The Digestive System

Overview

1. Food breakdown • CHO, Protein, Fat

2.

3. Digestive process

¾ Primary function: breakdown & transport

nutrients, H2O, & electrolytes 1. Motility – propulsive or mixing movements

2. – energy requiring secretion of H2O, electrolytes, & enzymes, salts, or

Lumen

Duct cells

Exocrine cells

Secretory product Figure 16.1 Page 592

Capillary

1 ¾ Primary function: breakdown & transport

nutrients, H2O, & electrolytes 3. –CHO

Carbohydrates Monosaccharides Polysaccharides (starch and glycogens) Maltose (glucose, galactose, fructose)

Amylase Maltase Disaccharides Sucrose

Sucrase

Lactose

Lactase Table 16.1 (1) Page 593

¾ Primary function: breakdown & transport

nutrients, H2O, & electrolytes 3. Digestion – proteins & fats

Fats Monoglyceride Triglyceride Free fatty acids

Lipase

¾ Primary function: breakdown & transport

nutrients, H2O, & electrolytes 4. Absorption – primarily in small intestines

2 General Digestive Anatomy 1. Digestive Tract 2. Accessory Digestive Organs

Nasal passages Salivary

Pharynx

Trachea

Esophagus

Liver Gallbladder Duodenum

Descending colon Transverse colon Ascending colon Jejunum

Cecum lleum

The Digestive Tract (inner layer to outer)

1. Mucosa Mucous membrane • Mucous membrane 9 Exocrine cells (digestive juices) 9 Endocrine cells () 9 Epithelial cells (absorption)

• Degree of folding depends on location 9 Highest in small intestine

3 The Digestive Tract (inner layer to outer)

2. Submucosa Submucosa

• Thick connective tissue Muscularis externa 9 Blood & nerve innervation

3. Muscularis externa • Inner circular layer • Outer longitudinal layer

4. Serosa • Outer connective tissue 9 Prevents friction

Digestive regulation

1. Smooth-muscle function • Slow-wave action potentials (basal electrical rhythm) 9 Interstitial cells of Cajal • Regulate and segmentation

2. Intrinsic nerve plexuses () – digestive nervous system • Respond to local stimuli for secretion (digestive juices & GI hormones) • Both excitatory & inhibitory

Digestive regulation

3. Extrinsic nerves • Modify intrinsic activity and other various digestive organs (generally sympathetic &/or parasympathetic)

4. Gastrointestinal hormones • Carried throughout blood influencing other areas of digestive tract

4 Sensory Receptors

1. Chemoreceptors • changes in chemical components within lumen

2. Mechanoreceptors • Respond to stretch and tension

3. Osmoreceptors • Changes in osmolarity

External Local changes in influence digestive tract

Receptors in digestive tract

Extrinsic Intrinsic Gastrointestinal automatic nerve plexuses hormones nerves

Smooth muscle (contraction for motility)

Exocrine gland cells (secretion of digestive juices)

Figure 16.4 cells (secretion of gastrointestinal Page 595 and pancreatic hormones)

The Digestive Process

5 The Digestion Process

1. Mouth • Chewing • secretion

¾ 3 major salivary glands • Salivary proteins 1. Amylase (CHO breakdown) 9 Polysaccharides to disaccharides 2. Mucus (lubrication) 3. Lysozyme (antibacterial)

Cerebral cortex Other inputs Salivary Regulation

Salivary center Conditioned in medulla reflex

Pressure receptors Autonomic nerves Unconditioned and chemoreceptors reflex in mouth

Salivary glands

Salivary

Figure 16.5 Page 601

The Digestion Process (cont.)

2. & • reflex

6 The Digestion Process (cont.)

2. Pharynx & Esophagus • Peristalsis

Bolus

Figure 16.7 Page 603

The Digestion Process (cont.)

Stomach Esophagus Fundus Gastroesophageal sphincter

Body

Pyloric sphincter

Figure

Antrum 16.8 Duodenum Page 604

Stomach Functions 1. Storage

2. Gastric mixing & mucous secretion

3. Production of chyme

4. Secretes hydrochloric acid (HCl) • Reduces large food particles • Kills microorganisms ingested in food

5. Initial stages of protein breakdown • Pepsinogen forming

7 Figure 16.9 (1) Page 605

Basal electrical rhythm ~ 3 per minute

Pyloric sphincter Duodenum

Direction of movement of peristaltic Peristaltic contraction contraction

Gastric emptying

Figure 16.9 (2) Page 605

Peristaltic contraction

Gastric mixing

Regulation of Gastric Emptying

¾ Amount of chyme

¾ Neural response • Intrinsic nerve plexus (short reflex) Enterogastric • Autonomic nerves (long reflex) Reflex

¾ Hormonal • Enterogastrones ( & cholecystoinin – CCK) released from duodenal mucosa 9 Inhibit antral contractions

8 Regulation of Gastric Emptying

¾ Duodenum 1. Fat ~ can only be processed in small intestine 2. Acid (unneutralized) 9 Excess HCl not buffered by sodium bicarbonate 3. Hypertonicity ~ increased osmolarity due to abundance of amino acids and glucose 4. Distension

¾ Emotions

Gastric Digestive Juices ~ 2 liters/day

¾ Responsibility of cells lining gastric mucosa

1. Oxyntic mucosa • Body • Fundus

2. Pyloric gland area (PGA) • Antrum

Table 16.4 (1) Page 609

Oxyntic Gastric mucosa pit

Mucosa

Stomach lumen

Pyloric gland area Submucosa

9 In oxyntic mucosa In pyloric gland area

Surface epithelial cells

Gastric pit Mucosa cells Alkaline mucus

Gastric G cells gland Chief cells pepsiongen D cells Parietal cells HCl

Enterochomaffin- like (ECL) cells

Chief & Regulation

¾ Acetylcholine (Ach) • Stimulates both • Also stimulates G and ECL cells

¾ Histamine (paracrine) • Released from ECL cells & increases HCl secretion

¾ Somatostatin • Released from D cells • Provides negative feedback

HCl & Pepsinogen

PlasmaHCl secretion into lumen Gastric lumen

Parietal cell

Cellular metabolism

Chief cell

10 ¾ HCl functions to: • Activate pepsinogen to form pepsin

Autocatalysis Digestion Pepsinogen Pepsin

Protein Gastric HCI lumen Peptide fragments

• Breakdown of connective tissue • Denatures proteins • Kills microorganisms

Gastric Mucosal Barrier

Mucus coating

Impermeable to HCI

Tight junction

Cells lining gastric mucosa (including those lining Submucosa gastric pits and glands)

Page 16.12 Page 614

Food leaving the Stomach… Mixed with secretions from pancreas and

11 Page 16.13

Bile Page 616 Duodenum from liver Stomach

Hormones (, glucagon) Blood

Endocrine portion of pancreas (Islets of Langerhans)

Duct cells Acinar cells secrete aqueous secrete digestive

NaHCO3 solution enzymes The glandular portions of Exocrine portion of panaceas the pancreas are grossly (Acinar and duct cells) exaggerated

Pancreas

¾ Exocrine & endocrine tissue 1. Exocrine: secretes enzymes capable of breaking down CHO, fat, & protein 9 Proteolytic enzymes: protein Regulated by secretin 9 Pancreatic amylase: CHO & 9 Pancreatic lipase: fat (CCK)

2. Endocrine (hormones): Insulin & glucagon

Hormonal Regulation

Acid in Fat and protein duodenal products in lumen duodenal lumen

Secretion release CCK release from duodenal from duodenal mucosa mucosa (Secretin carried Neutralizes(CCK carried Digests by blood) by blood)

Pancreatic duct Pancreatic acinar cells cells

Secretion of Secretion of aqueous pancreatic digestive NaHCO solution into 3 enzymes into duodenal lumen duodenal lumen

12 Liver ¾ Digestive role: secretion of bile salts • Aid fat digestion & absorption Hepatic Hepatic

Blood flow – Hepatic Portal System

Hepatic portal vein

Digestive

Digestive tract Page 16.15 Page 618

Liver ¾ Bile secretion

Bile salts Cholesterol

Common bile duct Gallbladder: Bile storage

Portal circulation

Colon Duodenum

Page 16.17 Terminal Page 620

Small Intestine ¾ Primary site of digestion & absorption Duodenum primary

Jejunum

Ileum

13 Segmentation of chyme ¾ Initiated by pacesetter cells

Small intestine – digestion & absorption

¾ Potential for increased surface area 1. Extensive folding 2. Villi & microvilli 9 Increased release

¾ Pancreatic enzymes • Fat reduced to FFA (with help of bile salts) 9 Lipase • Proteins to AA 9 Aminopeptidases • CHO to di- and monosaccharides 9 Maltase, sucrase, lactase

Lumen

Na+-and energy- dependent Facilitated secondary diffusion active transport

Energy required Epithelial cell of villus

14 Lumen

Na+-and energy- dependent absorption

Energy required

Epithelial cell of villus Capillary

Lipid emulsion

Micelles diffusion Lumen

Lumen Micelle Micelles Microvillus Fatty acids, monoglycerides

Aggregate and Short or Passive absorption coated with medium lipoprotien chain Basement Epithelial membrane cell of villus (Exocytosis) Central lacteal Capillary

Small intestine – digestion & absorption

1. Vitamin absorption (passive) • Water-soluble • Fat-soluble

2. Iron absorption – regulated • Absorbed into epithelial cells 9 Either used immediately for production of RBC or 9 Stored as ferritin

3. Calcium absorption – regulated • Active transport stimulated by Vitamin D

15 Biochemical Balance

Digestive tract Stomach Blood lumen parietal cell

Pancreatic duct cell

Intestinal epithelial cell

Large Intestine

Colon

Cecum

Drying & storage

Rectum

16