Gastrointestinal Functions & Digestive Diseases

Gastrointestinal Functions & Digestive Disease

• The gastrointestinal (GI) tract is primarily located in the abdomen.
• It is a muscular tube lined with epithelial cells.
• It extends approximately 10 meters from mouth to anus.
• The tract's structure is modified into specialized sections to support digestion and absorption.
• The GI tract is controlled by an intricate hormonal and neural network.
• It produces hormones like gastrin and secretin that act locally to influence intestinal and related organ function (pancreas, gallbladder).
• The lower digestive tract contains a large number of microorganisms that coexist with the body.
• These microorganisms offer nutrients like vitamin K.
• Changes in microbial flora or introduction of unusual microorganisms can lead to intestinal tract diseases.
• The image shows the location of major organs within the digestive tract.

Gastric Mucosa Cells

• Gastric mucosa contains various cells with specific functions.
• Mucous cells line the stomach and produce mucus to protect it from acidic enzymes.
• Parietal cells produce hydrochloric acid (HCl) and intrinsic factor.
• Chief cells produce pepsinogen, a proenzyme.
• G-cells in the antrum secrete gastrin.

Stomach Digestion

• Sight and smell of food trigger messages from the brain, stimulating HCl and gastrin production.
• Gastrin further stimulates HCl production.
• Chief cells contain receptors sensitive to acidic environments and secrete pepsinogen.
• Pepsinogen is rapidly converted to pepsin at pH 3.
• These actions convert food into chyme.

Duodenum

• Chyme enters the duodenum, where bile and pancreatic enzymes are secreted.
• Further enzymatic degradation occurs within the duodenum.
• Food material enters the rest of the small intestine.
• Food is broken down into amino acids, sugars, fatty acids, and glycerol during this process.

Small Intestine

• The small intestine, measuring 4 meters in length, includes the jejunum and ileum, in addition to the duodenum.
• Its villi and microvilli structures increase the absorption surface area.
• The undigested material enters the large intestine where water and electrolytes are selectively absorbed.
• The digestive process concludes with feces formation.

Portal Vein

• The portal vein collects absorbed substances and transports them to the liver.

Brain-Gut Axis

• The intestinal tract contains many endocrine cells.
• Peptide hormones produced by these cells regulate GI functions.
• The central nervous system has receptors for these hormones and influences the secretion of intestinal hormones.
• This relationship is often referred to as the brain-gut axis.

Carbohydrate Digestion & Absorption

• Most carbohydrates are ingested as starch, a long-chain polysaccharide of glucose.
• Starch digestion begins in the mouth with salivary amylase.
• Starch digestion continues in the duodenum with pancreatic amylase.
• Brush border enzymes, such as sucrase and lactase, further hydrolyze intermediate starch products into monosaccharides.
• Several transporter proteins on the epithelium facilitate monosaccharide absorption into the bloodstream (e.g., SGLT1, GLUT5, GLUT2).

Protein Digestion & Absorption

• Protein digestion begins in the stomach with pepsin, a proteolytic enzyme.
• Major products of pepsin digestion are short-chain polypeptides.
• Most protein digestion occurs in the duodenum and jejunum.
• Pancreatic enzymes (trypsin, chymotrypsin, elastase) cleave interior peptide bonds.
• Exopeptidases (e.g., carboxypeptidase and aminopeptidase) remove amino acids from the ends of polypeptides.
• Dipeptidases and tripeptidases from brush border enzymes help absorb dipeptides and tripeptides, hydrolyzing them to amino acids for uptake into the epithelial cells and the bloodstream.

Lipid Digestion & Absorption

• The salivary glands and stomach produce lipase in newborns. In adults, very little lipid digestion occurs in the mouth.
• Arrival of lipids in the duodenum stimulates bile secretion.
• Bile salt micelles emulsify fat droplets, increasing surface area for lipase action.
• This emulsification process is not a chemical digestion.
• Lipase hydrolyses triglycerides into glycerol, fatty acids, and monoglycerides.
• Absorbed lipids combine with proteins forming chylomicrons.
• Chylomicrons are secreted into lacteals and carried through the lymphatic system to the bloodstream.

Water & Sodium Absorption

• Sodium absorption is an active transport mechanism linked to the absorption of amino acids, glucose, and bicarbonate in the small intestine.
• Water is absorbed predominantly in the large intestine.
• Hormones (aldosterone, glucocorticoids, and somatostatin) influence water and electrolyte absorption.

Gastrointestinal Hormones (Gastrin, CCK, Secretin)

• Gastrin is produced by gastric G-cells and stimulates acid secretion and gastric motility.
• Gastrin release is triggered by vagal signals, gastrin-releasing peptide, and the presence of peptides, gastric distention and an elevated stomach pH.
• Cholecystokinin (CCK) is produced in the upper small intestine and regulates gallbladder and intestinal motility.
• CCK also stimulates pancreatic enzyme release. Amino acids and fatty acids stimulate CCK secretion.
• Secretin is released from S-cells in the duodenum and jejunum.
• Secretin inhibits smooth muscle and gastric acid secretion.
• It promotes water and bicarbonate secretion from the pancreas.
• Secretin works with CCK to influence gallbladder contraction and pancreatic enzyme release.

Pathological Conditions (Ulcers, Zollinger-Ellison Syndrome, Pernicious Anemia)

• Peptic ulcers are caused by excess acid action, while H. pylori is a common cause of chronic gastritis and peptic ulcers.
• Zollinger-Ellison syndrome involves an extreme form of peptic ulcer disease frequently triggered by a gastrin-secreting tumor.
• Pernicious anemia results from intrinsic factor deficiency due to autoantibodies blocking intrinsic factor.
• This intrinsic factor is essential for vitamin B12 absorption.

Small Intestine Diseases (Malabsorption, Celiac Disease, Lactose Intolerance)

• Malabsorption involves impaired nutrient digestion and absorption, leading to malnutrition.
• Celiac disease is an autoimmune response to wheat proteins in gluten, causing small intestinal inflammation.
• Lactose intolerance is a common carbohydrate malabsorption disorder resulting from reduced lactase activity, leading to undigested lactose, and associated complaints.

Gastrointestinal Function Tests

• Helicobacter pylori infection is determined using tests such as the urea breath test.
• A patient ingests a labeled urea solution; if H. pylori is present, the bacteria convert the urea into labeled CO2, detected in the breath.
• The definitive test for fat absorption is the quantitative measurement of fat in feces samples collected while the patient adheres to a diet with a known fat content.
• A more practical approach is a fat screening test, including stool evaluation for appearance and microscopic analysis with fat-specific stains.
• Lactose tolerance tests evaluate lactose digestion and absorption. Glucose levels are measured after lactose administration; a decreased glucose response indicates impaired lactose absorption.

Change of Analyte in Disease (Malabsorption Testing)

• Screening for malabsorption symptoms is often based on clinical signs and risk factors (e.g., elderly).
• Laboratory screening for malabsorption, while not highly sensitive, might assess serum albumin, calcium, vitamin B12, and iron levels, as well as peripheral blood smears.
• For suspected fat malabsorption, fecal microscopic evaluation is often initial. Protein malabsorption assessment is more complex.
• Evaluation of diarrhea primarily involves microbiology, if necessary.

Occult Blood in Stool and Carcinoembryonic Antigen (CEA)

• Detecting trace amounts of blood (hemoglobin) in stool using tests based on peroxidase activity of hemoglobin.
• Carcinoembryonic antigen (CEA) is a glycoprotein elevated in some GI tumors but not all, especially in colon cancers. Monitoring disease course, especially post-treatment, is one frequent use of CEA testing.

This quiz explores the functions of the gastrointestinal tract and the role of gastric mucosa cells in digestion. Learn about the intricate network that controls digestion, the importance of microorganisms, and how changes in this system can lead to diseases. Test your understanding of the anatomy and physiology of the digestive system.