NPB 101 Final (Digestive)

Digestive System

Primary function to transfer nutrients, water, and electrolytes from the food we eat into the body’s internal environment

Plant photosynthesis

Energy + CO2 + H2O → organic molecules + O2

Human cell metabolism

organic molecules + O2 → energy + CO2 + H2O

Nasal passages

Mouth

Salivary glands

Trachea

Esophagus

Muscular tube that extends between the pharynx and stomach

Stomach

J-shaped chamber located between the esophagus and SI

Gallbladder

Pancreas

Duodenum

Descending colon

Transverse colon

Jejunum

Cecum

Ileum

Appendix

Sigmoid colon

Rectum

Anus

Order of which food travels through the body

1. Mouth and salivary glands

2. Pharynx and esophagus

3. Stomach

4. SI

5. LI

Digestive tract wall major tissue areas

• Serosa

• Submucosa

• Muscularis externa

  • Mucosa

Serosa

Submucosa

Muscularis externa

Mucosa

Lumen

Continuous with the external environment

Enteric system of the Digestive tract (wall)

• Myenteric plexus

• Submucous plexus

Why is the Lumen continuous with the external environment?

• pH in the stomach can fall as low as 2. Inside the body the range of pH that is compatible with life = 6.8 - 8.0 (homeostatic range is 7.35 - 7.45)

• Harsh Enzymes that hydrolyze food could destroy the body’s own tissues. Enzymes are synthesized in an inactive form and are activated when they reach the lumen

• Millions of microorganisms inhabit the GI-tract, and these could be lethal if they entered the body properly

Basic digestive processes

1. Motility

2. Secretion

3. Digestion

4. Absorption

Motility (as a digestive process)

Muscular contractions mix and move forward the contents of the digestive tract through both propulsive & mixing movements

Propulsive motility movements

Move the contents of the GI tract forward

Mixing motility movements

• Aid digestion by mixing food with digestive juices

• Facilitate absorption by exposing food to absorbing surfaces

Secretion (as a basic digestive process)

Digestive juices are secreted into the digestive tract lumen by exocrine glands

Digestion (as a basic digestive process)

Biochemical breakdown process whereby structurally complex food is converted into smaller absorbable units

• carbohydrates → sugar

• proteins → AAs/peptides

• fat → FAs/glycerides

Absorption (as a basic digestive process)

The transfer of small absorbable units along with water, vitamins, and electrolytes from the digestive tract lumen into the blood or lymph

Mostly occurs in the SI

Small intestine (SI) components

• Duodenum

• Jejunum

• Ileum

Large intestine (LI) components

• Secum

• Appendix

• Colon

• Rectum

Palate

• Roof of the oral cavity

• Separates oral cavity from nasal passage

• Allows chewing and breathing to occur simultaneously

• Front-most palate = hard palate, back-most palate = soft

Cephalic phase response (CPR)

• Prepares food digestion and nutrient absorption efficiently

• Without, there would be nutrient absorption delay, slow digestion speed, and a longer digestion period

• Crucial for energy homeostasis and metabolism

Salivary glands

Secrete saliva in response to autonomic stimulation

Types:

• Sublingual

• Submandibular

• Parotid

Saliva contents

• Mucus — to moisten food and lubricate

• Lysozyme — to lyse bacteria

• Bicarbonate buffers — for neutralization of acid

• Amylase — which begins chemical digestion of carbohydrates by cleaving polysaccharides into maltose

Sublingual gland

Submandibular gland

Parotid gland

Start of carbohydrate digestion

Mouth

Pharynx

• at the rear of the mouth

• common passageway for the digestive system & respiratory system

Upper sphincter (pharyngoesophageal)

Normally closed to prevent air from entering esophagus

(prevents excessive eructation)

Lower sphincter (gastroesophageal)

Normally closed to prevent acid reflux from stomach (heartburn)

Swallowing

Refers to the entire process of moving food from the mouth through the esophagus and into the stomach

Which step in swallowing can you stop?

Oral

All-or-none reflex

Inititated when a food bolus is pushed by the tongie into the pharynx

Activation of the Swallowing center in medulla

Caused from pressure of bolus in pharynx

Swallowing center

Reflexively activates appropriate muscles to swallow (once swallowing is initiated, it cannot be stopped)

Swallowing stages

• Oropharyngeal

• Esophageal

Oropharyngeal stage

Movement of bolus from mouth through pharynx and into the esophagus

Duration: 1 second

Esophageal stage

“Ringlike” peristaltic wave moves bolus through esophagus and into stomach

Duration: 5-9 seconds

Which of the following is not a function of saliva?

Facilitates absorption of glucose across the oral mucosa

Stomach sections

• Fundus

• Body

• Antrum

Stomach Fundus

Located above the gastroesophageal sphinchter

Stomach Body

Middle part of stomach

Stomach Antrum

• Bottom portion

• Thick layer of smooth muscle

• Connected to small intestine by pyloric sphincter

Stomach fxns

• Store ingested food and delivers it to small intestine at appropriate rate

• Secrete HCl and enzymes to begin protein digestion

• Chyme- thick liquid mixture made by pulverizing ingested food and mixing it with gastric secretions

Empty stomach volume

~50 mL

Full stomach volume

~ 1k mL

unlike skeletal and cardiac muscle…

stomach smooth muscle maintains a constant tension over a range of lengths

Strong antral peristaltic contractions

• Mix food with gastric secretions to produce chyme

• Propel chyme towards pyloric sphincter where small amount is pushed into the duodenum

• Upon reaching the pyloric sphincter, cause the sphincter to close and the remaining chyme is tumbled back into the antrum

• Strength can vary depending on a number of factors

Gastric secretions per day

~ 2L

Gastric mucosa

Lining of the stomach, divided into 2 areas:

• Oxyntic mucosa – lines body and fundus

• Pyloric gland area - lines the antrum

Oxyntic mucosa

Stomach lining of the body and fundus

Pyloric gland area

Stomach lining of the antrum

Parietal (Oxyntic) cell

Secretes HCl (H+ & Cl- released independently) and intrinsic factor

Surface epithelial cell

Secretes thick alkaline mucus

Mucous neck cell

Secretes thin, watery mucus

Chief cell

Secretes pepsinogen (inactive precursor of pepsin)

Pepsinogen

• The major digestive constituent of gastric secretions

• Stored in vesicles “zymogen granules” inside of chief cells

• Secreted by exocytosis upon proper stimulation

• Inactive precursor of pepsin (inactive to prevent damage to the stomach lining)

• Once secreted, HCl cleaves off a small fragment to form pepsin

Pepsin

• Cleaved off of a small pepsinogen fragment via Parietal cell’s HCl

• Can act on other pepsinogen molecules converting them into pepsin (autocatalytic process)

• Initiates protein digestion by splitting certain amino acid linkages

  • Because it degrades protein, it must be secreted and stored in an inactive form

• Works best in an acidic environment!

Proton release in Parietal Oxyntic cells

• Actively transported against their concentration gradient

• Not transported from plasma, but derived from metabolic process.

• Whenever an ion is secreted, a new ion is generated from carbonic acid

Chlorine release in Parietal Oxyntic cells

Active secretion against a smaller concentration gradient

HCl major functions

1. Provides an acid environment optimal for pepsin via activation of pepsinogen

2. Aids breakdown of connective tissue

3. Kills microorganisms ingested with food

Intrinsic factor

• Secreted from parietal cells along with HCl

• Required for the absorption of vitamin B 12

  • intrinsic factor binds B12 to form a complex that then binds to the intestinalreceptor, cubulin, triggeringreceptor-mediatedendocytosis

  • vitamin B12 is essential for normal function of red blood cells.

  • without B12, pernicious anemia results

Degeneration of parietal cells most detrimental consequence

Loss of intrinsic factor

Gastric pit cells

• Chief

• Parietal

• Mucous neck

• Surface epithelial

Mucus

• Derived from surface epithelial cells and mucous neck cells

• Covers the surface of the gastric mucosa to protect it from several forms of injury:

  • mechanical injury- the lubricating properties of mucus protect the mucosa from mechanical injury self digestion- mucus inhibits pepsin activity and protects mucosa from self-digestion acid injury- being alkaline, mucus neutralizes HCl to protect mucosa from acid injury

What kinds of injury does the mucus protect the stomach from

• Mechanical injury- the lubricating properties of mucus protect the mucosa from mechanical injury

• Self-digestion- mucus inhibits pepsin activity and protects mucosa from self-digestion

• Acid injury- being alkaline, mucus neutralizes HCl to protect mucosa from acid injury

PGA (Pyloric gland area)

• Secretes mucus Pepsinogen-- and small amounts of into lumen

• Acid is not secreted in this area

• Releases gastrin (hormone) into blood

Gastrin hormone

• Secreted from special endocrine cells in the pyloric gland area into the blood

• After being carried by the blood back to the body and fundus of the stomach, stimulates parietal and chief cells to produce highly acidic gastric juice and pepsinogen

• Promotes mixing and emptying of stomach

Which factors control gastric mixing and emptying?

Stomach & Duodenum

Stomach influence on Gastric mixing/emptying

Promotion of gastric motility and emptying:

• Volume of Chyme:

  • distension stimulates motility and emptying via: 1) direct effects of stretch on smooth muscle 2) involve of the intrinsic plexuses and vagus nerve 3) gastrin (stomach hormone)

• Fluidity of Chyme: contents must be in a fluid form to be evacuated - increased fluidity allows more rapid emptying

Duodenum influence on Gastric mixing/emptying

Inhibits gastric motility and emptying until the duodenum has coped with factors already present:

• Fat

  • is only digested and absorbed within the small intestine.

  • when fat is present, further emptying from the stomach is prevented until SI processes fat already there.

• Acid

  • highly acidic chyme from the stomach needs to be neutralized by NaHCO3 (sodium bicarbonate) in the duodenum.

  • non-neutralized acid in the duodenum inhibits gastric Emptying

• Hypertonicity

  • molecules of amino acid and glucose build up as protein and starch are digested in the duodenum.

  • increased osmolarity within duodenum triggers a reflex to reduce gastric emptying

• Distension: too much chyme in the duodenum inhibits gastric emptying until the duodenum is ready to process it

Small intestine (SI)

• site where most digestion and absorption takes place (no digestion occurs after the small intestine) ~ 21 feet long (cadaver) and 1 inch diameter- arbitrarily divided into 3 regions

• Segmentation: small intestine’s primary method of motility-- chyme oscillating, ringlike contractions (not peristaltic wave) absent between meals, but vigorous during and after a meal

• Chyme: moves forward because the of contractions gradually along length of small intestine (duodenum ~12/min; ileum ~9/min)