Introductory Anatomy: Digestive System

Dr D.R.Johnson, Centre for Human Biology

Purpose

The digestive system prepares food for use by hundreds of millions of body cells. Food when eaten cannot reach cells (because it cannot pass through the intestinal walls to the bloodstream and, if it could would not be in a useful chemical state. The gut modifies food physically and chemically and disposes of unusable waste. Physical and chemical modification (digestion) depends on exocrine and endocrine secretions and controlled movement of food through the digestive tract.

Mouth

Mouth Food enters the digestive system via the mouth or oral cavity, mucous membrane lined. The lips (labia) protect its outer opening, cheeks form lateral walls, hard palate and soft palate form anterior/posterior roof. Communication with nasal cavity behind soft palate. Floor is muscular tongue. Tongue has bony attachments (styloid process, hyoid bone) attached to floor of mouth by frenulum.
Posterior exit from mouth guarded by a ring of palatine/lingual tonsils. Enlargement = sore throat, tonsillitis.
Food is first processed (bitten off) by teeth, especially the anterior incisors. Suitably sized portions then retained in closed mouth and chewed or masticated (especially by cheek teeth, premolars, molars) aided by saliva Ducted salivary glands open at various points into mouth. This process involves teeth (muscles of mastication move jaws) and tongue (extrinsic and intrinsic muscles). Mechanical breakdown, plus some chemical (ptyalin, enzyme in saliva). Taste buds allow appreciation, also sample potential hazards (chemicals, toxins)

Swallowing

In leaving the mouth a bolus of food must cross the respiratory tract (trachea is anterior to oesophagus) by a complicated mechanism known as swallowing or deglutination which empties the mouth and ensures that food does not enter the windpipe.
Swallowing involves co-ordinated activity of tongue, soft palate pharynx and oesophagus. The first (buccal) phase is voluntary, food being forced into the pharynx by the tongue. After this the process is reflex. The tongue blocks the mouth, soft palate closes off the nose and the larynx rises so that the epiglottis closes off the trachea. Food thus moves into the pharynx and onwards by peristalsis aided by gravity. If we try to talk whilst swallowing food may enter the respiratory passages and a cough reflex expels the bolus.

Oesophagus

The oesophagus (about 10") is the first part of the digestive tract proper and shares its distinctive structure. Basic tissue layers of the gut are
1. mucosa. Innermost, moist lining membrane. Epithelium (friction resistant stratified squamous in oesophagus, simple beyond) plus a little connective tissue and smooth muscle.
2. submucosa. Soft connective tissue layer, blood vessels, nerves, lymphatics
3. muscularis externa. Typically circular inner layer, longitudinal outer layer of smooth muscle
4. serosal fluid producing single layer.

Stomach

C shaped, left side abdominal cavity (because liver is on right). Cardioesophageal sphincter guarding entrance from oesophagus is of doubtful anatomical integrity (though functionally the diaphragmatic pinch cock serves). Pyloric sphincter guarding the outlet is much better defined. Fundus, body and pylorus recognised as distinct regions. Stomach secretes both acid and mucus (for self protection). Surface area increased by rugae. Serves as a temporary store for food which is also churned by muscular layers (three here) to form chyme, creamy substance voided via pyloric sphincter to duodenum

Duodenum

First part of small intestine. C shaped 10" long and curves around head of pancreas and entry of common bile duct (accessory organs of digestion, pancreas, liver see below). Chemical degradation of small controlled amounts of food controlled by pyloric sphincter begins here, enzymes secreted by pancreas and duodenum itself aided by emulsifying bile (which also lowers pH). Duodenal ulcers caused by squirting of acid stomach contents into duodenal wall opposite sphincter.

Small Intestine

Jejunum (8 feet) and ileum (12 feet) continue degenerative process. Surface area increased by plica circulares (circular folds) carrying villi: cells of villi carry microvilli. Each villus has a capillary and a lacteal (lymphatic capillary) Absorption of digested foodstuffs is via these to the rich venous and capillary drainage of the gut. Towards the end of the small intestine accumulations of lymphoid tissue (Peyer's patches) more common. Undigested residue of food is rich in bacteria.

Large Intestine

Jejunum terminates at caecum. Caecum is small saclike evagination, important in some animals as a repository for bacteria/other organisms able to digest cellulose. A blind ending appendix may give trouble (appendicitis) if infected. The large intestine has three longitudinal muscle bands (taenia coli) with bulges in the wall (haustra) between them. These may evaginate in the elderly to become diverticuli and infected in diverticulitis.
The large intestine resorbs water then eliminates drier residues as faeces. Regions recognised are the ascending colon, from appendix in right groin up to a flexure at the liver, transverse colon, liver to spleen, descending colon, spleen to left groin, then sigmoid (S-shaped) colon back to midline and anus. Anus has voluntary and involuntary sphincter and ability to distinguish whether contents are gas or solid. No villi in large intestine, but many goblet cells secreting lubricative mucus.

Accessory digestive organs

Salivary glands

Three pairs, parotid, submandibular, sublingual. Mumps begins as infective parotitis in the parotid glands in the cheek. The others open into the floor of the mouth. Saliva is a mixture of mucus and serous fluids, each produced to various extents in various glands. Also contains salivary amylase, (starts to break down starch) lysozyme (antibacterial) and IgA antibodies. In some mammals (and snakes!) saliva may be poisonous, quietening down living prey.

Pancreas

Endocrine and exocrine gland. Exocrine part produces many enzymes which enter the duodenum via the pancreatic duct. Endocrine part produces insulin, blood sugar regulator.

Liver and gallbladder

Bile, a watery greenish fluid is produced by the liver and secreted via the hepatic duct and cystic duct to the gall bladder for storage, and thence on demand via the common bile duct to an opening near the pancreatic duct in the duodenum. It contains bile salts, bile pigments (mainly bilerubin, essentially the non-iron part of haemoglobin) cholesterol and phospholipids. Bile salts and phospholipds emulsify fats, the rest are just being excreted. Gallstones are usually cholesterol based, may block the hepatic or common bile ducts causing pain, jaundice.

Liver

Multifunctional: important in this context since the capillaries of the small intestine drain fat and other nutrient rich lymph into it via the hepatic portal system.

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