
• PANCREAS lies within the loop of the duodenum.
• Pancreatic and bile ducts enter via a common duct near the pylorus called the papilla of Vater or major duodenal papilla.

Small Intestine - Structure

• JEJUNUM follows the duodenum; it is 0.9 meters long, containing more villi and highly developed plicae circulares.
• ILEUM, 2 meters long, is the final and longest section with prominent lymph nodes called PEYER’S PATCHES.
• The plicae become less obvious, and villi less numerous further along the small intestine.

Small Intestine - Villi

• The mucosa of the small intestine features a large number of finger-like projections called VILLI.
• The villi are continually contracting and lengthening.
• Villi greatly increase the surface area of the small intestine.

Small intestine - Villi

• Luminal face of villi - columnar epithelial cells with a brush border of MICROVILLI.
• Contain large numbers of mucus-secreting goblet cells.
• At the base of the villi are Crypts of Lieberkühn: short glands extending to the muscularis mucosa.
• Crypts are lined with:
  • Epithelial cells.
  • Goblet cells.
  • Stem cells.
  • PANETH cells contain zymogen granules containing lysozyme (bacteriocidal).
  • ENTEROENDOCRINE cells.

Small Intestine - Villi

• Lacteal – finger-like lymphatic projections.
• Arteriole – brings blood into the capillary and gives off multiple side branches capillaries.
• Venule – ferries blood away from the villus; all capillaries join the venule in the villus.
• Lamina propria.
• Enterocytes.

Small Intestine - Histology Layer

• Mucosa
  • Inner Tissue: Villi; crypts of Lieberkühn; columnar epithelium; microvilli
  • Outer Tissue: Lamina propria; lacteals; capillaries; Peyer’s patches in Ileum
• Muscularis Mucosa
  • Inner Tissue: Smooth muscle
• Submucosa
  • Inner Tissue: Connective tissue
  • Outer Tissue: Brunner’s glands in Duodenum
• Muscularis Externa
  • Inner Tissue: Smooth circular
  • Outer Tissue: Smooth longitudinal
• Serosa
  • Inner Tissue: Loose connective
  • Outer Tissue: Mesothelium

Small Intestine - Cytology

• Columnar epithelial cells (enterocytes):
  • Function: Absorption
  • Found: All parts of mucosa.
• Goblet cells:
  • Function: Mucus
  • Found: All parts of mucosa.
• Paneth cells:
  • Function: Lysozyme, Antimicrobial b- defensins
  • Found: Crypts of Lieberkühn
• Enteroendocrine cells:
  • Function: Secretin, Cholecystokinin (CCK)
  • Found: Duodenal crypts
• Brunner’s glands:
  • Function: Thin alkaline mucus
  • Found: Submucosa

Small Intestine - Physiology

• Function: digest food by enzymatic action and absorb products on the intestinal villi.
• CHYME enters the duodenum and is mixed with BILE and pancreatic secretions (1.8 liters/day).
• Secretions of the small intestine itself, the succus entericus, are approx. 2 liters / day.
• Brunner’s glands are compound tubular glands secreting an alkaline mucus with a pH of 8.8 – 9.3.
  • Raises the pH of the chyme to about 5.5.
  • Less than optimal for the operation of pancreatic enzymes.
• Lower regions of the ILEUM become more acidic again due to the production of organic acids by bacteria.

Small Intestine - Physiology

Two kinds of small intestine movements:

• SEGMENTING CONTRACTIONS: localized constrictions to cut the intestine into segments.
• PROPULSIVE MOVEMENTS: peristaltic waves to force the chyme towards the colon at speeds of up to 2cm / second.
  • Waves ease, so net movement averages 1cm / minute; time for chyme from the pylorus to the ILEOCAECAL VALVE is between 3 - 5 hrs.
• Movements lead to enzymatic breakdown of food and absorption of products by the enterocytes.

Pancreas

• The most important digestive gland, situated in the curve of the duodenum close to the stomach, with a distinct head, neck, body, and tail.
• Both an exocrine and an endocrine function
• Primarily exocrine tissue.
• Consists of many lobes covered in a loose connective tissue capsule.
• Exocrine tissue is composed of branching sets of tubes or ducts leading to blind secretory sacs called ACINI.
• Millions of acini: serous cells secreting enzymes.
• Each acinus is made of an irregular cluster of secretory cells, which drain into a minute central duct.
• Intercalated ducts drain into progressively larger interlobular ducts and eventually coalesce into the main pancreatic duct.
• Smallest ducts are lined by flat or low cuboidal lining cells called CENTROACINAR cells.
• The endocrine pancreas (10%) consists of a number of masses of tissue called the Islets of Langerhans, which secrete hormones into the bloodstream.

Pancreas - Arterial Supply

• Pancreatic branches of the splenic artery.
• The head is also supplied by the superior and inferior pancreaticoduodenal arteries, which are branches of the gastroduodenal (from the coeliac trunk) and superior mesenteric arteries, respectively.

Pancreas - Physiology

• Control of secretion is both hormonal and nervous, but mainly hormonal.
• Entry of chyme into the duodenum stimulates the enteroendocrine cells of the mucosa to release two hormones: Secretin & Cholecystokinin (CCK).
• These are carried in the circulation to the pancreas.

Pancreas - Structure

• SECRETIN, carried to the pancreas, causes the release of large quantities of alkaline fluid (pH 8.0) from the centroacinar cells; it contains bicarbonate ions.
• CHOLECYSTOKININ (CCK) stimulates the acinar cells to produce the digestive enzymes.
• Release of the pancreatic enzymes causes hydrolysis of the carbohydrates, proteins, and fats in the chyme into monosaccharides, amino acids, and fatty acids.
• Most of the ABSORPTION takes place in the jejunum and ileum through the microvillar surface of the columnar epithelial cells by active transport and facilitated diffusion.

Pancreas - Physiology

• Amino acids and sugars pass rapidly into the portal circulation.
• FATS are emulsified by the bile salts into small micelles and broken down by LIPASE into glycerol and fatty acids.
• On contact with the epithelial cells, the micelles discharge their contents through the membrane and into the cell.
• In the smooth E.R., triglycerides are re-synthesised, covered with a protein coat, and discharged as chylomicrons into the intercellular space, where they are taken up by the lacteals of the lymphatic system.

Pancreatic Enzymes & Secretions