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

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)

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

Oesophagus

Liver

Gallbladder

Small Intestines

Rectum

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

The ENS can operate independently of the CNS or PNS to increase/decrease contraction of the GIT or increase/decrease secretion in the GIT.

Stimulation includes;o Stretching of the GITo Changes in pH in the GITo 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 pelvic nerve – both of these nerves contain efferent (motor) and afferent (sensory) fibres.

o 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)

o The pelvic nerves, originating in the sacral spinal cord (S2-S4) innervate the GI tract from the splenic flexure to the anal sphincter

o The effect of this is increased motility, increased secretion and relaxation of sphincters

o 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 branches2. Spenic Artery – this gives the pancreatic arteries, short gastric arteries and

left gastro epiploic arteries3. 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

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 abdoo Efferents from the pre-aortic lymph nodes may be collected by a

single (gastro) intestinal lymph truck Para-aortic

o Also called the lumbaro Located on the side of the aortao 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.