U9 - Digestive System

Functions of the digestive system

• Ingestion (take in food)

• Propulsion (movement of food)

• Digestion/absorption (break down food and take in nutrients)

• Egestion (elimination of waste)

Layers of the alimentary canal (superficial to deep)

• Serosa (membrane that secretes a slippery/watery fluid, decr friction as digestive organs contract/move)

• Muscular (circular & longitudinal layer)

• Submucosa (has nerve fibers/glands/blood vessels)

• Mucosa (secretes mucus/hormones/digestive enzymes, absorbs nutrients, protects from pathogens w/ lymphatic tissue) _{^{(*has rugae folds)}}

Teeth

• accessory digestive organs

• begin mechanical digestion process

• Incisors: cut food

• Canines (cuspids): tear/pierce

• Premolars (bicuspids): crush

• Molars (tricuspids): grind

Tooth anatomy

• Crown

• Enamel (hard, protects tooth)

• Dentin (calcified connective tissue)

• Cementum (covers the dentin in the root)

• Pulp cavity (has blood vessels & nerves)

• Gingivae (gums) (provides seal around tooth)

• Bone of jaw

• Root

!!! alimentary canal = mouth, pharynx, esophagus, stomach, small intestine, large intestine
accessory organs = teeth, tongue, salivary glands, liver, gallbladder, pancreas

Child teeth - toddler’s only 20 compared to adult 32. Get incisors, canines, then molars (teens/early 20s), which are replaced w/ permanent teeth

epiglottis blocks larynx, uvula blocks nasal cavity → food travels to esophagus and not respiratory system

Other digestive structures in the mouth

• Salivary glands

  • make saliva

    • 99% water

    • mucus lubricates food for swallowing

    • amylase breaks down starch (carbs)

    • lysozymes kill bacteria, antibodies mark foreign invaders

• Hard palate - bony roof of the mouth

• Soft palate - rest of the mouth

• Uvula - prevents swallowed food from entering the nasal cavity

• Tongue - helps to push food down towards the esophagus

What happens during deglutition?

Swallowing

• Buccal phase - chew (mastication), food mix with saliva to make bolus (food lump), tongue forces into pharynx

• Pharyngeal-esophageal phase

Peristalsis

bolus propelled down esophagus in waves of muscle contractions
_{^{*involuntary, by longitudinal muscles, caused by PNS (parasympathetic nervous system) not gravity}}

Stomach and its anatomy (4)

• muscular sac w/ thick walls

• mechanical/chemical digestion

• Sphincters = thick rings of muscle, serve as gatekeepers to allow food to enter/leave at appropriate times

  • Cardiac (food enters, separates esophagus from stomach)

  • Pyloric (food leaves, separates stomach from small intestine)

Mechanical digestion: Slippery outer layer of stomach? Layers of muscle in stomach? What do the muscles do?

• serosa

• .

• Longitudinal muscularis

• Circular muscularis

• Oblique muscularis

• .

• muscles churn food (maceration) and propel towards small intestine

• rugae - folds of the mucosa that stretch when the stomach is full _{^{(in pic, bottom of stomach)}}

Chemical digestion: what are the specialized gastric gland cells (gastric juices) in the mucosa? What do they make?

• mucous cells - secrete mucus to protect stomach lining

• chief cells - secrete pepsinogen (inactive enzyme)

• parietal cells

  • secrete HCl to kill microbes in food

  • convert pepsinogen into the digestive enzyme pepsin, which breaks down food proteins

• .

• squeezing of stomach + gastric juices makes soupy mixture called chyme

Small intestine

• where most digestion and nutrient absorption happens

• Duodenum, jejunum, ileum

• Villi - tiny/finger-like projections in the lining of the small intestine, filled w/ blood vessels, absorb nutrients

• Joins large intestine at ileocecal sphincter _{^{(ileum+cecum)}}

Hormones/enzymes in the small intestine

_{^{many glands line the small intestine and secrete digestive hormones}}

• mucus - protects from pathogens

• secretin - inhibits release of gastric juices when chyme is very acidic

• cholecystokinin - causes gallbladder to release bile

• Intestinal juices

  • maltase/sucrase/lactase - break down sugars

  • peptidase/enterokinase - break down proteins _{^{(protein, think peptide (peptidase!))}}

What happens to chyme in small intestine?

• slowed down b/c small intestine needs time to absorb nutrients

• circular muscles in intestinal wall cause segmentation of the chyme _{^{(in contrast to longitudinal muscles involved in peristaltic contractions)}}

Large intestine

• Sections: Cecum, colon (parts: ascending, transverse, and descending), r3ctum

• Functions

  • absorb water back into body

  • remove waste (feces) thru the r3ctum/@nus

• Bacteria produce vit B & K

• Lumpy look b/c of haustra

• Haustral contractions - slow, segmenting, last ~30min after food moves into large intestine

• Mass movements - slower, powerful, push waste to r3ctum 3-4 times/day

R3ctum and anu$

• collects/stores undigested waste & bacteria

• r3ctum fills, pressure placed on internal @nal sphincter, it relaxes

• external @nal sphincter needs to voluntarily relax to release feces thru @nus

Liver

• major lobes=L & R lobes

• secretes bile (greenish fluid that breaks down fats)

• functions of liver cells

  • remove bacteria/old RBCs

  • detoxify blood from drugs/poisons

  • store glycogens/iron/copper

  • manufacture proteins/produce fats

• has hepatic sinusoids, which are full of blood

Gallbladder

• small green sac attached to liver by bile duct _{^{(liver makes bile, gallbladder stores it)}}

• stores bile before sending to small intestine to digest fats

• concentrates bile by removing water (too much water removed, gallstones made of cholesterol crystals may form)

Pancreas

• long/thin gland behind stomach

• secretes digestive enzymes & sodium bicarbonate (base that neutralizes stomach acid so the digestive enzymes are not affected by the lowered pH)

• secretes hormones that regulate blood sugar levels (insulin)

Digestive system/accessory organ anatomy

• liver

• gallbladder

• pancreas

• stomach

• small intestine _{^{(towards it; the opening)}}

Metabolism

life-sustaining chemical reactions of the body

• catabolism (break down complex molecules while releasing energy/ATP)

• anabolism (form complex molecules while using energy)

Nutrients needed by the body

nutrients provide the body with energy and can be used for growth

• macronutrients: (need in large amts)

  • carbs (sugars/glucose, main source of E)

  • lipids (fats, secondary source of E used for cell membranes/nerves/insulation/hormone production)

  • proteins (make up majority of cellular structures, some used as enzymes for chemical rxns)

  • water

• micronutrients: (need in small amts)

  • vitamins (organic molecules, broken down by heat/air/acid)

  • minerals (inorganic, ex: Ca, Fe, Mg, K, Zn, etc.)

How macromolecules are digested

• Carbs

  • broken down by

    • salivary amylase (mouth)

    • pancreatic amylase & epithelial enzymes (brush border enzymes) (small intestine)

  • process: cellular respiration breaks down carbs (glycolysis, citric acid cycle, electron transport chain). excess sugar stored as fat or glycogen

• Lipids

  • broken down by

    • bile salts (liver & small intestine)

    • pancreatic lipase (small intestine)

  • process: lipids are insoluble, must be emulsified first. broken into acetic acid then metabolized into ATP or stored for later use

• Proteins

  • broken down by

    • pepsin (stomach)

    • pancreatic enzymes & epithelial enzymes (aka brush border enzymes) (small intestine)

  • process: polypeptide chains are broken into indiv. amino acids then actively pumped into the cell for immediate use

Use of water

• chemical rxns

• dissolve foods for digestion

• maintain blood pH levels

• regulate body temp thru sweat

!!! SI - primarily absorbs nutrients, LI primarily absorbs water/electrolytes/vitamins

bariatric surgery - 40+ BMI qualify, most common types = sleeve gastrectomy (remove majority of stomach) and gastric bypass ('pouch' of stomach connected directly to small intestine, bypass duodenum)

homeostasis - eat carbs & blood glucose levels rise, pancreas makes more insulin, body cells incr glucose absorption, blood glucose levels drop, pancreas makes less insulin, glucose absorption in body cells slows, higher blood glucose levels (insulin tells body cells to take in glucose from blood)

type 1 diabetes - less insulin, cells absorb less glucose, more glucose in blood _{^{(less cellular respiration w/ glucose b/c cells take in less glucose, use fat instead, weight loss)}}

use antibiotics, can kill good bacteria that absorb vitamins, can cause vitamin deficiency

Meckel's Diverticulum - congenital pouch in small intestine (commonly confused w/ appendicitis)