The Digestive System

Alimentary Canal

Continuous food pathway (mouth to anus)

Accessory Organs

Help digestion: teeth, tongue, salivary glands, liver, gallbladder, pancreas

General functions of the digestive system

The tract takes in food, breaks it into nutrients, absorbs useful material, and removes waste

Ingestion

Bring food and water into the mouth

Secretion

Release mucus, enzymes, acid, and hormones into the tract.

Propulsion

Move food along, mainly by peristalsis (rhythmic muscle waves).

Mechanical Digestion

physically break/mix food

Chemical Digestion

enzymes break chemical bonds.

Absorption

Move nutrients, water, electrolytes, and vitamins into blood or lymph.

Defecation

Expel unusable material as feces.

Peritoneum (Peritoneal) Membrane

parietal layer lines body wall

Visceral (Peritoneal) Membrane

Layer that covers organs

Peritoneal Fluid

Fluid between lubricates to reduce friction

Mesentries

folds that support organs and carry vessels/nerves

Greater Omentum (Mesentery)

Drapes over organs

Lesser Omentum (Mesentery)

lesser omentum joins stomach to liver, mesocolon attaches to large intestine

Splanchnic Circulation

Supplies and drains abdominal digestive organs

Arteries come from?

Abdominal Aorta

Where do veins go through?

Hepatic Portal Vein to Liver, then Hepatic Veins to IVC

Mucosa

Innermost layer that lines the glands, vessels, and immune tissue

Submucosa

Second layer made up of Connective Tissue, Vessels, Glands, & ENS

Muscularis Externa

circular + longitudinal muscle for movement

Serosa (or Adventitia)

Serosa covers intraperitoneal organs; adventitia covers organs outside peritoneal cavity

Control of digestive processes

Nerves and hormones coordinate movement and secretion

Sympathetic Digestive Process

generally slows (inhibits) digestion

Parasympathetic Digestive Process

stimulates digestion

ENS controls what? (In digestive processes)

local short reflexes

ENS

Enteric nervous system: local nerve network in the tract that regulates secretion and movement

Oral Cavity

The mouth starts digestion and forms chewed food into a moist bolus

What processes occur in the oral cavity?

Ingestion, secretion, mechanical and chemical digestion, and propulsion.

What forms the roof of the oral cavity?

The palate: hard palate in front, soft palate behind.

Crown (Part of Tooth)

Visible; root sits in socket

Enamel (Part of Tooth)

covers crown and is the hardest body substance

Dentin (Part of Tooth)

lies beneath Enamel

Pulp (Part of Tooth)

contains vessels and nerves

Cementum (Part of Tooth)

covers root

Salivation

Saliva moistens food and contains enzymes and protective substances.

Which nervous system branch specifically controls salivation?

Parasympathetic reflex control

What is the function of salivary amylase?

Starts carbohydrate digestion

Parotid (Salivary Gland) Location

over masseter, duct opens near second molar; its the largest and is all serous; it provides about 25–30% of saliva.

Submandibular (Salivary Gland) Location

under/medial mandible

Sublingual (Salivary Gland) Location

under tongue; smallest salivary gland

What is the main job of the pharynx?

Pharynx propels the bolus toward the esophagus

Which portions of Pharynx are a part of the digestive system?

Oropharynx and Laryngopharynx are digestive; Nasopharynx is not.

Esophagus

Muscular tube carrying bolus to stomach; peristalsis moves it.

Upper Esophageal Sphincter is located where?

Pharynx - Esophagus

Lower/Gastroesophageal Sphincter is located where?

Esophagus – Stomach

Pyloric Sphincter is located where?

Stomach – Duodenum

Ileocecal Sphincter is located where?

Ileum – Cecum

Internal Anal (voluntary or involuntary)

Involuntary

External Anal (voluntary or involuntary)

Voluntary

Muscularis externa composition

• Upper third - skeletal muscle

• Middle - mix

• Lower third- smooth muscle

What is deglutition?

This means swallowing: voluntary oral, then reflex pharyngeal and esophageal phases.

Stomach

stores food, churns it into chyme, starts protein digestion, and controls delivery to small intestine.

What are the main functions of the stomach?

Storage/receptive relaxation, churning and mixing, acid/enzyme secretion, and regulated emptying.

Regions & Sphincters of Stomach

Cardia, fundus, body, pyloric antrum, pylorus; pyloric sphincter controls chyme leaving for duodenum

What is special about the muscularis externa?

It has a third, inner oblique smooth-muscle layer for strong churning.

Mucous neck cells secrete what?

Mucus

Parietal cells secrete what?

HCl and intrinsic factor

Chief cells secrete what?

Pepsinogen

G cells secrete what?

Gastrin

What does intrinsic factor do?

enables vitamin B12 absorption

What type of anemia occurs without sufficient intrinsic factor?

Too little can cause pernicious anemia.

What activates pepsin? 

Hydrochloric acid activates pepsinogen into pepsin

Cephalic Acid Secretion (what is the phase and what nerve stimulates it?)

sight, smell, thought (Vagus/Parasympathetic stimulation stimulates the first phases)

Gastric Acid Secretion (what is the phase and what nerve stimulates it?)

food in stomach (Vagus/Parasympathetic stimulation stimulates the first phases)

Intestinal Acid Secretion (what is the phase and what nerve stimulates it?)

protein enters duodenum (Vagus/Parasympathetic stimulation stimulates the first phases)

What hormone increases acid secretion?

Gastrin increases acid secretion (histamine also stimulates parietal cells).

What macromolecule’s digestion would be affected by inadequate amounts of stomach acid?

Protein digestion; acid activates pepsin.

How is acid secretion influenced by the enterogastric reflex?

Acid and lipids in duodenum trigger the enterogastric reflex, reducing stomach acid secretion and slowing gastric activity.

How is vomiting accomplished?

Stomach/intestine relax, then reverse contractions push contents upward; diaphragm and abdominal muscles contract as sphincters relax.

What do gastric pacemakers regulate?

Set Rhythmic stomach contractions/churning (about 3 waves per minute).

Small intestine consists of:

duodenum, jejunum, ileum; most chemical digestion and nutrient absorption occur here.

Segments of small intestine

Duodenum → Jejunum → Ileum

ileocecal valve does what?

lets contents enter cecum and prevents backflow.

Which structures increase surface area?

Circular folds, villi, and microvilli

General function of small intestine

Completes most chemical digestion, absorbs nutrients/water/vitamins, mixes and propels contents.

What is intestinal churning and which process accomplishes it?

Intestinal churning is segmentation: circular muscle contractions mix chyme with enzymes and bile.

Which nervous system controls a migrating motor complex?

Migrating motor complex during fasting: ENS and motilin control it.

Large Intestine consists of what?

cecum, ascending, transverse, descending, sigmoid colon, rectum, anal canal.

Flexures of Large Intestine are?

hepatic/right colic and splenic/left colic

Taeniae Coli of Large Intestine are?

three longitudinal muscle bands

Internal anal sphincter of Large Intestine is? (voluntary of involuntary)

smooth/involuntary

External anal sphincter of Large Intestine is? (voluntary or involuntary)

skeletal/voluntary

General functions of large intestine

Absorbs water, electrolytes, some vitamins; houses bacteria; forms and propels feces.

What is Normal Flora?

beneficial gut bacteria

What role does Normal Flora play in the digestive system?

they make vitamins (including K), metabolize fiber, inhibit harmful bacteria, and support immune development.

Mass movements are?

Strong periodic contractions move feces long distances through the colon, often after meals.

Defecation reflex is?

Rectal stretching triggers defecation; internal sphincter relaxes automatically, while external sphincter is under voluntary control.

Diarrhea is?

occurs when too little water absorbed/rapid passage

Constipation is?

occurs when there is prolonged passage with excess water absorption

Pancreas consists of?

endocrine hormone-secreting function and exocrine digestive-secretion function.

Endocrine functions of Pancreas

hormones to blood

Exocrine functions of Pancreas

Pancreatic juice to duodenum

Where does the main pancreatic duct empty into?

Duodenum at the major duodenal papilla (with bile duct).

Pancreatic juice

liquid that contains digestive enzymes and bicarbonate.

Which cells secrete pancreatic juice? Which hormone stimulates this process?

Acinar cells secrete enzymes- CCK stimulates them.

Duct cells secrete bicarbonate - Secretin stimulates it.

Why are pancreatic enzymes released in the inactive form?

Inactive enzymes help prevent digestion of pancreatic tissue; they are activated in the small intestine.

Bicarbonate ion does what?

Bicarbonate neutralizes acidic chyme in the duodenum.

Which hormone stimulates secretion in Pancreas?

Secretin