Digestive System Overview

Overview of the Digestive System

• The digestive system facilitates the breakdown and absorption of nutrients.
• The main components of the gastrointestinal tract are the mouth, oesophagus, stomach, small intestine, and large intestine.
• Associated organs include the salivary glands, liver, and pancreas.

Functions of the Digestive System

• Digestion breaks down large molecules into smaller, absorbable ones.
• Absorption is the passage of digested food into the blood via the GIT mucosa.
• Secretion involves the release of saliva and other digestive juices.
• Motility aids in the transportation of food from the mouth to all parts of the alimentary canal.

Control of the Gastrointestinal System

• Motility and secretion are controlled by the autonomic nervous system (ANS) and gastrointestinal hormones.
• The GIT has a specialized ANS called the Enteric Nervous System (ENS).
• Neurons of the ENS are found within the intestinal wall
• Sympathetic and parasympathetic systems can modify the activity of the ENS.
• Walls of the stomach and intestine secrete hormones that control their activity, including gastrin, cholecystokinin, secretin, and gastric inhibitory peptide (GIP).

Motility

• Motility refers to the movement of the stomach and intestines.
• The aim of motility is to mix food with digestive enzymes and push it down the GIT.
• Propulsive movement (peristalsis) is one type of motility
• Mixes and carries food down the GIT.
• It is stimulated by distension.
• The part of the intestine behind the food contracts, while the part in front dilates.
• Mixing-Movement (Segmentation-Movement) mixes food with digestive enzymes.

Saliva

• Food is mixed with saliva in the mouth; approximately 1.5 liters of saliva are secreted daily.
• Saliva is secreted by 3 pairs of glands in the oral cavity:
• Parotid glands produce a serous, watery secretion (25%).
• Submandibular glands generate a mixed serous and mucous secretion (70%).
• Sublingual glands mainly secrete mucous (5%).
• Saliva's composition includes water, electrolytes, amylase, and mucin.
• Is rich in bicarbonate, calcium, and phosphate.
• The alkaline pH, calcium, and phosphate in saliva protect teeth from demineralization.
• Saliva's functions include lubrication and binding to form a slippery bolus.
• It also coats the oral cavity and esophagus, protecting against solid food particles.
• It dissolves dry food to stimulate taste receptors.
• Saliva provides oral hygiene through mechanical flushing, lysozyme, and antibodies (IgA).
• It initiates starch digestion via amylase and helps in speech by facilitating movement of the lips, tongue, and teeth.
• Salivary secretion is under the control of the autonomic nervous system.
• Parasympathetic stimulation produces a large volume of saliva rich in electrolytes.
• Sympathetic stimulation produces a small, thick secretion rich in enzymes.
• Salivary secretion occurs in two phases:
• Conditioned (Acquired) reflex starts before food enters the mouth and is triggered by sight, smell,or thinking of food.
• Simple (Inborn/Unconditioned) reflex is stimulated by the presence of food or any object in the mouth.
• Conditioned reflexes are triggered by specific visual, auditory, olfactory, or memory stimuli,consciously interpreted in the cerebrum.
• Impulses are then sent to the salivary centre in the medulla, initiating saliva production.
• Simple reflexes involve food ingestion that stimulates mechanical and chemical receptors in the mouth.
• Sensory impulses are sent to the salivary centre in the medulla.
• Autonomic impulses from the medulla then initiate saliva production.
• Abnormalities in salivary secretion include excessive salivation, a symptom of lesions in the oral cavity and Xerostomia, which is decreased salivary secretion that leads to oral infections.

Stomach

• The main parts of the stomach are the fundus, body, antrum, and pylorus.
• Bolus turns into a semi-liquid form through grinding and gastric juices.
• Liquefied food passes through the pyloric canal into the small intestine.
• The stomach serves as a short-term storage reservoir.
• It grinds and mixes food with gastric secretions, resulting in liquefaction.
• It slowly releases food into the small intestine and starts the digestion of proteins, releasing iron from its compounds.
• The stomach has 4 major types of cells:
• Mucous cells secrete an alkaline mucus that protects the epithelium.
• Parietal cells secrete hydrochloric acid and intrinsic factor.
• Chief cells secrete pepsin.
• G cells secrete the hormone gastrin.
• About 2.5 liters of gastric juice is secreted into the stomach daily.
• Gastric juice includes bicarbonate-rich mucus for protection from acid.
• Hydrochloric acid from parietal cells provides an acidic environment (pH 1-2) that activates pepsinogen and inactivates ingested microorganisms.
• Pepsinogen, secreted by chief cells, becomes pepsin and initiates protein digestion.
• Intrinsic factor, secreted by parietal cells, is necessary for intestinal absorption of B12.
• Gastrin helps control acid secretion and gastric motility.
• Gastric secretion is divided into three phases:
• Cephalic phase: Seeing, smelling, or anticipating food triggers brain activity, sending signals to the stomach via the vagus nerve.
• Gastric phase: When food enters the stomach, distension and mucosal irritation cause the release of gastrin and histamine.
• Intestinal phase: Acidic or hyperosmolar contents in the small intestine trigger the release of inhibitory impulses to the stomach to reduce secretion.
• Vagus nerve stimulation releases ACH, stimulating gastrin and histamine.
• ACH, gastrin, and histamine stimulate the secretion of gastric juice.
• Two hormones, secretin and cholecystokinin (CCK), are responsible for forming enterogastrone.
• Abnormalities include peptic ulcers from excessive secretion or damage to the protective mucus layer caused by drugs like aspirin, alcohol, or bacteria (Helicobacter pylori).
• Acid secretion can be decreased by drugs like cimetidine, ranitidine, and omeprazole, with cimetidine and ranitidine acting as histamine antagonists.

Small Intestine

• The small intestine is the longest part of the digestive tube from the pylorus to the large intestine, with three segments: duodenum, jejunum, and ileum.
• It is the site of digestion and absorption of nutrients and is essential for life.
• Digestion occurs when food particles in the small intestine are exposed to pancreatic enzymes and bile.
• The final digestion happens on the small intestine or the brush border epithelium surface.
• Surface area is increased by mucosal folds.
• Villi protrude into the lumen.
• Microvilli densely stud the villi.
• Nutrients are absorbed into bloodacross the small intestine's mucosa, along with water and electrolytes.
• The small intestine absorbs massive amounts of water.
• Approximately 80% of fluid has been absorbed by the time the contents enter the large intestine.
• Water absorption is dependent on solute absorption, especially sodium.

Pancreas

• The pancreas is composed of exocrine and endocrine parts:
• The exocrine pancreas secretes pancreatic juice for digestion.
• The endocrine pancreas secretes insulin and glucagon for glucose metabolism.
• About 1.5 liters of pancreatic juice is secreted daily into the duodenum by the exocrine pancreas.
• The secretion and bile enter the duodenum through a common opening.
• The pancreatic juice is alkaline (rich in bicarbonate) and has enzymes to digest all nutrient types e.g. amylase, trypsin, chymotrypsin, and lipase.
• Sodium bicarbonate neutralizes acids and prevents development of duodenal ulcer.
• It adjusts pH for the action of pancreatic enzymes.
• Proteins are digested by trypsin, chymotrypsin, and carboxypeptidase.
• Lipids are digested when pancreatic lipase acts on triglycerides to form fatty acids.
• Carbohydrates are digested into disaccharides by pancreatic amylase.
• Pancreatic secretion is stimulated by secretin (producing HCO3), cholecystokinin (enzyme secretion), and the parasympathetic (vagus) nerve (enzyme secretion)

Liver

• The liver is essential for carbohydrate metabolism (glucose ↔ glycogen), fat metabolism (desaturation of fat), and protein metabolism (deamination and transamination of amino acids.
• Synthesis of plasma proteins and blood clotting factors.
• The liver aids in breakdown of erythrocytes, defence against microbes (by Kupffer cells), detoxification of drugs, inactivation of hormones, and synthesis of vitamin A.
• The liver aids in the production of heat, the secretion of bile, and the storage of fat-soluble vitamins (A, D, E, K).
• Bile is a yellow secretion formed by liver cells, collected by bile ducts, and secreted into the duodenum.
• Between meals, bile is stored in the gall bladder.
• Bile contains bile salts (for emulsification of fat) and bile pigments (bilirubin, a waste product).
• Bile secretion is triggered by the vagus nerve and cholecystokinin when a meal enters the duodenum.
• Obstruction results in jaundice as bile pigments increase.

Digestion and Absorption of Carbohydrates

• Polysaccharides and disaccharides must be digested to monosaccharides for absorption.
• The main dietary carbohydrate is starch, which begins digestion in the mouth via salivary amylase.
• Pancreatic amylase continues the digestion of starch.
• Amylase breaks down starch into maltose.
• Dietary disaccharides (lactose, sucrose, maltose) are broken down by enzymes in the small intestine.
• Maltase cleaves maltose into two molecules of glucose.
• Lactase cleaves lactose into glucose and galactose.
• Sucrase cleaves sucrose into glucose and fructose.
• The monosaccharides (glucose, galactose, fructose) are absorbed into the intestinal cells.
• Lactase deficiency may cause lactose intolerance, leading to abdominal discomfort and diarrhea after milk intake.

Digestion and Absorption of Proteins

• Proteins must be digested into amino acids.
• Protein digestion starts in the stomach.
• The stomach secretes pepsinogen (activated to pepsin by HCl).
• The pancreas secretes proteases (trypsin, chymotrypsin).
• Proteases are secreted as inactive and activated after reaching the lumen of the intestine.
• Gastric and pancreatic proteases digest dietary proteins into medium and small peptides in the small intestine.
• Peptidases on the surface of the small intestine complete the breakdown of peptides into free amino acids.
• The free amino acids are absorbed into the blood.

Digestion and Absorption of Lipids

• The main dietary lipid is triglyceride (glycerol with 3 fatty acids), with cholesterol and fat-soluble vitamins also present.
• Two processes are needed for triglyceride absorption.
• Emulsification breaks down large aggregates of dietary triglyceride.
• Digestion enzymatically digests triglyceride molecules, producing monoglyceride and fatty acids.
• Bile salts solubilize fats for digestion.
• Combination of bile salts with lipids form micelles.
• Pancreatic lipase digests triglycerides into monoglycerides and fatty acids.
• Lipids are combined with proteins to form chylomicrons inside the cells.
• The chylomicrons enter the lymphatics before reaching the blood.

Functions of the Large Intestine

• Absorption occurs when water, mineral salts, vitamins, and some drugs are absorbed until the feces is semisolid.
• Microbial activity occurs when certain bacteria synthesize vitamin K and folic acid.
• Gases result from bacterial activity.
• Mass movement occurs when a strong peristalsis sweeps along the transverse colon.
• Defecation starts when the a mass movement transfers the contents to the rectum to stimulate the nerve endings in the walls.
• The external sphincter allows for conscious control, through the pudendal nerve
• It involves defecation by the muscle of the rectum and internal sphinter relaxation.
• Abdominal contraction and diaphragm lowering assist in defecation.
• Normal feces consist roughly of 75% water and 25% solids,
• The bulk of which consists of bacteria, organic matter, and undigested fibers.
• The brown color of feces results from stercobilinogen.
• The odor is due to skatole, mercaptans, and hydrogen sulfide.
• A considerable amount of gas is often present in the gastrointestinal tract, with
• Five major intestinal gases often present: N₂, O₂, CO₂, and H₂.