Digestive System: Overview and Control

Overview of the Digestive System

• The digestive system's primary function is to break down large molecules into smaller, absorbable ones through digestion
• Absorption is the passage of digested food into the blood by crossing the mucosa of the gastrointestinal tract (GIT)
• Secretion involves the release of saliva and other GIT juices
• Motility aids in transporting food from the mouth to all parts of the alimentary canal

Control Mechanisms

• Motility and secretion in the gastrointestinal system (GIS) are controlled by the autonomic nervous system (ANS) and gastrointestinal hormones
• The Enteric Nervous System (ENS) is a special part of the ANS within the GIT
• Neurons are located within the wall of the intestine
• Sympathetic and parasympathetic systems can modify the activity

Gastrointestinal Hormones

• The stomach and intestine walls secrete hormones to control activity
• Examples are Gastrin, Cholecystokinin, Secretin, and Gastric Inhibitory Peptide (GIP)

Motility in the Stomach and Intestine

• Motility aims to mix food with digestive enzymes and push it down the gastrointestinal tract (GIT)
• Two types of motility are propulsive movement (peristalsis) and mixing movement (segmentation)
• Peristalsis carries food down the GIT and is stimulated by distension
• During peristalsis, the intestine part behind the food contracts, while the part in front dilates
• Segmentation mixes food with digestive enzymes

Saliva Production and Composition

• Saliva mixes with food in the mouth
• Approximately 1.5 liters of saliva are secreted daily
• Saliva is produced mainly by three pairs of glands in the oral cavity:
• Parotid glands produce a serous, watery secretion (25%)
• Submandibular glands produce a mixed serous and mucous secretion (70%)
• Sublingual glands secrete mainly mucous secretion (5%)
• Saliva consists of water, electrolytes, enzymes (amylase), and mucin
• It is rich in bicarbonate, calcium, and phosphate
• Alkaline pH, calcium, and phosphate help protect teeth from demineralization

Functions of Saliva

• Lubrication and binding: Mucus binds masticated food into a slippery bolus, aiding swallowing.
• Coating the oral cavity and esophagus: Protects against solid food particles.
• Dissolving dry food: Stimulates taste receptors.
• Oral hygiene: Mechanical flushing, lysozyme (lyses bacteria), and antibodies (IgA) are present.
• Initiates starch digestion: contains enzyme amylase, which starts digestion of dietary starch into maltose
• Speech assistance: facilitates movement of the lips, tongue, and teeth

Control of Salivary Secretion

• The autonomic nervous system controls salivary secretion
• Parasympathetic stimulation produces a large saliva volume, rich in electrolytes
• Sympathetic stimulation produces a small, thick secretion rich in enzymes
• Salivary secretion has two phases: conditioned (acquired) reflex and simple (inborn) reflex
• Conditioned reflex starts before food enters the mouth, triggered by sight, smell, or thought of food
• Specific visual, auditory, olfactory, or memory stimuli, interpreted in the cerebrum, initiate saliva production via the medulla and autonomic nerves
• A simple (inborn) reflex is stimulated by food or any object's presence in the mouth through sensory impulses sent to the medulla, leading to saliva production

Abnormalities in Salivary Secretion

• Excessive salivation is a symptom of lesions in the oral cavity
• Xerostomia is decreased salivary secretion that may lead to oral infections

The Stomach

• The main parts are fundus, body, antrum, and pylorus
• Food enters the stomach as a semi-solid bolus and is turned into a semi-liquid form by the grinding action of the stomach and gastric juice
• Liquidified food passes through the pyloric canal into the small intestine

Functions of the Stomach

• Serves as a short-term storage reservoir
• It grinds and mixes food with gastric secretions, resulting in liquefaction
• Slowly releases food into the small intestine for further processing
• Starts digestion of some foods, particularly proteins
• Releases iron from its compounds

Stomach Cell Types

• The stomach wall contains four main types of cells:
• Mucous cells secrete alkaline mucus that protects the epithelium.
• Parietal cells secrete hydrochloric acid and intrinsic factor
• Chief cells secrete pepsin, a proteolytic enzyme
• G cells secrete the hormone gastrin

Gastric Secretion

• About 2.5 liters of gastric juice are secreted into the stomach daily
• Gastric juice consists of mucus (bicarbonate-rich to protect and lubricate), hydrochloric acid (for acidic medium and pepsinogen activation), pepsinogen (inactive protease activated into pepsin), and intrinsic factor (for vitamin B12 absorption)
• The stomach secretes hormones (like gastrin) into the blood, which controls acid secretion and gastric motility

Control of Gastric Secretion

• Gastric secretion is divided into three phases:
• Cephalic phase: triggered by seeing, smelling, or anticipating food and prepares the stomach for a meal ; employs parasympathetic stimuli via the vagus nerve
• Vagus nerve stimulation releases ACH, stimulating gastrin and histamine secretion, which then stimulates gastric juice secretion
• Gastric phase: occurs when a meal enters the stomach, causing distension and mucosal irritation, which stimulates further gastric juice secretion by increasing gastrin and histamine
• Intestinal (inhibitory) phase: the small intestine detects highly acidic or hyperosmolar contents and sends inhibitory impulses and hormones to the stomach to inhibit secretion

Abnormalities of Gastric Secretion

• Excessive secretion or damage to the protective alkaline mucous barrier may result in peptic ulcer
• Drugs like aspirin, alcohol, and bacteria (Helicobacter pylori) can damage the protective layer
• Drugs like cimetidine, ranitidine, and omeprazole can decrease acid secretion
• Cimetidine and ranitidine are histamine antagonists

The Small Intestine

• The small intestine, extending from the pylorus to the large intestine, is the longest section of the digestive tube
• It consists of the duodenum, jejunum, and ileum
• Digestion and absorption of almost all nutrients occur here
• It is essential for life

Digestion and Absorption in the Small Intestine

• Exposure to pancreatic enzymes and bile enables the digestion of all food particles
• The final stages of digestion occur on the surface of the small intestinal epithelium (the brush border)
• Intestine’s absorptive surface area (brush border) is about 250 square meters
• Mucosal folds, villi, and microvilli increase the surface area

Absorption in the Small Intestine

• It is the primary site for nutrient absorption, where all nutrients are absorbed into the blood across the mucosa
• Absorbs water and electrolytes

Absorption of Water and Electrolytes

• The small intestine absorbs great quantities of water
• A normal person ingests roughly 1 to 2 liters of dietary fluid every day
• The small intestine receives another 6 to 7 liters daily from secretions
• Approximately 80% of the fluid is absorbed by the time the content enters the large intestine
• Water absorption is dependent on the absorption of solutes, particularly sodium
• Interference with water absorption can cause diarrhea

The Pancreas

• The pancreas consists of two main parts:
• Exocrine pancreas that secretes pancreatic juice important for digestion
• Endocrine pancreas that secretes hormones (insulin and glucagon) for glucose metabolism

Pancreatic Juice

• About 1.5 liters of pancreatic juice are secreted daily into the duodenum by the pancreas exocrine
• Pancreatic secretion and bile enter the duodenum through a common opening
• Pancreatic juice is alkaline (rich in bicarbonate)
• Pancreatic juice contains amylase, trypsin, chymotrypsin, and lipase

Functions of Pancreatic Juice

• Neutralizes acids coming from the stomach to the duodenum via Sodium Bicarbonate
• Prevents development of duodenal ulcer
• Adjusts the PH for the action of pancreatic enzymes
• Responsible for Protein digestion (trypsin, chymotrypsin, carboxypeptidase enzymes)
• Responsible for Lipid digestion (pancreatic lipase)
• phospholipase A2 is also secreted
• Responsible for Carbohydrate digestion via pancreatic amylase

Control of Pancreatic Secretion

• Secretin, produced by the duodenum, stimulates HCO3 secretion
• Cholecystokinin(CCK), produced by the duodenum, stimulates enzyme secretion
• Parasympathetic (Vagus) stimulates enzyme secretion

Functions of the Liver

• Carbohydrate metabolism: glucose to glycogen and vice versa
• Fat metabolism: desaturation of fat
• Protein metabolism: deamination of amino acids
• Removing nitrogenous portion to form urea
• Transamination: Remove the nitrogenous portion to form carbohydrate molecules
• Synthesis of plasma proteins and most of the blood clotting factors
• Breakdown of erythrocytes and defense against microbes (Kupffer cells)
• Detoxification of drugs and noxious substances (ethanol)
• Inactivation of hormones (insulin and sex hormones)
• Synthesis of vitamin A from carotene
• Production of heat
• Responsible for Bile secretion and synthesis
• Storage: Fat-soluble vitamins, Iron and copper, and some water-soluble vitamins

Bile

• Bile is a yellow secretion formed in the liver
• It is collected by the bile ducts and secreted into the duodenum
• Bile is stored in the gall bladder between meals

Composition of Bile

• The bile consists of bile salts and bile pigments
• Bile salts emulsify fat
• Bile pigments (bilirubin) are waste products excreted in stools

Control of Bile Secretion

• Bile secretion is stimulated by the vagus nerve and the hormone cholecystokinin
• Secretion starts when a meal enters the duodenum
• Obstruction in the bile ducts causes jaundice due to increased bile pigments in the blood

Digestion and Absorption of Carbohydrates

• Polysaccharides and disaccharides must be digested to monosaccharides
• Starch, the main dietary carbohydrate, digestion starts in the mouth by the salivary amylase
• Pancreatic amylase continues the digestion of carbohydrates
• Amylase breaks down starch into maltose
• Enzymes on the wall of the small intestine (the brush border): lactose (milk sugar), sucrose (table sugar), and maltose
• Maltase cleaves maltose into two molecules of glucose
• Lactase cleaves lactose into a glucose and a galactose
• Sucrase cleaves sucrose into a glucose and fructose
• Monosacharides (glucose, galactose, and fructose) are absorbed into the intestinal cells and then into the blood
• Lactose intolerance (abdominal discomfort and diarrhea following intake of milk) is caused by the deficiency of lactase

Digestion and Absorption of Proteins

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

Digestion and Absorption of Lipids

• Triglyceride is a dietary lipid with glycerol and 3 fatty acids
• The foodstuffs contain cholesterol, fat-soluble vitamins
• The two processes that must occur for the triglyceride to be absorbed: emulsification and digestion
• Emulsification is dietary triglyceride , that broken down due to aqueos enviornment
• Triglyceride molecules undergo enzymatical breakdown to produce monoglyceride and fatty acids
• Emulsification is done by bile salts
• Bile salts have water and fat-soluble poles
• Dietary lipids are micelles
• Digestion involved pancreatic

Absorption of Lipids

• Digestion of triglycerides into monoglycerides and fatty acids occurs
• Monoglycerides, fatty acids form structures with bile salts and other lipids called micelles

Chylomicrons

After entry into the intestinal cells, lipids combine with proteins to form structures inside cells

• Chylomicrons enter the lymphatics before, and chort chain fatt acids may enter the bloodstream directly

Functions of the Large Intestine, Rectum, and Anal Canal

• Involves water absorption until semisolid feces are produced

• Mineral salts, vitamins, and some drugs are absorbed Involves microbial activity- certain types of bacteria are synthesized by bacterial and synthesized form vitamin K and folic acid

• Gases produced by bacterial fermentation pass out as flatus

Mass Movement and Defecation

• Strong peristalsis sweeps along the transverse colon
• external anal sphincter is under conscious control through the pudendal nerve
• Defecation involves themuscle contraction of the rectum
• Abdominal mmuscles and lowering of the diaphragm assist the process of defecation

Feces

• Normal feces consist roughly of 75% water and 25% solids

• Buld is fecal solids are made up of bacteria and undigested organic matter/fibers

• Brown cholor is due to stercobilinogen by bacterial degradation

• Odor is due to gases proudced by bacterial metabolism

Intestinal Gas

• Is present in the gastrintestinal tract
• Common complaints of excessive gas by people is common
• Five major intestinal gases are nitrogen, oxygen, carbon dioxide, hydrogen, and methane