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Summary BBS2041 - Human Intermediary Metabolism (all practicals included)
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Case 1Gastrointestinal tract
The GI tract consists of the mouth, pharynx, oesophagus, stomach, small intestine and large
intestine. The accessory digestive organs are the teeth, tongue, gallbladder, and a number
of large digestive glands (the salivary glands, liver, and pancreas).
Mouth (salivary glands)
There are major and minor salivary glands:
a. the major, or extrinsic, glands are the parotid, submandibular and the sublingual
glands
b. the minor, or intrinsic, glands are the buccal glands (and many more)
The salivary glands are composed of two types of secretory cells: serous and mucous.
a. Serous cells produce a watery secretion containing enzymes (amylase), ions, and a
tiny bit of mucin (parotid, submandibular, buccal glands)
b. Mucous cells produce mucus, a stringy, viscous solution (sublingual, buccal glands)
,Esophagus
The esophagus is a 25 cm-long muscular tube and is collapsed when not involved in food
propulsion. It is connected to the stomach via the cardiac orifice within the abdominal cavity.
The cardiac orifice is surrounded by the gastroesophageal or cardiac sphincter. The orifice is
closed when food is not being swallowed. Mucous cells on both sides of the sphincter help
protect the esophagus from reflux of stomach acid. In the esophagus peristalsis takes place,
which is described in the picture below:
Stomach
In the stomach the chemical breakdown of proteins begins and food is converted into a
creamy paste called chyme. An empty stomach has a volume of about 50 ml and a full one
can hold about 4 L of food and may extend nearly to the pelvis. When empty, the stomach
collapses inward, throwing its mucosa (and submucosa) into large, longitudinal folds called
rugae.
The cardia surrounds the cardiac orifice through which food enters the stomach from the
esophagus. The fundus is dome-shaped and bulges superolaterally to the cardia. The body,
or the midportion of the stomach, is continuous inferiorly with the funnel-shaped pyloric part.
The wider and more superior area of the pyloric part, the pyloric antrum narrows to form the
pyloric canal, which terminates at the pylorus. The pylorus is continuous with the duodenum
through the pyloric sphincter or valve, which controls stomach emptying. The stomach has a
greater and a lesser curvature. Extending from these curvatures are two mesenteries, called
omenta, that help tether the stomach to other digestive organs and the body wall.
Small intestine
In the small intestine, the macronutrients are absorbed, but this cannot happen without the
aid of secretions from the liver (bile) and pancreas (digestive enzymes).
The small intestine is a convoluted tube extending from the pyloric sphincter to the ileocecal
valve (sphincter) where it joins the large intestine. The small intestine has three subdivisions:
, 1. the duodenum, which is mostly retroperitoneal. The bile duct, delivering bile from the
liver, and the main pancreatic duct, carrying pancreatic juice from the pancreas, unite
in the wall of the duodenum in a bulblike point called the hepatopancreatic ampulla.
The ampulla (of Vater) opens into the duodenal papilla. A smooth muscle valve called
the hepatopancreatic sphincter controls the entry of bile and pancreatic juice.
2. the jejunum, which is intraperitoneal. It extends from the duodenum to the ileum.
3. ileum, which is intraperitoneal. It joins the large intestine at the ileocecal valve.
Liver
The digestive function of the liver is to produce bile for export to the duodenum. Bile is a fat
emulsifier that breaks down fats into tiny particles to make them more readily digestible. The
liver has four primary lobes.
- the right lobe → largest one and separated from the smaller left lobe by a deep
fissure.
- the left lobe → separated from the right lobe by the falciform ligament
- the caudate lobe → most posterior
- the quadrate lobe → lies inferior to the left lobe
Running along the inferior edge of the falciform ligament is the round ligament, or
ligamentum teres, a fibrous remnant of the fetal umbilical vein. Bile leaves the liver through
several bile ducts that ultimately fuse to form the large common hepatic duct, which travels
downward toward the duodenum. Along its course, that duct fuses with the cystic duct
draining the gallbladder to form the bile duct.
Gallbladder
The gallbladder is a thin-walled green muscular sac about 10 cm long. It is located in the
inferior surface of the right liver lobe from which its rounded fundus protrudes. The
gallbladder stores bile that is not immediately needed for digestion and concentrates it by
absorbing some of its water and ions. When empty, its mucosa is thrown into
honeycomb-like folds that, like the rugae of the stomach, allow the organ to expand as it fills.
, Composition of Bile
Bile is a yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol,
triglycerides, phospholipids (lecithin and others), and a variety of electrolytes. Of these, only
bile salts and phospholipids aid the digestive process.
- Bile salts, primarily cholic and chenodeoxycholic acids, are cholesterol derivatives.
Their role is to emulsify fats into smaller accessible fatty droplets that provide large
surface areas for the fat-digesting enzymes to work on. Bile salts also facilitate fat
and cholesterol absorption. In addition, they help solubilize cholesterol. Most bile
products are secreted in the feces, except for bile salts which are recycled. In this
process, bile salts are:
(1) reabsorbed into the blood by the ileum
(2) returned to the liver via the hepatic portal blood
(3) resecreted in newly formed bile.
This pool of bile salts recirculates two or three times for a single meal. The chief bile pigment
is bilirubin, a waste product of the heme of hemoglobin formed during the breakdown of
worn-out erythrocytes. The globin and iron parts of hemoglobin are saved and recycled, but
bilirubin is absorbed from the blood by liver cells, excreted into bile, and metabolized in the
small intestine by resident bacteria. One of its breakdown products, stercobilin, gives feces a
brown color.
Pancreas
The pancreas is a soft, tadpole-shaped gland that is retroperitoneal and lies deep to the
greater curvature of the stomach. The pancreas has exocrine and endocrine functions:
a. exocrine: it produces enzymes (pancreatic juice) via the acini that break down all
categories of foodstuffs. Pancreatic juice drains from the pancreas into the
duodenum via the centrally located main pancreatic duct. The pancreatic duct
generally fuses with the bile duct just as it enters the duodenum (at the
hepatopancreatic ampulla). A smaller accessory pancreatic duct empties directly into
the duodenum just proximal to the main duct.
b. endocrine: scattered amid the acini are the lightly staining pancreatic islets. These
mini-endocrine glands contain several cell types:
1. alpha cells → glucagon, plays an important role in carbohydrate metabolism.
2. beta cells → insulin, plays an important role in carbohydrate metabolism.
3. delta cells → somatostatin, inhibits digestive system
4. PP cells → pancreatic polypeptide, regulates pancreatic secretion activity
Composition of pancreatic juice
Consists of water, enzymes and electrolytes. The epithelial cells lining the smallest
pancreatic ducts release the bicarbonate ions (HCO3+) that make it alkaline (about pH 8).
The high pH of pancreatic fluid helps neutralize acid chyme entering the duodenum and
provides the optimal environment for intestinal and pancreatic enzymes. Like pepsin of the
stomach, pancreatic proteases (protein-digesting enzymes) are produced and released in
inactive forms, which are activated in the duodenum, where they do their work. This protects
the pancreas from digesting itself. Other pancreatic enzymes (amylase, lipases, and
nucleases) are secreted in active form, but require that ions or bile be present in the
intestinal lumen for optimal activity.
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