A&P CH23 Digestive System

Function of Digestive System

• Main functions of the digestive system

1. Take in food

2. Break it down into nutrient molecules (DIGESTION)

3. ABSORB molecules into the bloodstream

4. Rid body of any indigestible remains

Part 1 – Overview of Digestive System

• Organs of the digestive system fall into two groups:

  • Alimentary canal (gastrointestinal or GI tract or gut)

    • Continuous muscular tube that runs from the mouth to anus

    • Digests food: breaks down into smaller fragments

    • Absorbs fragments through lining into blood

    • Organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

  • Accessory digestive organs

    • Teeth

    • Tongue

    • Gallbladder

    • DIGESTIVE GLANDS: produce secretions that help break down foodstuffs

      • Salivary glands

      • Liver

      • Pancreas

What are 5 chemicals used in digestion?

1. HYDROCHLORIC ACID (HCl) → creates acidic environment

   1. activates pepsin to denature proteins and prepare for digestion

2. PEPSIN → protease enzyme breaks down proteins into smaller peptides

3. BILE → produced in liver & stored in gallbladder

   1. Emulsifies fat

4. AMYLASE →  breaks down carbs into simple sugars (maltose)

5. LIPASE → secreted by pancreas & stomach

   1. Breaks down fats (triglycerides) into fatty acids & glycerol

23.1 Digestive Processes

Processing of food involves six essential activities:

1. INGESTION: eating

2. PROPULSION: movement of food through the alimentary canal, which includes:

   1. Swallowing

• PERISTALSIS: major means of propulsion of food that involves alternating waves of contraction and relaxation

3. MECHANICAL BREAKDOWN: includes chewing, mixing food with saliva, churning food in stomach, and

   1. SEGMENTATION: local constriction of intestine that mixes food with digestive juices

4. DIGESTION: series of CATABOLIC STEPS that involves enzymes that break down complex food molecules into chemical building blocks

5. ABSORPTION: passage of digested fragments from lumen of GI tract into blood or lymph

6. DEFECATION: elimination of indigestible substances via anus in form of feces

23.2 Organization of Digestive System

Relationship of Digestive Organs to Peritoneum

• PERITONEUM: serous membranes of abdominal cavity that consists of:

  • VISCERAL peritoneum: membrane on external surface of most digestive organs

  • PARIETAL peritoneum: membrane that lines body wall

• Peritoneal cavity

  • Fluid-filled space between two peritoneums

  • Fluid lubricates mobile organs

• MESENTERY: double layer of peritoneum; layers are fused back to back

  • Extends from body wall to digestive organs

  • Provides routes for blood vessels, lymphatics, and nerves

  • Holds organs in place and also stores FAT

• INTRAPERITONEAL (peritoneal) ORGANS: organs that are located within the peritoneum

• RETROPERITONEAL ORGANS: located outside, or posterior to, the peritoneum

  • Includes most of pancreas, duodenum, and parts of large intestine

Histology of the Alimentary Canal

All digestive organs have the same four basic layers, or tunics

• Mucosa – 3 layers

  • Secretes MUCUS, digestive enzymes, and hormones

  • Absorbs end products of digestion

  • Protects against INFECTIOUS DISEASE

• Submucosa

• Muscularis externa

  • Muscle layer responsible for SEGMENTATION & PERISTALSIS

• SEROSA → (squamous epithelial cells) lines outer surface of intestinal lining (protective layer)

Blood Supply: Splanchnic Circulation

• SPLANCHNIC CIRCULATION includes:

  • Arteries that branch off aorta to serve digestive organs

    • Hepatic, splenic, and left gastric arteries

    • Inferior and superior mesenteric arteries

  • HEPATIC PORTAL CIRCULATION

    • Drains nutrient-rich blood from digestive organs

    • Delivers blood to liver for processing

23.3 Control of Digestive System

Enteric Nervous System

• GI tract has its own nervous system, referred to as ENTERIC NERVOUS SYSTEM

  • Also called the GUT BRAIN

  • Contains more neurons than spinal cord

• Gut brain is made up of neurons that communicate extensively with each other

  • SUBMUCOSAL NERVE PLEXUS

• Regulates GLANDS and SMOOTH MUSCLE in mucosa

• MYENTERIC NERVE PLEXUS

  • Controls GI tract MOTILITY

• SHORT REFLEXES: mediated by enteric nerve plexuses (gut brain); respond to stimuli in GI tract

• LONG REFLEXES: respond to stimuli arising inside or outside of gut, such as from autonomic nervous system

  • PARASYMPATHETIC SYSTEM enhances digestive process

  • SYMPATHETIC SYSTEM inhibits dig estion

Basic Concepts of Regulating Digestive Activity

Three key concepts regulate GI activity

1. Digestive activity is provoked by a range of mechanical and chemical stimuli

   1. Receptors located in walls of GI tract organs

   2. Respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion

2. Effectors of digestive activity are SMOOTH and GLANDS.

   1. When stimulated, receptors initiate reflexes that stimulate smooth muscle to mix and move lumen contents

   2. Reflexes can also activate or inhibit digestive glands that secrete digestive juices or hormones

3. Neurons (intrinsic and extrinsic) and hormones control digestive activity

• Nervous system control

  • INTRINSIC CONTROL: involve short reflexes (ENTERIC (gut brain) nervous system) 

  • EXTRINSIC CONTROLS: involve long reflexes (AUTONOMIC nervous system)

• Hormonal controls

  • Hormones from cells in stomach and small intestine stimulate target cells in same or different organs to secrete or contract

Part 2 – Functional Anatomy of the Digestive System

23.4 Mouth and Associated Organs

• Mouth is where food is chewed and mixed with enzyme-containing saliva that begins process of digestion, and swallowing process is initiated

• Associated organs include:

  • MOUTH - Also called the ORAL (BUCCAL) CAVITY

  • TONGUE

  • SALIVARY GLANDS

  • TEETH

Tongue

• Occupies floor of mouth

• Composed of interlacing bundles of skeletal muscle

• Functions include:

  • Gripping, repositioning, and mixing of food during chewing

  • Formation of BOLUS, mixture of food and saliva

  • Initiation of SWALLOWING, SPEECH, & TASTE

• Intrinsic muscles change shape of tongue

• Extrinsic muscles alter tongue’s position

• LINGUAL FRENULUM: attachment to floor of mouth

Salivary Glands

• Functions of saliva

  • Cleanses mouth

  • Dissolves food chemicals for taste

  • Moistens food; compacts into bolus

  • Begins breakdown of starch with enzyme AMYLASE

• Mostly water (97–99.5%) so hypo-osmotic

• Slightly acidic pH → 6.5 – 7.00

Control of salivation

• 1500 ml/day can be produced

• Minor glands continuously keep mouth moist

• Major salivary glands are activated by parasympathetic nervous system when:

  • Ingested food stimulates chemoreceptors and mechanoreceptors in mouth, sending signals to:

    • SALIVATORY NUCLEI → in brain stem that stimulate parasympathetic impulses along fibers in cranial nerves VII and IX to glands

• Strong sympathetic stimulation inhibits salivation and results in DRY MOUTH → (XEROSTOMIA)

• Smell / sight of food or upset GI can act as stimuli

The Teeth

• Lie in sockets in gum-covered margins of mandible and MAXILLA

• MASTICATION: Chewing

• DEGLUTITION: Swallowing

• Teeth are classified according to SIZE:

  • INCISORS: chisel shaped for cutting

  • CANINES: fang like teeth that tear or pierce

  • PREMOLARS (bicuspids): broad crowns with rounded cusps used to grind or crush

  • MOLARS: broad crowns, rounded cusps: best GRINDERS

    • During chewing, upper and lower molars lock together, creating tremendous crushing force

• Dentition and the dental formula

  • PRIMARY DENTITION consists of 20 DECIDUOUS TEETH, or milk or baby teeth, that erupt between 6 and 24 months of age

  • 32 deep-lying PERMANENT TEETH enlarge and develop while roots of milk teeth are resorbed from below, causing them to loosen and fall out

    • Occurs around 6–12 years of age

  • All but 3rd molars (WISDOM TEETH) are in by end of adolescence

    • Third molars may or may not emerge around 17–25 years of age

Tooth and Gum Disease

• PERIODONTITIS (periodontal disease)

  • Neglected gingivitis can escalate to disease

  • IMMUNE CELLS attack not only bacterial intruders, but also body tissues

    • Can destroy periodontal ligament

    • Can activate OSTEOCLASTS, which leads to dissolving of bone and possible tooth loss

  • May increase heart disease and stroke two ways:

    • Promotes ATHEROSCLEROTIC PLAQUE FORMATION

    • Bacteria entering blood can cause CLOT FORMATION in coronary and cerebral arteries

• Risk factors: SMOKING, diabetes mellitus, oral piercings

23.5 Pharynx and Esophagus

The Pharynx

• Food passes from mouth into oropharynx and then into laryngopharynx

  • Allows passage of food, fluids, and air

  • Stratified squamous epithelium lining with mucus-producing glands

  • External muscle layers consists of two skeletal muscle layers

    • Inner layer of muscles runs longitudinally

    • Outer pharyngeal constrictors encircle wall of pharynx

The Esophagus

• Flat muscular tube that runs from laryngopharynx to stomach

  • Is collapsed when not involved in food propulsion

• Pierces diaphragm at ESOPHAGEAL HIATUS

• Joins stomach at CARDIAL ORIFICE

• GASTROESOPHAGEAL (cardiac) SPHINCTER surrounds cardial orifice

  • Keeps orifice closed when food is not being swallowed

  • Mucus cells on both sides of sphincter help protect esophagus from acid reflux

• GASTROESOPHAGEAL REFLUX DISEASE (GERD) → stomach contents/acids flow back up esophagus leading to heartburn & regurgitation (ACID REFLUX)

23.6 The Stomach

Gross Anatomy of the Stomach

• STOMACH is a temporary storage tank that starts chemical breakdown of PROTEIN DIGESTION

  • Converts bolus of food to paste-like CHYME

  • Empty stomach has ~50 ml volume but can expand to 4 L

  • When empty, stomach mucosa forms many folds called RUGAE

• Autonomic nervous system supplies stomach

  • SYMPATHETIC FIBERS from thoracic splanchnic nerves are relayed through celiac plexus

  • PARASYMPATHETIC FIBERS are supplied by VAGUS NERVE (X) 

• Blood supply

  • Celiac trunk (gastric and splenic branches)

  • Veins of HEPATIC PORTAL SYSTEM 

Microscopic Anatomy of the Stomach

• Mucosa layer

• Consists of simple columnar epithelium entirely composed of mucous cells

  • Secrete two-layer coat of ALKALINE MUCUS 

  • TIGHT JUNCTIONS between epithelial cells

    • Prevent juice seeping underneath tissue

  • Damaged epithelial cells are quickly replaced by division of stem cells

    • Surface cells replaced every 3–6 days

  • Dotted with gastric pits, which lead into gastric glands

    • Gastric glands produce gastric juice

Types of gland cells

• Glands in fundus and body produce most gastric juice

• Glands include secretory cells

  • Parietal cells

    • Secretions include:

      • Hydrochloric acid (HCl)

        • pH 1.5 – 3.5; DENATURES PROTEIN (harder to breakdown protein) , activates pepsin, breaks down plant cell walls, and kills many bacteria

• Intrinsic factor (product)

  • Glycoprotein required for absorption of VITAMIN B12 in small intestine

• Chief cells

  • Secretions include:

    • PEPSINOGEN: inactive enzyme that is activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism)

    • LIPASES - Digests ~15% of LIPIDS

• Enteroendocrine cells

  • Secrete chemical messengers into lamina propria

  • Act as PARACRINES

    • SEROTONIN & HISTAMINE

  • HORMONES

    • SOMATOSTATIN (also acts as paracrine) and GASTRIN

Clinical – Homeostatic Imbalance 23.7

Gastritis

• Inflammation caused by anything that breaches stomach’s MUCOSAL BARRIER

Peptic or gastric ulcers

• Can cause erosions in stomach wall

  • If erosions perforate wall, can lead to peritonitis and hemorrhage

• Most ulcers caused by BACTERIAL INFECTION & ANTI–INFLAMMATORIES

  • Can also be caused by non-steroidal anti-inflammatory drugs (NSAIDs), such as ASPIRIN

Digestive Processes in the Stomach

• Carries out breakdown of food

• Serves as holding area for food

• Delivers CHYME to small intestine

• Denatures PROTEINS by HCl

• Only stomach function essential to life is secretion of INTRINSIC FACTOR for vitamin B12 absorption

  • B12 needed for RED BLOOD CELLS to mature

  • Lack of intrinsic factor causes PERNICIOUS ANEMIA

    • AUTOIMMUNE DISEASE

      • Treated with B12 injections

Regulation of Gastric Motility and Emptying

• Duodenum can prevent overfilling by controlling how much chyme enters

  • Duodenal receptors respond to stretch and chemical signals

  • Enterogastric reflex and enterogastrones inhibit gastric secretion and duodenal filling

• Stomach empties in ~4 HRS, but increase in FATTY CHYME entering duodenum can increase time to 6 hours or more

• Carbohydrate-rich chyme moves QUICKLY through duodenum

23.7 Liver, Gallbladder and Pancreas

• Liver, gallbladder, and pancreas are accessory organs associated with small intestine

• LIVER: digestive function is production of BILE

  • BILE: fat emulsifier

• GALLBLADDER: chief function is STORAGE OF BILE

• PANCREAS: supplies most of enzymes needed to digest chyme, as well as bicarbonate to NEUTRALIZE stomach acid

The Liver

• Largest gland in body; weighs ~ 3.3 LBS

• Consists of four primary lobes: right, left, caudate, and quadrate

• Hepatic artery and vein enter liver at porta hepatis

  • Hepatic → liver

Bile ducts

• Common hepatic duct leaves liver

• Cystic duct connects to gallbladder

• Bile duct formed by union of common hepatic and cystic ducts

Liver lobules

• Hexagonal structural and functional units

• Composed of plates of hepatocytes (liver cells) that filter and process nutrient-rich blood

• Hepatocytes have increased rough and smooth ER, Golgi apparatus, peroxisomes, and mitochondria

• Hepatocyte functions

  • Produce ~ 900 mL bile per day

  • Process bloodborne nutrients

    • Example: store glucose as GLYCOGEN and make plasma proteins

  • Store FAT–SOLUBLE VITAMINS

    • Perform detoxification

      • Example: converting ammonia to urea

Bile: Composition and enterohepatic circulation

• Yellow-green, alkaline solution containing:

  • BILE SALTS: cholesterol derivatives that function in fat emulsification and absorption

  • BILIRUBIN: pigment formed from heme

    • Bacteria break down in intestine to stercobilin that gives brown color of feces

  • Cholesterol, triglycerides, phospholipids, and electrolytes

• ENTEROHEPATIC CIRCULATION - RECYCLING mechanism that conserves bile salts

• Bile salts are:

  • Reabsorbed into blood by ILEUM (the last part of small intestine)

  • Returned to liver via HEPATIC PORTAL BLOOD 

  • Resecreted in newly formed bile

• About 95% of secreted bile salts are recycled, so only 5% is newly synthesized each time

Homeostatic imbalance of liver

• Hepatitis

  • Usually VIRAL infection, drug toxicity, wild mushroom poisoning

• Cirrhosis

  • Progressive, chronic inflammation from chronic hepatitis or alcoholism

  • Liver → fatty, fibrous → PORTAL HYPERTENSION

• Liver transplants successful, but livers are scarce

  • Liver can regenerate to its full size in 6–12 months after 70% removal

The Gallbladder

• Gallbladder is a thin-walled muscular sac on ventral surface of LIVER

• Functions to STORE and concentrate bile by absorbing water and ions

• Contains many honeycomb folds that allow it to expand as it fills

• Muscular contractions release bile via cystic duct, which flows into bile duct

• GALLSTONES (biliary calculi): caused by too much CHOLESTEROL or too few BILE SALTS

  • Can obstruct flow of bile from gallbladder

  • Painful when gallbladder contracts against sharp crystals

  • OBSTRUCTIVE JAUNDICE: blockage can cause bile salts and pigments to build up in blood, resulting in jaundiced (YELLOW) skin

    • Jaundice can also be caused by liver failure

  • Gallstone treatment: crystal-dissolving drugs, ultrasound vibrations (lithotripsy), laser vaporization, or surgery

The Pancreas

• Location: mostly retroperitoneal, deep to greater curvature of stomach

• Head is encircled by DUODENUM; tail abuts SPLEEN

• ENDOCRINE FUNCTION: secretion of INSULIN & GLUCAGON by pancreatic islet cells

• EXOCRINE FUNCTION: produce PANCREATIC JUICE

  • 1200–1500 ml/day is produced containing:

    • Watery, ALKALINE SOLUTION (pH 8) to neutralize acidic chyme coming from stomach

• Electrolytes and Digestive enzymes

  • Proteases (for PROTEINS): secreted in inactive form to prevent self-digestion

  • Amylase for CARBOHYDRATES

  • Lipases for LIPIDS

  • Nucleases for NUCLEIC ACIDS

Bile and Pancreatic Secretion into the Small Intestine

• Bile and pancreatic juice secretions are both stimulated by neural and hormonal controls

• Hormonal controls include:

  • Cholecystokinin (CCK)

  • Secretin

Bile secretion is increased when:

• Enterohepatic circulation returns large amounts of bile salts

• Secretin, from intestinal cells exposed to HCl and fatty chyme, stimulates gallbladder to release bile

• Hepatopancreatic sphincter is closed, unless digestion is active

  • Bile is stored in gallbladder and released to small intestine only with contraction

23.8 The Small Intestine

Small intestine is the major organ of DIGESTION (breakdown)  & ABSORPTION (taking into)

• 2–4 m long (7–13 ft) from pyloric sphincter to ILEOCECAL VALVE point at which it joins large intestine

• Small diameter of 2.5–4 cm (1.0–1.6 inches)

• Subdivisions (3)

  • DUODENUM: mostly retroperitoneal; ~25.0 cm (10.0 in) long; curves around head of pancreas

  • JEJUNUM: ~2.5 m (8 ft) long; attached posteriorly by mesentery

  • ILEUM: ~3.6 m (12 ft) long; attached posteriorly by mesentery; joins large intestine at ileocecal valve

Blood supply:

• SUPERIOR artery brings blood supply

• Veins (carrying nutrient-rich blood) drain into superior mesenteric veins, then into hepatic portal vein, and finally into LIVER

Nerve supply

• Parasympathetic innervation via VAGUS NERVE (X) , and sympathetic innervation from thoracic splanchnic nerves

Intestinal Juice

• 1–2 L secreted daily in response to distension or irritation of mucosa

• Major stimulus for production is hypertonic or acidic chyme

• Slightly ALKALINE and isotonic with blood plasma

• Consists largely of water but also contains mucus

  • Mucus is secreted by duodenal glands and goblet cells of mucosa (PEYER’S PATCHES)

Motility (movement/contraction) of the small intestine

• After a meal

• SEGMENTATION → most common motion of small intestine

  • Initiated by intrinsic pacemaker cells

  • Mixes/moves contents toward ileocecal valve

  • Intensity is altered by long and short reflexes and hormones

    • Parasympathetic INCREASES motility

    • Sympathetic DECREASES it

Between meals

• PERISTALSIS increases, initiated by rise in hormone motilin in late intestinal phase (every 90–120 minutes)

• Meal remnants, bacteria, and debris are moved toward large intestine

• Complete trip from duodenum to ileum takes ~2 HRS 

23.9 The Large Intestine

• CECUM: first part of large intestine

• APPENDIX: masses of lymphoid tissue

  • Part of MALT (mucosa associated lymphoid tissue) of immune system

  • Bacterial storehouse capable of recolonizing gut when necessary

  • Twisted shape of appendix makes it susceptible to blockages

• Colon: has several regions, most which are retroperitoneal (except for transverse and sigmoid regions)

  • ASCENDING COLON: travels up RIGHT SIDE of abdominal cavity to level of RIGHT KIDNEY

    • Ends in right-angle turn called right colic (hepatic) flexure

  • TRANSVERSE COLON: travels ACROSS abdominal cavity

    • Ends in another right-angle turn, left colic (splenic) flexure

  • DESCENDING COLON: travels down left side of abdominal cavity

  • SIGMOID COLON: S-shaped portion that travels through pelvis

• RECTUM: 3 RECTAL VALVES rectal valves stop feces from being passed with gas (flatus)

• ANAL CANAL: last segment of large intestine that opens to body exterior at ANUS

• Has two sphincters

  • INTERNAL ANAL SPHINCTER:  smooth muscle

  • EXTERNAL ANAL SPHINCTER:  skeletal muscle

Digestive Processes in the Large Intestine

• Residue remains in large intestine 12–24 HRS 

• No food breakdown occurs except what enteric bacteria digest

• VITAMINS (made by bacterial flora), WATER, and electrolytes (especially Na+ & Cl–) are reclaimed

• Major functions of large intestine is propulsion of FECES to anus and DEFECATION

• Large intestine is important for our COMFORT, but it is NOT essential for life

Defecation

• Mass movements force feces toward RECTUM

• DISTENSION initiates spinal defecation reflex

• Parasympathetic signals:

  • Stimulate contraction of SIGMOID COLON and rectum

  • Relax INTERNAL ANAL SPHINCTER 

• CONSCIOUS CONTROL allows relaxation of external anal sphincter

• MUSCLES of rectum contract to expel feces

• Assisted by VALSALVA’S MANEUVER

  • Closing of GLOTTIS, contraction of DIAPHRAGM and abdominal wall muscles cause increased INTRA–ABDOMINAL PRESSURE

Clinical – Homeostatic Imbalance 23.15

• DIARRHEA: watery stools, results when large intestine does not have sufficient time to absorb remaining water

  • Causes include irritation of colon by bacteria or JOSTLING of digestive viscera (occurs in marathon runners)

  • Prolonged diarrhea may result in DEHYDRATION and electrolyte imbalance (acidosis and loss of potassium)

• CONSTIPATION → occur when food remains in colon for extended periods of time and too much water is absorbed

  • Stool becomes HARD and difficult to pass

May result from insufficient FIBER or FLUID the diet, improper BOWEL HABITS, lack of EXERCISE, or LAXATIVE abuse