Gastrointestinal and Liver Physiology

What does the Gastrointestinal system consist of

GI tract and accessory organs and tissues

GI tract

mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus

Accessory organs and tissues

salivary glands, liver, gall bladder, and exocrine pancreas

Major role of GI system

handle ingested food pat

How does water absorption along the lumen occur

by process of osmosis

Four major processes GI tract

digestion, absorption, secretion, motility

Digestion

Mechanical Disruption of food particles

Absorption

Movement of nutrient across gut wall: GI lumen → epithelial GI cell → blood vessel or lymph vessel

Secretion

Addition of substance (e.g. water, ion, metabolite) to the lumen of the gastrointestinal tract

Motility

Movement of the GI tract: 1) local segmentation for mixing, 2) wave-like contractions to propel contents

Structure of GI tract top down

mucosa, submucosa, submucosal plexus, muscularis externa, myenteric plexus, serosa

Mucosa structure

epithelial cells, lamina propria, circular and longitudinal muscle

lamina propria

connective tissue, small blood vessels, neurons, and lymphatic vessels

sub-mucosa

blood + lymph vessels, nerve plexus (submucosal plexus)

muscularis externa

circular muscle, nerve plexus (myenteric plexus), longitudinal muscle

Circular muscle function

narrows the lumen

Longitudinal muscle function

shorten length of tract

Stomach muscle layers

3 of them: circular, longitudinal, and oblique

Serosa

connective tissue

Purpose of GI tract invagination and evagination

to increase surface area for secretion and absorption

Mucous cells

secrete mucus

Parietal cells

secrete acid (HCl) and intrinsic factors

Chief cells

pepsinogen secretion

Enteroendocrine cells

secrete hormones

Chyme

digested food mixed with gastric juices that passes from stomach to small intestine

Boles

ingested food after being chewed in the mouth and mixed with saliva that passes from mouth to stomach

What stimulates enteroendocrine cells to secrete hormones

nutrients in chyme (glucose, small peptides, aa’s, fatty acids), distention of GI wall, and vagal inputs

GI hormone cell targeting function

affect a variety of cell types (2 or more target cells)

GI hormone GI lumen environment function

regulate environment of GI lumen

GI hormone GI organ function

maintain growth and structure of GI organs

GI hormones effect on other hormone

permissive effects

Enteric Nervous System composition

Submucosal and Myenteric nerve plexus

What is the enteric nervous system a part of

autonomic nervous system

What neurons form nerve plexus

afferent, interneurons, and motor

What reflexes does the enteric nervous system use to regulate motility and gland secretory activities

reflexes that originate within the tract (short reflexes)

Short reflex stimuli

distension of wall, chyme osmolarity, chyme acidity, and nutrients in chyme

Peristalsis

a progressive wave-like contraction and relaxation throughout the GI tract, resulting in a net propulsion of ingested food down the GI tract

Segmentation

a rhythmical contraction and relaxation mix luminal contents to enhance mixing and absorption of intestinal contents that occurs in the small intestine (predominate motility) and large intestine

Migrating Myoelectrical Complex (MMC)

special pattern of peristaltic motility occurring about every 2 hours between meals (during fasting)

Where does MMC start and propagate

MMC starts from lower portion of the stomach and propagates along the length of the small intestine

Purpose of MMC

move food residues, undigested materials, bacteria, and dead cells into the large intestine

What hormone initiates MMC

motilin- feeding inhibits motilin secretion stopping MMC

Salivary glands

secrete saliva, which contains water, mucus, bicarbonate, lysozyme, amylase, and lipase

Mouth/pharynx/salivary glands exocrine secretion

ions and water: food dissolution

mucus: food lubrication

salivary amylase: CHO digestion

lysozyme, antibodies, immune factors: anti-microbial

Esophagus structures

upper 1/3 made of skeletal muscle, middle 1/3 made of smooth and skeletal muscle, lower 1/3 made of skeletal muscle

upper and lower esophageal sphincters

Gastrointestinal reflux disease cause

weak lower esophageal sphincter

Esophagus exocrine secretion

mucus: lubrication

Esophagus function

swallowing reflux (swallowing center in brain medulla)

moving food to stomach by peristalsis

Stomach purpose

store ingested food and slowly releases food to the small intestine

Stomach exocrine secretion

Parietal cells- secrete HCl stimulated by aa’s, peptides, gastrin, histamine, and ACh

Chief cells- secrete pepsinogen which activates to pepsin in acidic environment

Mucus: lubrication and protection

Intrinsic factor: binds B12

Stomach endocrine secretion

Gastrin- secretion in response to amino acids peptides and PNS

• Stimulate acid secr, growth of stomach pancreas and small intestine, stim motility in stomach and intestines

Ghrelin- secretion in response to hunger

• sends feedback to hypothal to stimulate food intake

• stim secr of GH to stimulate fatty acid break down

Stomach funcitons

• Store mix dissolve and continue food digestion

• HCl secr- solubilization of food

• Protein digestion of proteins

• Some water absorption

• Intrinsic factor bings vit B12 for absorption

• Endocrine functions

Pancreas purpose

Secretes enzymes specific for digestion of each class of macromolecules

How do exocrine secretions of pancreas empty

empty into the duodenum through the pancreatic duct

Pancreas exocrine secretion

• Protein digestion enzymes- trypsin, chymotrypsin, elastase, carboxypeptidase

• Lipase- digest lipid

• Amylase- digest CHO

• Ribonuclease, deoxyribonuclease- digest nucleic acid

Bicarbonate- neutralize acid from stomach in small intestine

What form does the pancreas secrete enzymes in

zymogen- inactive form

Pancrease endocrine secretion

insulin- secreted by beta cell

• key hormone to lower blood glucose

• target tissues: muscle, adipose cells, and liver

glucagon- secreted by alpha cell

• glucagon- elevates blood glucose by stimulating liver release of glucose during post reabsorptive state

somatostatin- secreted by delta cells

• paracrine communication among pancreatic cells

Pancreas function

Enzymes digest major macromolecules

Insulin and glucagon maintain blood glucose

Bicarbonate-rich fluid neutralizes acidic chyme that enters small intestine

Portal circulation in liver

Absorbed substances from the GI tract enter the liver before going into systemic circulation

Liver exocrine secretion

Bile salts, bicarbonate, organic waste, and trace metals

Liver endocrine secretion

IGF-1: growth promoting hormone
Vit D3- 1st hydroxylation
Hepcidin- important for iron homeostasis

Liver function

• bicarb neutralize acidic chyme in small intestine

• bile solubilizes fats

• elimination of waste products

• larges production of lymph

Gallbladder function

strores and concentrates bile between meals

Small intestine secitons

duodenum, jejunum, and ileum

Enterogastrones

group of hormones released from the duodenum to inhibit emptying of the stomach

Incretin hormones

group of hormones released by the small intestine that stimulates insulin secretion (e.g. GIP and GLP-1)

Small intestine exocrine secretion

• ions and water- maintain fluidity of luminal content

• mucus- lubrication and protection

• brush border enzymes- group of enzymes tethers to microvilli involved in final step of digestion of cho, proteins, and lipis

Small intestine endocrine secretion

CCK-

• inhib stomach acid secr

• Stim secr of digestive enzymes from pancrea

• stim contraction of gallbladder

Secretin-

• inhib stomch acid secretion

• stim secr of bicarb fluid from pancreas

GIP + GLP-1-

• stimulate insulin secr

Small intestine function

• nutrient digestion by brush border enzymes

• absorption of macromolecules

• absorption of water

• absorption of trace minerals + ions

• segmentation

• MMC

• Peyer’s patches- area of tract w immune function

Large intestine portions

ascending, transverse, descending

Large intestine exocrine secreiton

mucus for lubrication

Large intestine function

House microbes

Water and sodium reabsorption

Store and concentrate undigested matter

Where do capilaries from GI drain

hepatic portal vein which branches to form capillaries again in the liver

Where do absorbed substances from GI tract enter before going into systemic circulation

Absorbed substances from the GI tract enter the liver before going into systemic circulation

Exception for substances entering hepatic portal system

Fats and fat-soluble nutrients do not enter the hepatic portal system. Instead, they enter the lymphatic system through the small intestine’s lacteals. The lymphatic system then drains into large veins near the heart

Advantages of hepatic portal system

allows absorbed substances to be processed by the liver first before entering the systemic circulation

Delivering nutrients to the liver hepatic portal system advantage

Processing the absorbed products of CHO and protein digestion, as well as water, minerals, and water-soluble vitamins

pancreatic hormones hepatic portal system advantafe

Delivering pancreatic hormones to the liver where they regulate the metabolic processing of nutrients

Absorbed toxins hepatic portal system advantage

Delivering absorbed toxins to the liver for detoxification

Lobule

functional unit of liver

Hepatocytes

primary epithelial cell population in the liver; carry most of the liver’s functions

Glycogenesis

Formation of glyxogen from glucose

Glycogenesis major endocrine stimulus

insulin

Glycogenesis enzyme in liver

glycogen synthase

Glycogenolysis

breakdown of glycogen into glucose molecules

Glycogenolysis endocrine stimulus

glucagon, sns, cortisol

Glycogenolysis key enzyme in liver

glycogen phosphorylase

Gluconeogenesis

synthesis of glucose from non-carbohydrate sources, including lactate, pyruvate, amino acids, and glycerol

Major endocrine stimulus of gluconeogeneiss

glucagon, sns, cortisol

Components of Bile

bicarbonate, cholesterol, phospholipids, bile salts, bile pigments

What do bile salts do

solubilize dietary fats

What does bicarbonate do

helps neutralize acid from the stomach

Why is bile secretion important to maintain choleserol homseostasis

liver secretes cholesterol extracted from the blood into the bile

predominate bile pigment

bilirubin

Enterohepatic Circulation

dietary fat is present in the small intestine, the bile from the liver and gall bladder is secreted through the common bile ducts to the duodenum. 95% of secreted bile salts is reabsorbed from the lumen of the ilium into the liver through the hepatic portal system. The rest of bile salts (5%) is lost in feces.