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Physiology of the Digestive Tract
1. Outline diagrammatically the basic gross structure of the digestive tract,
showing relationships between components and including the organs which
secrete digestive juices
Oesophagus
Liver
Gallbladder
Small
Intestines
Rectum
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Digestion begins in the oral cavity, which contains the tongue, teeth, salivary
glands. Three paired salivary glands are associated with the oral cavity – parotid,
submandibular and sublingual glands.
Within these glands, there are 2 types of cells;
1. serous (protein secreting)
2. mucous (mucous secreting)
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The pharynx is a transitional zone conveying food to the oesophagus
Food leaves the oesophagus and passes through the lower oesophageal
sphincter and enters the stomach
Emptying into the gastric pits of the stomach, are gastric glands, whose function
is different in each region of the stomach;
1. The cardia, a narrow band at the junction of the distal oesophagus and the
stomach, contains glands that secrete mucous and lysozome
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2. Parietal cells exists mainly in the fundus and body of the stomach. These
secrete hydrochloric acid (gastric acid – stimulated by gastrin)
3. Chief cells populate the lower half of the gastric glands in the fundas and the
body of the stomach. Chief cells secrete and produce enzymes for the
digestion of protein and fat, specifically, pepsinogen and lipase
A very small amount of absorption takes place in the stomach, but the majority
occurs when the chime passes the pyloric sphincter into the small intestine
The small intestine is the last and most important place for the absorption of
food (but not water)
The pancreas and liver are important secretary organs which secrete their
products into the duodenum
- the pancreas is both an exocrine organ (secreting substances out of the body,
into the GI lumen), and endocrine (secretes hormones into the body via the
bloodstream). The exocrine portion participates in digestion by producing
and secreting a host of important enzymes.
o Secretions of the Pancreas:
 Exocrine secretions: various enzymes including pancreatic
amylase (for digestion of starches), trypsin, carboxypepiydase
and chymotrypsin (proteases) and pancreatic lipase (digestion
of fats). Sodium bicarbonate is also produced, which makes
the pancreatic juice alkaline (neutralises the HCl in the chyme
and makes the environment optimal for the action of the
enzymes)
 The exocrine cells, called acini, produce the hormones insulin,
glucagon, somatostatin (an inhibitory hormone which inhibit
the release of growth hormone, and thyroid-stimulating
hormone) and pancreatic polypeptide
- The liver produces and secretes bile acids into the duodenum. Bile acids are
essential for the digestion of lipids
- Once bile is produced, the gallbladder concentrates and stores it. The
gallbladder also has mucous-secreting cells which produce the mucous found
in the bile
Microvilli in the small intestine act to increase the surface area of the small
intestine to maximize absorption of nutrients from ingested food. They also bear
enzymes bound to their membranes which act to hydrolyse complex
carbohydrates and peptides into simple sugars and amino acids
In between the villi, tubular glands (also called the crypts of Lieberkuhn) open
into the intestinal lumen. These consist primarily of stem cells that divide and
replace lost epithelial cells and other intestinal cells of the villi (i.e. mucous
producing cells (goblet cells), lysozome-producing cells (Paneth cells) and
enteroendocrine cells
At the tip of each villi is a capillary network that drains into veins of the
submucosal plexus. These veins drain into the mesenteric veins and then the
portal vein to the liver.
Lymphatics are also located within each of the villi
2. Describe the intrinsic and extrinsic innervation of the GIT
Intrinsic Innervation
 The ENS (Enteric Nervous System), provides intrinsic innervation of the GI tract
structures
 The submucosal and myenteric plexus’ of the gastrointestinal tract are composed
of ganglia
 The myenteric ganglia form a continuous network from the upper oesophagus to
the internal anal sphincter. These act to regulate GIS motility (the movement
along the tract)
 The submucosal ganglia are
concentrated in the small and large
intestine and regulate glands, smooth
muscle cells and mucosal cells
 The ganglia within each layer are
connected allowing for signals to be sent
across circular muscle layers, which is
critical for the coordination of activity
along the intestinal wall
 The enteric neurons connect with the
intestinal mucosa, secretory cells, blood
vessels, smooth muscle cells,
sympathetic neurons, parasympathetic
neurons and enteroendocrine cells (specialised endocrine cells of the GIT)
 Both excitatory and inhibitory motorneurons, secretormotor neurons and
vasodilator neurons are found in the ENS
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The ENS can operate independently of the CNS or PNS to increase/decrease
contraction of the GIT or increase/decrease secretion in the GIT.
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Stimulation includes;
o Stretching of the GIT
o Changes in pH in the GIT
o Detection of fates, acids etc in the GIT
Extrinsic Innervation
 Extrinsic innervation links to the CNS via the visceral afferent fibres (vagus and
splanchnic nerves)
 CNS can control either end of the GIT tract (i.e. chewing and swallowing) and
excretion/relaxation of the external sphincter
 Between the pharynx and the external anal sphincter, the GI tract is extrinsically
innervated only by the autonomic nervous system and therefore, the brain has
little voluntary control over most of the GI tract.
 The parasympathetic and sympathetic divisions of the Autonomic Nervous
System regulate the function of the GIT tract
o The parasympathetic nervous system: connects the medulla to the
myenteric and submucosal plexuses through the vagus nerve and the
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pelvic nerve – both of these nerves contain efferent (motor) and
afferent (sensory) fibres.
The vagus innervates the GI tract from the upper oesophageal
sphincter to the transverse colon and creates local effects (as
opposed to the system effects of the sympathetic nervous system)
The pelvic nerves, originating in the sacral spinal cord (S2-S4)
innervate the GI tract from the splenic flexure to the anal sphincter
The effect of this is increased motility, increased secretion and
relaxation of sphincters
The sympathetic nervous system innervates the entire GI tract.
Innervation of fibres from several ganglia with extensive branching
will cause widespread inhibitory effects, which in turn, will reduce
motility, blood flow towards the intestine (blood flow will be
redirected to the muscles in order to prepare for the fight or flight
response).
3. Describe the major vessels of arterial supply, venous and lymphatic
drainage of the GIT
Arteries of the GIT
Branches of the Coeliac trunk (foregut)
The branches of the Coeliac Trunk supply the terminal oesophagus, stomach,
duodenum (to the bile duct), liver, pancreas and spleen.
1. Left Gastric Artery – this then divides into the oesophageal branches
2. Spenic Artery – this gives the pancreatic arteries, short gastric arteries and
left gastro epiploic arteries
3. Common Hepatic Artery – this divides into the hepatic artery and the
gastroduodenal artery.
 The hepatic artery
gives off the Right
Gastric Artery before
dividing into the Left
and Right Hepatic
Branches
 The Gastroduodenal
Artery divides into the
Right Gastro-Omental
Artery and Superior
Pancreatico-Duodenal
Artery (which further
divides into an anterior and posterior branch)
Superior Mesenteric Branches (mid gut)
The branches of the Superior Mesenteric Artery supply the distal duodenum,
jejunum, ileum, caecum, appendix, ascending colon and most of the transverse
colon.
The branches include;
 Inferior pancreatico-duodenal artery – divides into anterior and posterior
branches
 Jejunal arteries
 Ileal arteries
 Ileocolic artery – this gives off the ascending colic, anterior and posterior
caecal branches. The appendicular artery usually arises from the posterior
caecal artery
 Right colic artery
 Middle colic artery
Inferior Mesenteric Branches (hidegut)
The branches of the Inferior Mesenteric supply the left colic flexure and descending
colon, sigmoid colon and rectum.
The branches include;
 Left colic artery – this divides into the left and right inferior branches. The
inferior mesenteric artery continues to the pelvis as the superior rectal artery
 Sigmoid arteries
Veins of the GIT
Tributaries of the Inferior Vena Cava
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Common Illiac veins
Lumbar veins (3rd and 4th)
Right gonadal veins
Right and left renal veins
Right suprarenal veins
Inferior phrenic veins
Hepatic veins (right, middle and left)
Portal Venous Drainage
The Hepatic Portal Venous System drains all the unpaired abdominopelvic viscera
and corresponds to the arterial supply to the foregut, midgut and hindgut. The
Hepatic Portal Venous System drains the GIT, abdominopelvic cavity, pancreas and
the spleen. It also carries all the venous blood (including all the nutrients absorbed in
the small intestine and the hormones secreted by the pancreas), to the liver.
The Portal Vein
The Portal Vein is formed by the union
of the superior mesenteric and splenic
veins.
Tributaries;
 Splenic
 Superior mesenteric
 L. gastric (receives oesophageal
veins)
 R. gastric
 Cystie
 Paraumbilical veins
Tributaries of the Splenic Vein
 Short gastric veins
 Left gastro-omental
 Pancreatic veins
 Inferior mesenteric (arises from
the pelvis and is a continuation
of the rectal vein (which receives
the sigmoid and left colic veins))
Superior Mesenteric Tributaries
 Jejunal vein
 Ileal vein
 Ileocolic vein
 R. colic vein
 Middle colic vein
 R. gastro-epiploic vein
 Pancreatico-duodenal veins
Lymph Vessels and Lymph Nodes of the GIT
If possible, superficial lymph vessels accompany superficial veins and deep lymph
vessels accompany deep lymph veins. This rule applies for the anterior abdominal
wall, however the abdominal cavity is an exception. In the abdominal cavity, the
lymph vessels accompany the arteries to their origins at the front of the aorta, rather
than the tributaries of the partal vein.
There are no major lymph nodes in the abdo cavity (remember, the major ones are:
cervical, axillary and inguinal).
Aortic lymph nodes are divided into 2 groups;
 Pre-aortic
o In front of the Aorta around the origins of the inferior mesenteric,
superior mesenteric and coeliac arteries
o This group of lymph nodes drain the unpaired viscera of the abdo
o Efferents from the pre-aortic lymph nodes may be collected by a
single (gastro) intestinal lymph truck
 Para-aortic
o Also called the lumbar
o Located on the side of the aorta
o Drains paired viscera (and the testes)
o Efferents from the para-aortic (lumbar) lymph nodes are collected by
the paired lumbar lymph trunks
Both the (gastro) intestinal lymph trunk and the paired lumbar lymph trunks drain
into the cisterna chyli (lymph sac located in front of bodies of L1 – 2 below the aortic
opening in the diaphragm) which then drained into the venous system.