Human Digestion and Absorption

Learning Outcomes

• Outline the roles played by the gastrointestinal tract and the related accessory organs (liver, gallbladder, and pancreas) in digestion and absorption.

• Describe how foods are moved along the digestive tract.

• Explain the 4 main types of absorption.

Learning Outcomes

• Identify the key enzymes and hormones involved in digestion and absorption and their functions.

• Identify major nutrition-related gastrointestinal diseases and disorders and typical approaches to prevention and treatment.

• Explain why diarrhea represents a serious health challenge to infants and young children around the world.

Organization of the Human Body

• Chemical Level– Atoms combine to form molecules

• Cell Level– Molecules form organelles– Use ATP

• Tissue Level– Similar cells make up tissues

• Organ Level– Different tissues combine to form organs

• Organ System Level– Organs make up an organ system

• Organism Level– Organ systems make up an organism

Human Tissues

• Epithelial– Cells that cover surfaces inside and outside of the body

• Connective– Support and protect the body by holding structures

together• Muscle

– Contract and relax to permit movement• Nervous

– Transmit nerve impulses

Digestive Organ System

• GI tract–Mouth, esophagus, stomach, small

intestine, large intestine• Liver, pancreas and gall bladder

Digestive System Overview

• Digestion– Process of breaking down foods into a form the

body can use

• Absorption– Uptake of nutrients from the GI tract into the

blood or lymph

• Excretion of waste matter• Immune system function

Anatomy of the GI Tract

• GI tract = the alimentary canal– 15’ long, hollow, muscular tube

• Four layers– Mucosa

• Innermost layer: forms hollow area of the lumen– Submucosa

• Contains blood vessels (carry nutrients), nerves, and glands– Muscle (as double layers)

• Moves food forward– Serosa

• Outside layer;-protects the tract

Sphincters

• Ring like muscles that control the flow of contents in the GI tract– Lower esophageal sphincter– Pyloric sphincter– Sphincter of Oddi– Ileocecal sphincter– Anal sphincter

GI Motility:Mixing and Propulsion

• Peristalsis– Contractions

• Segmentation– Back and forth movement

• Mass movements– Peristalsis over widespread area

• Elimination• Vomiting

Digestive System Secretions

• Saliva• Mucus• Digestive enzymes• Hydrochloric acid• Bile• Bicarbonate ions• Hormones

Mouth (Oral Cavity)

• Chewing increases surface area• Mixed with saliva food becomes a bolus• Saliva

– Lysozyme• Breaks down bacteria

– Mucus• Lubricates and hold bolus together

– Amylase• Breaks down starch

– Enhances perception of flavor

Taste and Smell

• Taste buds on the tongue and soft palate – contain taste-receptor cells– Salty– Sour– Sweet – Bitter– Umami

• Olfactory cells in nose (smell)– Stimulated with chewing

Esophagus

• Swallowing– Moves bolus from the mouth to the esophagus

• Epiglottis– Prevents food from entering the trachea– Closes over the larynx

Stomach

• Food bolus enters stomach through lower esophageal sphincter

• Holding & mixing tank– Mixed with stomach secretions food becomes

chyme

Stomach Secretions

• Parietal Cells– Hydrochloric Acid

• Inactivates proteins• Destroys bacteria and viruses• Dissolves minerals to aid in absorption• Converts pepsinogen into pepsin

– Pepsinogen• Protein digesting enzyme

• Gastrin– Hormone that controls release of HCl and pepsinogen

Other Stomach Secretions

• Chief cells– Gastric lipase

• Mucus– Protects the stomach from being digested– Production relies on prostaglandins

Small Intestine• Most digestion and absorption occurs here• Pyloric sphincter allows chyme into the

small intestine– Gastric inhibitory peptide (hormone) slows

release of chyme• Sections

– Duodenum, jejunum and ileum

Small Intestine (2)

• Circular folds• Villi – lined with:

– Goblet cells - make mucus– Endocrine cells - produce hormones– Enterocytes - produce digestive enzymes and

absorb nutrients• Contain a brush border of microvilli covered with

glycocalyx

Liver, Gallbladder and Pancreas

• Liver– Provides bile– Enterohepatic circulation (recycling of bile)

• Gallbladder– Bile storage

• Pancreas– Produces sodium bicarbonate, lipases, proteases

and pancreatic amylase

Hormones of the GI Tract

• Gastrin– Stomach release of HCl and pepsinogen

• Cholescystokinin (CCK)– Release of bile

• Secretin– Release of pancreatic bicarbonate

• Gastric Inhibitory Peptide– Limits release of gastric juices

Absorption

• Primarily occurs in small intestine• Passive diffusion

– Concentration gradient• Facilitated diffusion

– Concentration gradient + carrier protein• Active

– Carrier protein + energy (regardless of concentration)• Endocytosis

– Engulfment of compounds or liquids

Moving Nutrients around the Body

• Cardiovascular System– Includes heart, blood

vessels and blood– Water-soluble

nutrients transported via capillaries in villi to portal vein

• Lymphatic System– Includes lymph– Fat-soluble nutrients and large particles

transported via lacteals into the lymph vessels to thoracic duct

Large Intestine

• Ileocecal valve• Colon

– Cecum, ascending colon, transverse colon, descending colon and sigmoid colon

• Rectum• Anus

Functions of Large Intestine

• Absorption of water and electrolytes• Formation and expulsion of feces• Housing of bacteria (microbiota)

Microbiota (bacterial flora)• Beneficial bacteria

– Control pathogenic bacteria– Synthesize Vitamin K and Biotin– Aid lactose digestion and fermentation of dietary fibers

• Probiotics– Live bacteria in food and supplements– Health benefits

• Prebiotics– Non digestible carbohydrates in food that promote the

growth of bacteria-– E.g. inulin, resistant starch

When Digestive Processes Go Awry

• Heartburn and Gastroesphogeal reflux disease (GERD)– Foods that increase reflux:

• Citrus, caffeine, chocolate, fatty foods, spicy foods, onion, garlic and tomato based foods

• (Peptic) Ulcers– Causes: H. pylori and NSAID medications– Treatment:

• Medications • Avoid foods that increase symptoms

When Digestive Processes Go Awry (2)

• Food Intolerances• Intestinal Gas

(Flatulence)• Constipation

– Fiber, fluid and exercise

– Laxative use

• Diarrhea– Replace fluid and

electrolytes

• Irritable Bowel Syndrome (IBS)– Cause unknown

• Inflammatory Bowel Disease (IBD)– Ulcerative colitis and Crohn’s disease

• Hemorrhoids• Gallstones

Celiac Disease

• Intolerance to gluten• 1 in 133 people affected

– Many undiagnosed

• Can affect many body systems– GI and others

• Gluten-free diet for life– Corn, rice, quinoa, and buckwheat OK