SILVERTHORN
CHAPTER 21 – DIGESTIVE SYSTEM
DIGESTIVE FUNCTION AND PROCESSES
The primary function of the gastrointestinal tract (GI) is to move nutrients, water and electrolytes
from the external environment into the body’s internal environment. It is a long tube through the
body with muscular walls with epithelium and is closed off by a skeletal-muscle sphincter at each
end. Secreted enzymes digest macronutrients into small
molecules that are small enough to be absorbed.
To maintain homeostasis, the volume of fluid entering the GI
tract by intake or secretion must equal the volume leaving the
lumen. Fluid input in the lumen comes from ingestion and
secretion (saliva, bile from liver, gastric-, pancreatic- and
intestinal secretions (total of 9.0 L put into lumen). Fluid removal
from the digestive system is done via absorption (small and
large intestine) and secretion (feces) (total of 9.0 L removed
from lumen)
The four basic processes of the digestive system:
1. Digestion - chemical and mechanical breakdown of food into absorbable units
2. Absorption - movement (active or passive) of material from GI lumen to ECF
3. Motility - movement of material through the GI tract as a result of muscle contraction.
4. Secretion – trans-epithelial transfer of water and ions from the ECF to the digestive tract
lumen and release substances synthesized by GI epithelial cells.
ANATOMY OF THE DIGESTIVE SYSTEM
The route of food through the digestive system:
1. Oral cavity – Mouth contains salivary glands (sublingual under tongue, submandibular
under jawbone, parotid near hinge of jaw that secretes -amylase) and the pharynx.
2. Oesophagus – consists of skeletal muscle that transitions to smooth muscle.
3. Stomach – three sections (upper fundus, central body, lower antrum). Can hold 2L of food
and fluid. Mixing of food with acid and enzymes to create chime. Pylorus is opening
between stomach and small intestine and is guarded by pyloric valve, that regulates the
amount of food entering the duodenum.
4. Small intestine – Consists of duodenum, jejunum and ileum. Digestion is carried out by
intestinal enzymes, aided by exocrine secretion from accessory glandular glands, the
pancreas and the liver. sphincter keeps pancreatic fluids and bile (gal) from entering the
small intestines except during a meal. Almost all nutrients and secreted fluids are absorbed.
1.5L of chyme passes into the large intestine.
5. Large intestine – watery chyme is converted to semisolid feces in the colon as water and
electrolytes are absorbed out of the chyme into the ECF. Feces entering the rectum triggers
a defecation reflex.
Stomach Small intestine
Silverthorn – chapter 21: Digestive tract Page 1 of 12
CHAPTER 21 – DIGESTIVE SYSTEM
DIGESTIVE FUNCTION AND PROCESSES
The primary function of the gastrointestinal tract (GI) is to move nutrients, water and electrolytes
from the external environment into the body’s internal environment. It is a long tube through the
body with muscular walls with epithelium and is closed off by a skeletal-muscle sphincter at each
end. Secreted enzymes digest macronutrients into small
molecules that are small enough to be absorbed.
To maintain homeostasis, the volume of fluid entering the GI
tract by intake or secretion must equal the volume leaving the
lumen. Fluid input in the lumen comes from ingestion and
secretion (saliva, bile from liver, gastric-, pancreatic- and
intestinal secretions (total of 9.0 L put into lumen). Fluid removal
from the digestive system is done via absorption (small and
large intestine) and secretion (feces) (total of 9.0 L removed
from lumen)
The four basic processes of the digestive system:
1. Digestion - chemical and mechanical breakdown of food into absorbable units
2. Absorption - movement (active or passive) of material from GI lumen to ECF
3. Motility - movement of material through the GI tract as a result of muscle contraction.
4. Secretion – trans-epithelial transfer of water and ions from the ECF to the digestive tract
lumen and release substances synthesized by GI epithelial cells.
ANATOMY OF THE DIGESTIVE SYSTEM
The route of food through the digestive system:
1. Oral cavity – Mouth contains salivary glands (sublingual under tongue, submandibular
under jawbone, parotid near hinge of jaw that secretes -amylase) and the pharynx.
2. Oesophagus – consists of skeletal muscle that transitions to smooth muscle.
3. Stomach – three sections (upper fundus, central body, lower antrum). Can hold 2L of food
and fluid. Mixing of food with acid and enzymes to create chime. Pylorus is opening
between stomach and small intestine and is guarded by pyloric valve, that regulates the
amount of food entering the duodenum.
4. Small intestine – Consists of duodenum, jejunum and ileum. Digestion is carried out by
intestinal enzymes, aided by exocrine secretion from accessory glandular glands, the
pancreas and the liver. sphincter keeps pancreatic fluids and bile (gal) from entering the
small intestines except during a meal. Almost all nutrients and secreted fluids are absorbed.
1.5L of chyme passes into the large intestine.
5. Large intestine – watery chyme is converted to semisolid feces in the colon as water and
electrolytes are absorbed out of the chyme into the ECF. Feces entering the rectum triggers
a defecation reflex.
Stomach Small intestine
Silverthorn – chapter 21: Digestive tract Page 1 of 12
