Physiology Of The Digestive System

PHYSIOLOGY OF THE DIGESTIVE SYSTEM The major function of the gastrointestinal system is to provide nutrients for the cells of the body. Nutrients derived from the food we ingest rendered absorbable by dissolving or breaking it down into simpler substance with the aid of secretions containing enzymes like, saliva, gastric, pancreatic and intestinal juices. These substances are then absorbed into the blood and lymph which carry them to the cells of the body. Unabsorbed food residues and a number of waste products are moved to the end of the tract and eliminated from the body. I. BRIEF ANATOMICAL CONSIDERATIONS A. GROSS: It is essentially a tube about 5 meters in length of variable cross-sectional are which include the mouth, pharynx. Esophagus, stomach, the small and large intestine, rectum and anus. It is collectively known as the digestive or the gastrointestinal tract. Other components of the gastrointestinal system which lie outside of the digestive tract, are the salivary glands, pancreas and the biliary system (liver, gallbladder and bile ducts). B. MICROSCOPIC: From inside to outside. (a)

mucus membrane with the characteristic of: - being thrown into folds - contains glands for mucus, (enzyme and hormone secretions

(b)

a thin layer of smooth muscle, usually longitudinal; the submucus muscle.

(c)

a layer of nerve cells and fibers; the submucus plexus of the Meissner’s plexus

(d)

a middle layer of smooth muscles arranged in a circular manner

(e)

another layer of nerve cells and fibers; the Myenteric plexus or the Auerbach’s plexus

(f)

outer layer of smooth muscles arranged longitudinally .

(g)

connective tissue layer - consists of fibrous and elastic tissue fibers.

(h)

Blood vessels, nerves and lymphatics

** The smooth muscles are condensed at strategic points to form the sphincters. II. INNERVATION OF THE GIT A. The gastrointestinal tract receives a dual extrinsic innervation from the autonomic nervous system: (a) Parasympathetic cholinergic - generally excitatory to GIT activity. - fibers are preganglionic and generally end on the nerve cells of the myenteric and meissner’s plexus (b) Sympathetic noradrenergic - generally inhibitory to GIT activity while stimulating contraction of sphincters - fibers are postganglionic, many of them end on postganglionic cholinergic neurons inhibiting acetylcholine secretion. - Some innervate blood vessels producing vasoconstriction. - Some appear to end directly on intestinal smooth muscle B. Major network of nerve fibers that are intrinsic to the GIT (a) Myenteric plexus - located between the outer longitudinal and middle circular smooth muscle layers. - excitatory mainly on the motor function of the gut - major stimulus is stretch of intestinal wall - mucosal receptor are mainly mechanoreceptors (b) Meissner’s plexus - located between the middle circular smooth muscle layer and the mucosa. - excitatory mainly on the secretory function of the gut

- major stimulus is irritation of the intestinal mucosa - mucosal receptors are chemorecptors that sense the composition of intestinal contents. III. ASPECTS OF GASTROINTESTINAL FUNCTIONS (a) Motility (b) Secretion (c) Digestion (d) Absorption IV. MOTOR FUNCTION OF THE GIT (a) Peristalsis - for propulsion of food in the different segments of the GIT in analward or aboral direction (b) Segmenting contractions - for mixing of food particles with the different secretions of the GI glands (c) Tonus of the GIT - continuous degree of contraction of the smooth muscles for the prevention of too much distention of the gut especially in the region of stomach, intestine and rectum. Control of the Motion Functions of the GIT: 1. Possible myogenic origin - the smooth muscles of the gut exhibits spontaneous activity 2. The myenteric plexus - believed to the responsible for the different local motor reflexes in the GIT 3. The autonomic nervous system – exercise some degree of control on the myenteric plexus or a direct effect on the smooth muscles and glands. 4. Hormones - these are humoral agents secreted by certain parts of the gut mucosa and the transported in the circulation to influence the function of certain components of the GIT. Movements of the GIT: 1. Mastication – a process wherein a reasonable number of opposing teeth interdigitate to adequately breakdown food particles and mix them with saliva to facilitated swallowing.

The masseter and temporalis muscle are the “power muscles” that produce the crushing force between the teeth when they occlude or contact a tough bolus of food. The medial pterygoid is a mandibular elevator. The lateral pterygoid and suprehyoid muscles open the mouth by pulling forward on the mandibular condyles and down backward on the symphysis. 2. Deglutition – movement responsible for transporting material from the mouth to the stomach. The act has been divided into three stages: Oral, Pharyngeal and Esophageal. The oral component is voluntary; pharyngeal and esophageal stages are involuntary. 3. Movements of the stomach: Functions: a) Volume adaptation (temporary storage) b) Grinding, mixing and emulsification of foodstuffs c) Propulsion of chime to small intestine at a rate suitable for optimal absorption. A) Passage of bolus from esophagus into the stomach results in a relaxation of the upper part of the stomach called receptive relaxation; probably neurally mediated. B) Digestive contractions are peristaltic in nature. C) The entire terminal portion of the amtrum constitutes a functional motor unit of which the pylorus is the most distal part. D) Gastric emptying, probably during the time interval before the impulses reaches the antrum. Factors affecting gastric emptying: 1. Rate of emptying is dependent upon amount ingested and the physical and chemical properties of the ingested material. 2. Recptors responsive to osmolarity, pH, and fatty acids are located in the upper duodenum; this inhibit gatric emptying by neural (enterogastric reflex) and/or hormonal (secretin enterogastrone) mechanism.

4. Movement of the small intestine: Functions: - Propulsion of luminal contents at a rate suitable for optimal digestion and absorption.

5. Movements of the large intestine: Functions: a) Storage b) Slow movements for optimal absorption 1. Haustral segmentstion (haustration) non -propulsive 2. Mass movement – peristaltic in nature, preceded by the disappearance of haustral segmentation. Delivers stool to rectum after perhaps 3 to 4 stops along the way---- the “call” or the urge to defecate. Eating stimulates mass movement; called the gastrocolic reflex; probably due to the release of the gastrointestinal hormone, gastrin or neurally mediated. Digestive juices: A. Saliva: Glands: parotid (serous) w/ Stensen’s duct Submaxillary (mixed) w/ Wharthon’s duct Sublingual (mucous) w/ Rivinus duct The parotid glands secretes the enzymes ptyalin which necessary for digestion, while the submaxillary and sublingual secretions are needed more for lubrication. Functions: a) digestive b) lubrication c) solvent action d) cleansing action – lysozyme e) helps in articulation f) for excretion – like in poisons

Gastric juices: Types of Gastric secretory cells. 1. Parietal cell – elaborates hydrochloric acid and the gastric intrinsic factor which is necessary for the optimal absorption of erythrocyte maturing factor (vit. B 12) 2. Chief cell – secretes the proteolytic enzyme pepsin. Most of these cell (parietal and chief cells) are present in the body and fundus of the stomach. 3. Mucous cells – all regions of the stomach possesses cells which secrete mucous and an alkaline fluid. 4. Antral “G” cells – secrete the hormone gastrin. 5. Surface epithelial cells of the stomach mucosa – secretes also an alkaline fluid and insoluble mucus. Pancreatic juice; Acinar cells – secretes the different pancreatic digestive enzymes. Cells of the intercalated ducts (ductal cells) – secretes a pancreatic juice, containing no enzymes, but mostly water and electrolytes, mainly bicarbonates. Functions: 1. Digestive – amylase which hydrolyzes starch and glycogen -lipase responsible for the gastrointestinal digestion of fats.

Bile Bile is approximately isotonic solution containing pigments, mixed bile acid-lecithin-cholesterol micelles and iorganic electrolytes. It is manufactured by the polygonal cells of the liver continuously, wether the person is feeding or fasting. The hepatic cells secrete it into the bile canaliculi, from which it passes into the hepatic duct and then the common bile duct. If the bile is not needed, the bile is stored in the gallbladder. Function of bile: a) vehicle for excretion of pigments and other waste products of metabolism. b) Bile salts are necessary as solvents for cholesterol c) Bile salts are also necessary for fat absorption and digestion d) Bile salts are important for optimal absorption of fat soluble vitamins (A,D,E,K) 6. Intestinal juices (small intestine) a) Volume and composition: around 3000 cc per day. Collectively known as succus entericus. Composed of two important constituents – mucus secretion for lubrication and protection of the intestinal mucosa. Most of those mucus secretion is coming from the Brunner’s gland (duodenum) Microbial Flora of the Digestive Tract a) mouth – enteroccocci, staphylococci, spirals, vibriones, amoeba and actinomyces b) stomach and proximal intestine – sterile c) last part of the small intestine – Escherichia coli d) large intestine – almost all kinds of bacteria in large amount