Ross & Wilson Anatomy and Physiology in Health and Illness (129 page)

Small intestine
293

       Functions of the small intestine
295

       Chemical digestion in the small intestine
295

       Absorption of nutrients
296

Large intestine, rectum and anal canal
297

       Functions of the large intestine, rectum and anal canal
299

Pancreas
299

Liver
300

       Functions of the liver
301

Biliary tract
303

       Bile ducts
303

       Gall bladder
304

Summary of digestion and absorption of nutrients
304

Metabolism
305

       Carbohydrate metabolism
307

       Protein metabolism
308

       Fat metabolism
310

Diseases of the mouth
311

Diseases of the pharynx
312

Diseases of the salivary glands
312

Diseases of the oesophagus
313

Diseases of the stomach
314

Diseases of the intestines
316

Diseases of the pancreas
323

Diseases of the liver
324

Diseases of the gall bladder and bile ducts
326

ANIMATIONS

12.1
The alimentary canal
278

12.2
Oesophagus
288

12.3
Mouth: chewing and preparation for swallowing
288

12.4
Pharynx
289

12.5
Peristalsis
289

12.6
Stomach
291

12.7
Stomach: secretion of pepsinogen
291

12.8
Small intestine
294

12.9
Summary of digestion
297

12.10
Large intestine
298

12.11
Hepatic portal circulation
301

12.12
Biliary tract and secretion of bile
303

12.13
Factors influencing metabolic rate
306

12.14
Glycolysis
308

The digestive system is the collective name used to describe the
alimentary canal
, its
accessory organs
and a variety of digestive processes that take place at different levels in the canal to prepare food eaten in the diet for absorption. The alimentary canal begins at the mouth, passes through the thorax, abdomen and pelvis and ends at the anus (
Fig. 12.1
). It has a general structure which is modified at different levels to provide for the processes occurring at each level (
Fig. 12.2
). The digestive processes gradually break down the foods eaten until they are in a form suitable for absorption. For example, meat, even when cooked, is chemically too complex to be absorbed from the alimentary canal. Digestion releases its constituents: amino acids, mineral salts, fat and vitamins. Digestive
enzymes
(
p. 24
) that effect these changes are secreted into the canal by specialised glands, some of which are in the walls of the canal and some outside the canal, but with ducts leading into it.

Figure 12.1 
The digestive system.

Figure 12.2 
General structure of the alimentary canal.

After absorption, nutrients are used to synthesise body constituents. They provide the raw materials for the manufacture of new cells, hormones and enzymes, and the energy needed for these and other processes and for the disposal of waste materials.

The activities in the digestive system can be grouped under five main headings.

Ingestion

This is the taking of food into the alimentary tract, i.e. eating and drinking.

Propulsion

This mixes and moves the contents along the alimentary tract.

Digestion

This consists of:

•
mechanical breakdown
of food by, e.g. mastication (chewing)

•
chemical digestion
of food into small molecules by enzymes present in secretions produced by glands and accessory organs of the digestive system.

Absorption

This is the process by which digested food substances pass through the walls of some organs of the alimentary canal into the blood and lymph capillaries for circulation and use by body cells.

Elimination

Food substances that have been eaten but cannot be digested and absorbed are excreted from the alimentary canal as
faeces
by the process of
defaecation
.

The fate of absorbed nutrients and how they are used by the body is then explored. In the final section disorders of the digestive system are considered.

Organs of the digestive system (
Fig. 12.1
)

Learning outcomes

After studying this section, you should be able to:

identify the main organs of the alimentary canal

list the accessory organs of digestion.