lecture 7: digestive system

Digestive System Components

Digestive System Components

consists of the mouth, pharynx, oesophagus, stomach, small intestine, and large intestine (including the rectum and anal canal). Accessory organs include the salivary glands, pancreas, liver, and gall bladder, which support digestive functions.

Digestion Processes

Involves ingestion, secretion of digestive juices, mixing and propulsion, digestion (mechanical and chemical), absorption, and elimination.

Gastrointestinal Tract (GIT) Layers

Consists of mucosa, submucosa, muscularis, and serosa (peritoneum) with specific functions and structures.

Epithelium

he innermost layer of the GIT. It varies by location: protective epithelium in the mouth, pharynx, esophagus, and anus, and columnar epithelium in the stomach and intestines. This epithelium includes microvilli for absorption, goblet cells for mucus secretion, and enteroendocrine cells for hormone release. cells renew every 5-7 days.

Lamina Propria

connective tissue with blood and lymphatic vessels for nutrient absorption. It contains MALT (mucosa-associated lymphoid tissue), which is a collection of immune cells. In the GIT, MALT is referred to as GALT (gut-associated lymphoid tissue), and it's found in areas like the tonsils, esophagus, small intestine, appendix, and large intestine.

GALT

contains 70% of the body’s immune cells and is part of the first line of defense against ingested pathogens. It makes up around 25% of the intestinal mucosal mass.

Muscularis Mucosa

a thin layer of smooth muscle that creates small folds (villi) to increase surface area for absorption and digestion. Its movement ensures that absorptive cells are fully exposed to the GIT contents.

Submucosa layer

a connective tissue layer containing blood and lymph vessels that absorb food molecules. It has a network of neurons (submucosal plexus), and may contain glands and lymphatic tissue like Peyer's patches in the ileum.

Muscularis Externa

contains skeletal muscle in the mouth, pharynx, upper oesophagus, and anal sphincter for voluntary control. The rest of the GIT has smooth muscle, with inner circular and outer longitudinal layers. It aids in mixing food, propelling it via peristalsis, and has neurons in the myenteric plexus.

Peritoneum

the body's largest serous membrane, with two layers: parietal (covering the abdominal and pelvic walls) and visceral (covering the organs). It provides a barrier against infection and contains a lubricating fluid in the peritoneal cavity.

Ascites

Fluid accumulation in the peritoneal cavity, caused by liver cirrhosis, GIT malignancies, heart failure, or pancreatitis.

Peritonitis

Acute inflammation of the peritoneum, often caused by bacterial infection, ruptured appendix, friction, or surgical wounds.

Greater Omentum

The largest peritoneal fold, draping over the transverse colon and small intestine. It stores fat, contains lymph nodes for immune defense, and has four layers due to folding.

Lesser Omentum

A peritoneal fold that connects the stomach and duodenum to the liver. It contains the hepatic portal vein, common hepatic artery, bile duct, and lymph nodes.

Enteric Nervous System

The "brain of the gut" with 100 million neurons, controlling digestive functions from the oesophagus to the anus. It operates independently but is influenced by the autonomic nervous system, with parasympathetic stimulation increasing activity (e.g., peristalsis), and sympathetic stimulation decreasing it.

Plexuses (networks)

The enteric nervous system contains two of these: the myenteric plexus, located between the circular and longitudinal muscle layers, which controls gut motility by regulating the strength and frequency of muscle contractions, and the submucosal plexus, found in the submucosa, which regulates digestive secretions and detects sensory information in the digestive tract.

Enteric nervous system: neurons

controlls peristalsis (myenteric) and secretions (submucosal), sensory neurons detecting changes via chemoreceptors and stretch receptors, and interneurons linking the plexuses.

GIT blood supply

receives blood from abdominal aorta branches, including mesenteric arteries. Nutrient-rich blood returns via the portal vein to the liver for filtering and processing or through iliac veins from the lower rectum and anal canal.

Oral cavity

lined with mucous membranes, includes the hard and soft palate, uvula, and tongue. It supports chewing, speech, taste, and swallowing by moving the bolus into the pharynx.

Teeth

Adults have 32 teeth (20 temporary), each with a crown, neck, and root. Teeth consist of dentin, covered by enamel (hardest tissue). They sit in the gum and periodontal membrane and aid in chewing, supporting digestion.

Saliva

produced by salivary glands, is controlled by the autonomic nervous system. Parasympathetic activity promotes lubrication, while sympathetic reduces it, causing dryness. Dehydration stops salivation, triggering thirst, and sensory cues stimulate it.

Saliva composistion

It is 99.5% water with mineral salts, enzymes (amylase, lipase, lysozymes), mucus, IgA, blood clotting factors, and a mildly acidic pH of 6.35–6.85.

Saliva functions

it aids digestion, lubricates food, cleanses the mouth, defends (IgA, lysozymes), enhances taste, buffers acidity, and removes waste (urea, uric acid).

Oesophagus

The 25cm muscular tube connects the larynx to the stomach, moving food via peristalsis. The epiglottis blocks the trachea, and the lower sphincter prevents stomach reflux.

Stomach

A J-shaped organ with three muscle layers and regions (cardia, fundus, body, pyloric). It secretes acidic gastric juice and mucus, with two sphincters. Neural feedback regulates peristalsis and juice flow. The lining renews every three days.

Pariteal cells

Secrete intrinsic factor (IF) for vitamin B12 absorption and hydrochloric acid (HCl). HCl activates pepsin, acts as an antimicrobial, aids iron absorption, and supports bile and pancreatic juice flow. Vitamin B6 and zinc are used to produce HCl.

Cheif cells

Secrete pepsinogen and gastric lipase. Pepsinogen is converted to pepsin by HCl, which digests proteins, while gastric lipase aids in lipid digestion.

Goblet cells

Secrete mucus, which protects the stomach lining from acid.

Stomach functions

Mixes and stores food, provides defense through acidity, absorbs limited substances (water, alcohol, drugs), aids in mechanical and chemical digestion (proteins, lipids), makes iron soluble, and signals satiation.

Stomach hormones

Ghrelin stimulates hunger, and gastrin (from G cells) increases gastric juice secretion and motility in response to stomach distension.

Pancreas

about 15cm long and retroperitoneal, connects to the duodenum via the pancreatic duct. It has exocrine functions, secreting 1.2-1.5L of pancreatic juice with enzymes and bicarbonate for digestion, and endocrine functions, releasing hormones like insulin and glucagon to regulate blood sugar.

Pancreatic enzymes

include amylase, which breaks down starches into sugars; lipase, which digests fats; trypsin and chymotrypsin, both responsible for protein digestion; ribonuclease, which digests RNA; and deoxyribonuclease, which digests DNA.

Gallbladder

7-10 cm long, stores bile produced by hepatocytes. It releases bile into the duodenum via the common bile duct when fat is ingested, aiding fat digestion. Bile, composed of bile salts, cholesterol, and bilirubin, is mostly reabsorbed in the ileum and returned to the liver.

Small intestine

about 6.5m long, is where most digestion and absorption occur. It has three regions: the duodenum (30cm) for emulsification and digestion, the jejunum (2.5m) for absorption, and the ileum (3.5m) where vitamin B12 is absorbed. The small intestine moves food, digests and absorbs nutrients, regulates hunger, and supports immunity.

Villi

Finger-like projections in the small intestine that increase surface area for absorption.

Brush border enzymes

Enzymes attached to the intestinal lining for digestion, including maltase, sucrase, and lactase. Microvilli (brush border) are projections of absorptive cell membranes. They increase the surface area for absorption.

Cholecystokinin (CCK)

a hormone secreted by enteroendocrine cells in the duodenum in response to high-fat or protein chyme. It stimulates the pancreas to secrete digestive enzymes, increases bile production in the liver, and stimulates gallbladder contraction for fat digestion. CCK also helps mediate satiety.

Small Intestine: Absorption

absorbs 90% of nutrients. Carbs and amino acids enter capillaries, fats and vitamins enter lacteals, and vitamin B12 is absorbed in the ileum. Water, vitamins, and minerals go to the liver, while bile salts are recycled.

Fluid movement

Water absorption in the GI tract occurs via osmosis, depending on electrolyte, monosaccharide, and amino acid levels to maintain an osmotic balance with the blood.

Large intestine

it absorbs water, minerals, vitamins, and some drugs, and forms stool. It has four regions: caecum (with appendix), colon, rectum, and anal canal. The ileocaecal valve controls flow from the small intestine.

Large intestines: microbes

it hosts trillions of microbes, mostly symbiotic, that aid in nutrient extraction and fermentation. They produce short-chain fatty acids, gases (like CO2 and methane), and contribute to faecal odor. Some bacteria also produce vitamins (B12, K2) and fatty acids, with faeces being 30–50% bacteria.

Defecation

occurs when mass peristalsis moves faecal matter into the rectum, stimulating stretch receptors. This triggers a reflex, causing rectal, abdominal, and diaphragm muscles to contract, increasing pressure and opening the internal sphincter. The external sphincter is voluntarily relaxed.

Liver

the body's heaviest gland, filters 1.4 liters of blood per minute. It receives oxygenated blood from the hepatic artery and nutrient-rich blood from the portal vein. Kupffer cells remove foreign bodies, and blood is filtered and metabolized before entering systemic circulation.

Liver functions

it has over 500 functions, including cleansing blood, detoxification, bile production, and synthesis of plasma proteins. It regulates hormones, metabolizes glucose, fats, and amino acids, produces heat, synthesizes vitamins, and stores vitamins, minerals, and glycogen.

Liver metabolism

stores excess glucose as glycogen, synthesizes cholesterol and triglycerides, and converts essential amino acids to non-essential ones. It also forms urea from amino acids and uric acid from nucleotides for excretion.

Liver detoxification

detoxifies toxins by turning them into non-toxic metabolites for excretion. Hydrophilic toxins are excreted directly, while lipophilic ones are changed into water-soluble compounds in two phases. Phase I uses CYP450 enzymes, and Phase II adds molecules to make toxins easier to excrete.

Abdominal quadrants

it is divided into nine regions to help pinpoint the location of symptoms. These are split by three lines: the upper (right/left hypochondrium, epigastric), middle (right/left flank, umbilical), and lower (right/left groin, pubic).

Monosaccharides

Single sugar units, e.g., glucose, fructose, galactose.

Disaccharides

Two sugar units, e.g., maltose, sucrose, lactose.

Polysaccharides

Many sugar units, e.g., starch, glycogen (digestible), cellulose (indigestible fiber).

Cellulose

A plant structural material found in cell walls. Humans can't digest it, so it acts as fiber to aid intestinal movement.

Glycogen

The primary short-term energy storage, made of around 60,000 glucose molecules. It is stored in the liver (for blood sugar regulation) and muscles (for muscle energy use).

Dietary lipids

include triglycerides, the main fat in our diet, made of glycerol and three fatty acid chains (saturated or unsaturated); phospholipids, which have two fatty acid tails and a phosphate head; and cholesterol, a steroid vital for cell membranes, vitamin D, and hormone production. Lipids provide energy, insulation, protect organs, and support cell membranes.

Cis and trans fatty acids

have hydrogen atoms on the same side of the double bond, while trans fatty acids have them on opposite sides. Most natural fats are cis, which the body can use, but trans fats, often created in labs, are linked to heart disease. Look for hydrogenated vegetable oil on food labels.

Proteins

made of amino acids, make up 15% of body mass. They need to be denatured before being broken down and absorbed. Functions include immunity, structure, enzymes, hormones, neurotransmitters, and energy.

Enzymes

biological catalysts that speed up reactions. They are substrate-specific (lock and key), often named after their substrate with "-ase" (e.g., lactase), or "-in" for some protein-digesting enzymes (e.g., pepsin). They remain unchanged in reactions but can be denatured by pH or temperature changes.

Peristalsis

Involuntary muscle contractions that propel food along the digestive tract.

Enteric

Refers to the enteric nervous system, which controls gut functions independently but is regulated by the autonomic nervous system.

Mesenteric Arteries

Arterial blood supply to the intestines, returning nutrient-rich blood via the portal system or iliac veins.

Saliva Composition

Contains water, mineral salts, enzymes, mucus, immunoglobulins, and blood clotting factors, aiding in digestion and oral health.

Protease enzymes

Enzymes involved in protein digestion, including trypsin, chymotrypsin, ribonuclease, and deoxyribonuclease.

Pancreatic lipase

Enzyme responsible for lipid/fat digestion.

Pancreatic amylase

Enzyme that breaks down starches into sugars.

Insulin and glucagon

Hormones involved in regulating blood sugar levels.

Somatostatin

Growth-hormone-inhibiting hormone secreted by the pancreas.

Small intestine functions

Includes digestion, absorption of nutrients and water, hunger/satiety regulation, and immunity.

Intrinsic factor

Substance needed for vitamin B12 absorption, produced by the stomach.

Dietary Lipids

Predominant dietary lipid composed of glycerol and three fatty acid chains, which can be saturated or unsaturated.

Phospholipids

Lipids composed of two fatty acid tails and a phosphate head, digested to free fatty acids and absorbed.

Cholesterol

Steroid found in animal foods, essential for cell membrane integrity, vitamin D synthesis, and sex hormone synthesis.

Proteins

Long molecules of amino acids making up 15% of total body mass, crucial for immunity, structural support, and various functions.

Enzymes

Biological catalysts, usually proteins, that speed up chemical reactions, highly specific and not consumed in reactions.