HUMB C3

Digestive System Anatomy

Continuous tube with accessory organs (liver, gallbladder, pancreas, salivary glands)

Oral cavity (mouth) with salivary glands

Pharynx (throat)

Esophagus

Stomach

Small intestine (duodenum, jejunum, ileum,) with liver, gallbladder and pancreas

Large intestine (cecum, colon [ascending, transverse, descending, and sigmoid], rectum and anal canal)

Anus

Digestive System Functions

Ingestion

Mastication

Secretion

Digestion

Absorption

Elimination

Ingestion

introduction of food into stomach via oral cavity

Mastication

chewing to facilitate chemical digestion via increasing surface area

Secretion

lubricate, liquefy, digest (mucus lubricates food along entire digestive tract)

Digestion

Mechanical and chemical digestion of food into nutrients

Absorption

Movement of nutrients from digestive tract to cells

Elimination

Waste products removed from body as faeces

Digestive Tract Histology

Mucosa

Submucosa

Muscularis

Serosa/Adventitia

Mucosa

Innermost layer, avascular, mostly simple columnar but stratified squamous in anus and oral cavity (abrasive)

Submucosa

Connective tissue, vascular

Muscularis

circular and longitudinal smooth muscle layers for peristalsis (food movement), stomach also has oblique layer for circular churning motion

Serosa/Adventitia

Outermost layer, stabilises tract in place, attaches organs to other organs or parts of body

Peristalsis

Unidirectional wave-like contractions that propels food through the gastrointestinal tract

Segmentation

Back and forth contractions that mixes and breaks down food, primarily in the intestines

Peritoneum

Epithelial tissue

Visceral peritoneum: Covers organs

Parietal peritoneum: Covers interior surface of body wall

Mesenteries: double fold in peritoneum connecting organs together and small intestine to body wall, route for vessels and nerves to pass from body wall to organs

Omentums

Greater omentum: connects stomach to transverse colon

Lesser omentum: connects stomach to liver & diaphragm

Oral cavity

Mastication

Hard Palate: Hard bone, anterior mouth roof

Soft palate: Soft muscle, posterior mouth roof

Uvela: back of soft palate, stop food entering nasal cavity

Teeth

Sets: Primary/milk in childhood and permanent/secondary as adult (32 teeth)

Types:

Molars - heavy grinding

premolars - crushing

canines - tearing

incisors - slicing, biting

Oral cavity – salivary glands

secrets saliva to protects oral cavity as well as moisten, lubricate, and digest food

Amylase: enzyme that breaks down carbohydrates into simpler sugars

Lysozyme: antibacterial enzyme

Major pairs: parotid (back of mouth), sublingual (under tongue), submandibular (under mandible)

Pharynx

connects oral cavity to esophagus

Uvula (back of soft palate) prevents food/drink from entering the nasopharynx

nasopharynx (behind nose), oropharynx (behind oral cavity), laryngopharynx (behind larynx)

Esophagus

connects pharynx to stomach

25cm long

posterior to trachea

Epiglottis prevents food from entering the trachea

End at gastro-esophageal sphincter/cardiac sphincter

Swallowing

Voluntary phase - Tongue pushes bolus to back of oral cavity towards oropharynx

Pharyngeal phase - Uvula closes off nasopharynx, bolus touches oropharynx receptors and swallowing reflex moves bolus down pharynx into esophagus, Epiglottis covers trachea

Esophageal phase - peristalsis moves bolus down esophagus towards stomach

Stomach

churns food into chyme (thick liquid) via oblique layer of smooth muscle (muscularis)

produces mucus, hydrochloric acid, protein digesting enzymes (pepsin)

mucus layer lubricates and protects epithelial cells on stomach wall from acid

Stomach Openings/Sphincters and Parts

Openings/Sphincters:

Gastroesophageal/cardiac: to esophagus

Pyloric: to duodenum (first part of small intestine)

Parts:

Cardiac (closest to esophagus)

Fundus

Body (main)

Pyloric: antrum and canal (attached to small intestine)

Stomach Layers

Visceral peritoneum, peritoneal fluid, parietal peritoneum, then body wall

Muscularis - Outer longitudinal, Middle circular, Inner oblique

Submucosa

Mucosa - arranged into gastric pits

Rugae: folds in stomach wall that allow stomach to stretch after eating

Stomach Movements (Gastric Phase)

80% mixing waves (blending), 20% peristaltic waves (propulsion)

esophageal and pyloric sphincters are closed

Water takes ~1-2hrs to exit

Stomach empties every ~4hrs - depending on food

Small intestine

~6m long

small diameter

folds increase surface area for efficient nutrient absorption

Connected to posterior body wall by mesenteries

Small intestine Divisions

Duodenum

Jejunum

Ileum

Duodenum

25 cm

starts at pyloric sphincter

in epigastric and umbilical regions

curves around pancreas

chyme mixes with digestive enzymes

mucous from Brunner’s glands (in submucosa) neutralise acidic chyme

Jejunum

2.5 m

in Left lumbar and umbilical regions

extensive villi for nutrient absorption

Ileum

3.5 m

in hypogastric region

Ends at iliocaecal junction (first part of large intestine)

iliocaecal junction

transition from end of small intestine (ileum) to start of large intestine (cecum)

Small intestine adaptations

Adaptations increase surface area to improve absorption

Plicae circulares- largest, circular folds in the wall of the small intestine

Villi– finger like folds of epithelium, contain capillaries and lacteals

Microvilli: small extensions on epithelial cell surface

Lipids move into lacteals go to lymph

Carbs and proteins move into capillaries go to blood

Liver

makes ~100ml of bile per day which enters gallbladder via cystic ducts then into duodenum via common bile duct to emulsify fats and lipids

Filters nutrient rich blood from intestine via portal system

Stores glucose (as glycogen) and lipids for energy

Detoxifies toxins

Gallbladder

inferior side of liver’s right lobe

muscular sac

stores and concentrates bile which is released after stimulation to duodenum

Pancreas

Produces insulin and glucagon for blood sugar homeostasis

produces digestive enzymes which move into duodenum via ducts

What enzymes does pancreas produce?

Lipase – breakdown lipids

Pancreatic amylase – breakdown carbohydrates

Trypsin – breakdown proteins

Large intestine

Absorption of water + electrolytes (e.g NaCl)

Extends from ileocaecal junction to anus

Consists of cecum, colon (ascending, transverse, descending, sigmoid), rectum, anal canal

Bacteria/microbes synthesise vitamin B & K

18-24hr for chyme to faeces

~1500 mL chyme enters the cecum daily, 90% reabsorbed

Defecation reflex triggered by rectal distention (full of faeces)

Goblet cells secrete mucous

Enzymes

protein catalyst that increases rate of chemical reaction without being permanently changed

Highly specific – active site can only bind to specific reactant so many different enzymes needed in the body for different chemical reactions

Often named with -ASE suffix to their reactant (lipase is enzyme breaking down lipids)

Nutrients

Chemicals taken into body to make ATP for energy and provide building blocks to build other molecules (skeletal muscle)

Macromolecules

carbohydrates, proteins, lipids

organic (contain carbon) nutrients

Other nutrient classes

vitamins, minerals, water

taken into body without being digested

Essential nutrients

must be taken into the body, because we can’t make them ourselves

some amino acids/fatty acids/carbs, water, most vitamins & minerals

Vitamins

organic

found in small quantities

essential

can be fat or water soluble

Too much - diarrhea (C), toxic during pregnancy (A)

Too little - scurvy (C), rickets (D)

Water soluble vitamins

Vitamin C

All vitamin Bs

Minerals

Inorganic

Major minerals >100 mg/day (Ca, Na)

Trace minerals < 100 mg/day (Zn, Cu)

Components of co-enzymes, haemoglobin

Used for resting membrane potential

Bone and teeth strength

Deficiencies - muscle weakness, anaemia

Carbohydrates

mono (glucose, fructose)

di (sucrose = glucose + fructose)

poly: glycogen (animals) = many glucose

starch (plants) - break down for energy

cellulose (fibre) - can’t break down

-saccharides (sugars)

Most come from plants

Contain 2H and 1O for each C

Carbohydrate absorption

polysaccharide digested into disaccharide by saliva and pancreatic amylase in duodenum, then digested to monosaccharide by enzymes in intestine to be absorbed into blood via villi/microvlli and transported to liver via hepatic portal vein

Carbohydrate Uses

Glucose produces ATP to produce energy (cell function, maintain homeostasis, contract muscles, brain activity)

Excess glucose stored as glycogen in muscle and liver cells, beyond this converts to fat

Sugars form nucleic acids (DNA, RNA) and glycoproteins + glycolipids for cell membranes

Lipids

triglycerides (glycerol + 3 fatty acids) make up 95% of fats in body

Fatty acid can be saturated (only C-C), unsaturated (C=C bond/s), or trans (artificially altered unsaturated)

Contain mostly C, H, O, sometimes N, P

mostly insoluble in water as lower ratio of O to C

oils, dairy, animal fat, eggs

Lipid absorption

digestion begins in duodenum where bile from gall bladder emulsifies

pancreatic lipase further breakdown into short chain fatty acids (mono/tri glycerides)

These are absorbed into lymphatic system via lacteals (centre of villi)

moved into cardiovascular system

excess stored in adipose tissue and liver

Lipid Uses

triglyceride produces ATP

Cholesterol: Component of plasma membranes, used to form bile

Phospholipids: major components of plasma membranes, myelin sheath (neuron covering to speed up info transmission), part of bile

Eicosanoids: inflammation, blood clotting, tissue repair, smooth muscle contraction

Proteins

chains of amino acids (polypeptides) linked by peptide bonds (C-N)

Contain amine group (NH2), carboxyl group (COOH), H and side group

Side group differentiates amino acids

Not stored in body

Essential (body can’t produce) and non-essential (synthesized from essential)

Complete (contain all 9 essential aa) and Incomplete food sources

Protein Functions

Globular proteins (haemoglobin)

Structural (muscle proteins or CT)

Cell membrane transport

Enzymes

Hormones

Antibodies

Protein absorption

Protein digested by pepsin in stomach

Polypetides digested by trypsin in duodenum

Peptides and individual amino acids absorbed into blood via intestine’s villi/microvilli

Water absorption

~9L enters digestive tract daily

99% is absorbed

Water can move in either direction across intestine wall

Transports ions (e.g Na, P, Ca)