BIOL 22000 Lecture Notes - Lecture 9: Coagulation, Thoracic Duct, Hepatic Portal System
8 Jun 2018
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Chapter 9 The Digestive System
9.1 Anatomy of the Digestive System
• First, intracellular digestion, as a part of metabolism, involves the oxidation of glucose
and fatty acids for energy
• Our diets do not consist of pure glucose and fatty acid, we must extract from our food
o The process occurs within the lumen of the alimentary canal and is known as
extracellular digestion
o This is technically “outside” the body, as it is outside the cell borders
• The alimentary canal runs from the mouth to the anus and is sectioned off by
sphincters, circular smooth muscles around the canal that can contract to allow
compartmentalization of function
• The most basic functional distinction is between digestion and absorption
o Digestion involves the breakdown of food into its constituent organic molecules
▪ Lipids (fats) into free fatty acids and glycerol
▪ Starches and other carbohydrates into monosaccharides
▪ Digestion can be subdivided into mechanical and chemical processes
• Mechanical digestion is the physical breakdown of large food
particles into smaller food particles, no breaking chemical bonds
• Chemical digestion is the enzymatic cleavage of chemical bonds,
such as the peptide bonds of proteins or the glycosidic bonds of
starches
o Absorption involves the transport of products of digestion from the digestive
tract into the circulatory system for distribution to the body’s tissues and cells
• The digestive tract begins with the oral cavity (mouth) followed by the pharynx, a
shared pathway for both food and air
• From the pharynx, food enters the
esophagus, which transports food to the
stomach
• From the stomach, food travels to the
small intestine, and then the large
intestine
• Finally, waste products of digestion enter
the rectum, where feces are stored until
the appropriate time of release
• Within the digestive tract, salivary
glands, pancreas, liver, and gallbladder
help to provide the enzymes and
lubrication necessary to aid the digestion
of food
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• The enteric nervous system is a collection of neurons that governs the function of the
gastrointestinal system
• These neurons are present in the walls of the digestive tract and trigger peristalsis,
rhythmic contractions of the gut tube, to move materials through the system
o Function independently of the brain and spinal, but heavily regulated by the
autonomic nervous system
• The parasympathetic division is involved in stimulation of digestive, activities, increasing
secretions from exocrine glands and promoting peristalsis
o We feel sleepy after eating a large meal because the blood flow is increased for
digestion function
• The sympathetic division is involved in inhibition of these activities
o during high sympathetic activity, blood flow is decreased to the digestive tract,
and gut motility slows significantly
9.2 Ingestion and Digestion
ORAL CAVITY
• The oral cavity plays a role in both mechanical and chemical digestion of food
o Mechanical digestion in the mouth involves the breaking up of large food
▪ This process is called mastication (chewing)
▪ Chewing helps to increase the surface-area-to-volume ratio of the food,
more surface area for enzymatic digestion
▪ Food particles that are too large create an obstructive risk in the tract
o Chemical digestion begins the breakdown of chemical bonds in the
macromolecules that make up food
▪ Begins in the in mouse and complete in the small intestine
▪ Relies on enzymes from saliva produced by the three pairs of salivary
glands
▪ Saliva aids mechanical digestion by moistening and lubricating food
▪ The salivary glands are innervated by the parasympathetic nervous system
▪ Saliva contains salivary amylase(ptyalin), and lipase
• Salivary amylase is for hydrolyzing starch into smaller sugars
• Lipase catalyzes the hydrolysis of lipids
PHARYNX
• The pharynx is the cavity that leads from the mouth and posterior nasal cavity to the
esophagus
• The pharynx connects to the esophagus and to the larynx
• The pharynx can be divided into three parts:
o Nasopharynx (behind the nasal cavity)
o Oropharynx (at the back of the mouth)
o Laryngopharynx (above the vocal cords)
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• Food is prevented from entering the larynx during swallowing by the epiglottis, a
cartilaginous structure that folds down to cover the laryngeal inlet
o Failure can lead to aspiration of food and choking
ESOPHAGUS
• The esophagus is a muscular tube that connects the pharynx to the stomach
• The top third of the esophagus is composed of skeletal muscle, the bottom third is
composed of smooth muscle, and the middle third is a mix of both
o This means the top of the esophagus is under somatic (voluntary) motor control,
the bottom is under autonomic (involuntary) nervous control
• The rhythmic contraction of smooth muscle that propels food toward the stomach is
called peristalsis
o Emesis = vomiting, which is the reversal of peristalsis
• Swallowing is initiated in the muscles of the oropharynx, which constitute the upper
esophageal sphincter
• As the bolus approaches the stomach, a muscular ring known as the lower esophageal
sphincter (cardiac sphincter) relaxes and opens to allow the passage of food
STOMACH
• The stomach is in the upper left quadrant of the abdominal cavity
o Uses hydrochloric acid and enzymes to digest food, creating a harsh environment
o Mucosa (membrane) is thick to prevent autodigestion
• The stomach can be divided into four main anatomical
divisions
o Fundus and body contain mostly gastric glands
o Antrum and pylorus contain mostly pyloric
glands
o The internal curvature of the stomach is called the
lesser curvature
o The external curvature is called the greater
curvature
o The lining of the stomach is thrown into folds
called rugae
• The mucosa of the stomach contains gastric glands and
pyloric glands
• The gastric glands respond to signals from the vagus
nerve of the parasympathetic nervous system
o Activated by the brain in response to the sight, taste, and smell of food
o Gastric glands have three different cell types
▪ Mucous cells produce the bicarbonate-rich mucus that protects the
muscular wall from the harshly acidic (pH = 2) and proteolytic
environment of the stomach
Document Summary
Hcl: decrease ph, kill microbes, denature proteins, some chemical digestion; intrinsic factor: absorption of vitamin. Increase pancreatic secretions, especially bicarbonate, reduce hcl secretion, decrease motility. Brush-border enzyme; break dipeptides into free amino acids. Recruit secretions from gallbladder and pancreas; promote satiety. The functions of the various digestive enzymes (and bile) Duodenum hydrolyzes specific peptide bonds; converts chymotrypsinogen to chymotrypsin; activated by enteropeptidase. Duodenum hydrolyzes specific peptide bonds; activated by trypsin. Duodenum hydrolyzes terminal peptide bond at carboxy end; activated by enteropeptidase. Duodenum hydrolyzes terminal peptide bond at amino end. Duodenum converts trypsinogen to trypsin and procarboxypeptidases a and b to carboxypeptidases a and b. Bile (bile is not an enzyme, but is involved in mechanical digestion of fats) 9. 4 absorption and defecation: absorption of nutrients primarily occurs in the small intestine, especially in the jejunum. 10 and ileum: the large intestine largely absorbs water.
