The Digestive System ["A man does not live on what he eats, an old proverb says, but on what he digests." Jean Anthelme Brillat-Savarin, The Physiology of Taste: Or Meditations on Transcendental Gastronomy]

Objectives      

1. Describe stimuli and controls of digestive activity.

2. Describe the location and function of the peritoneum.

3. Define retroperitoneal and name the retroperitoneal organs.

4. Define splanchic circulation.

5. Indicate the importance of the hepatic portal system.

6. Describe the tissue composition and the general function of each of the four layers of the

alimentary canal.

7. Identify structural modifications of the wall of the stomach that enhance the digestive process.

8. Name the cell types responsible for secreting the various components of gastric juice and

indicate the importance of each component in stomach activity.

9. Describe stomach structure and indicate changes in the basic alimentary canal structure that

aid its digestive function.

10. Explain how gastric secretion and stomach motility are regulated.

11.  Identify and describe structural modifications of the wall of the small intestine that enhance the digestive process.

12.  Differentiate between the roles of the various cell types of the intestinal mucosa.

13.  Describe the function of local intestinal hormones and paracrines.

14.  State the role of bile in digestion and describe how its entry into the small intestine is regulated.

15.  Describe the role of the gallbladder.

16.  State the role of pancreatic juice in digestion.

17.  Describe how entry of pancreatic juice into the small intestine is regulated.

18.  List the major functions of the large intestine.

19.  List the enzymes involved in chemical digestion; name the foodstuffs on which they act.

20.  Describe the process of absorption of breakdown products of foodstuffs that occurs in the small intestine.

21.  Describe some of the structural and functional properties of smooth muscle.

Part 1: Overview of the Digestive System (pp. 852–858; Figs. 23.1–23.6)

A.    Digestive system organs fall into two main groups: the alimentary canal and the accessory organs.

1.     The alimentary canal, or gastrointestinal (GI) tract, is the continuous muscular digestive tube that winds through the body digesting and absorbing foodstuff; its organs include: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

2.     Accessory digestive organs aid digestion physically and produce secretions that break down foodstuff in the GI tract; the organs involved are the teeth, tongue,

gallbladder, salivary glands, liver, and pancreas.

I. Digestive Processes

A.    Ingestion is the simple act of putting food into the mouth.

B.    Propulsion moves food through the alimentary canal and includes both swallowing andperistalsis.

C.    Mechanical digestion is the physical process of preparing the food for chemical digestion and involves chewing, mixing, churning, and segmentation.

D     Chemical digestion is a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by enzymes.

E.    Absorption is the passage of digested end products from the lumen of the GI tract through the mucosal cells into the blood or lymph.

F.     Defecation eliminates indigestible substances from the body via the anus as feces.

II.  Basic Functional Concepts (p. 854; Fig. ]23.4)

A.    The digestive system creates an optimal internal environment for it's functioning in the lumen of the GI tract, an area that is technically outside of the body.

1.     Digestive activities within the GI tract are triggered by mechanical and chemical stimuli [e.g., stretching of organ by food, osmolarity, pH, etc.]

2.     The activities of the digestive system are regulated by

                a. Neural mechanisms [Enteric Nervous System (ENS)]: the ENS has roughly as                                many neurons as the spinal cord, and as many neurotransmitters as the brain,                              but whose functions remain largely unknown...

                                Short[myenteric] reflexes control smooth muscle contraction and                                            glandular secretion as relatively localized activities involving small                                          segments of the digestive tract; this mechanism is usually                                                             considered parasympathetic, but the plexus also contains sensory                                         neurons, motor neurons and interneurons for local reflexes whose                                    internal workings operate entirely outside the control of the CNS.

                                Long reflexesinvolve interneurons and motor neurons in the CNS and                                   provide a higher level of control over digestive and glandular activities.

                b. Hormonal mechanisms involve as many as 18 hormones, affecting almost                           every aspect of digestive function...

                c. Local mechanisms involve prostaglandins, histamine and other chemicals                           released into the interstitial fluid and affecting adjacent cells...