Gastrointestinal Physiology
PHG217
DR. O.O. MEDUBI
DEPARTMENT OF PHYSIOLOGY
FACULTY OF BASIC MEDICAL SCIENCES
COLLEGE OF MEDICINE
UNIVERSITY OF LAGOS
Outline

Organs of the digestive system.

Basic structure and functions of different parts of the digestive system.

Composition, secretion and functions of saliva.

Digestion and absorption of the major foods.

Classes of food and functions.

Balanced diet.

Metabolism of food.
Introduction / Organs of the GI system.
Learning Objectives
At the end of this lecture, students should be able to:
 Explain the importance of the gastrointestinal and how it
contributes to body homeostasis.

Name the organs of the gastrointestinal (GI) systems.

Explain the functions of the different part of the GI system.

Know how the gastrointestinal system is controlled.
INTRODUCTION
 The
GI tract functions as a major link in the
exchange of material and energy between the
body and the external environment.
 It
is composed of a long muscular tube (GI tract/
alimentary canal) extending from the mouth to the
anus, and a set of accessory organs.
INTRODUCTION
 The
alimentary canal consists of the oral cavity,
pharynx, oesophagus, stomach, small intestine, large
intestine, rectum, and anal canal.
 The
GI tract is unique among the organs of the body
in having a large degree of freedom from central
control.
Figure 1: The Gastrointestinal Tract
INTRODUCTION cont’d

The accessory organs include:

Tongue,

Teeth,

Salivary glands,

Pancreas,

Liver,

Gall bladder.
INTRODUCTION cont’d

The GI system provides the body with a continuous supply of water,
electrolytes and nutrients, and to achieve this requires:

Movement of food through the alimentary tract.

Secretion of digestive juices and digestion of the food.

Absorption of the digestive products, water, and the various electrolytes.

Circulation of blood through the GI organs to carry away the absorbed
substances.

Control of all these functions by the nervous and hormonal systems.
GENERAL ORGANISATION OF GIT

From the mouth to the anus, digestive canal is lined by a mucous
membrane (mucosa).

In the mouth, esophagus and anus, the epithelial layer is of a
stratified squamous variety about 10-15 cells thick.

In the region of absorption and secretion, a single -layered columnar
epithelium is found. This region is also characterized by folding and
the presence of finger-like projections called, villi.
GENERAL ORGANISATION OF GIT cont’d

From the outer(serosal) to the inner (luminal), the tissue layers of the GI
tract are:

Serosa

Longitudinal muscle

Myenteric plexus

Circular muscle

Submucosa

Mucosa
NB: each muscle layer functions as a syncytium.
Fig 2: Layers of the Gastrointestinal Tract.
Control of GI function


In general, the motor and secretory activity of the GI
tract is regulated and coordinated by nerves and
hormones.
The nervous control can be divided into two
components:
1.
Intrinsic nervous control (INC) (Enteric NS)
(a)
Myenteric plexus
(b)
Submucosal plexus
-
Can act independently of the ENC
- involves local reflexes confined to the wall of the
gut itself e.g peristaltic reflex.
Control of GI function (cont,d)
2. Extrinsic nervous control (ENC)
(a)
Parasympathetic nerve fibers: vagus and the pelvic nerve
-
Increases motility and secretion
-
Decreases sphincter tone
(a)
Sympathetic nerve fibers: T5 - L2
-
Innervate all levels of the GIT
-
Increases sphincter tone
-
Reduces blood flow to the GIT
3. Sensory fibers
-
Sensory nerve fibers (mechanical and chemical) feeds the CNS
-
Initiates reflexes that control the motility and secretion of the
gut.
Blood Supply of GIT

Splanchnic bed (superior and inferior mesenteric arteries), with the
capillary system extending into all of the villi.

Blood flows out of the tract through the portal veins, directly to the
liver.

This is the “first pass” effect, whereby absorbed substances are routed
directly to the liver (without entering into the general circulation),
where much processing occurs.
Lymphatics

Lymph lacteals (lymphatic capillaries) are found throughout the
intestines and extend into the villi of the small intestine, allowing
absorption of lipids.

The lymphatic system is also extensive in the liver, where it absorbs
fluids and proteins and transports them to the systemic venous blood.

Lymphatic transport of proteins contributes substantially to plasma
oncotic pressure and provides binding proteins for circulating
substances including hormones, calcium, and iron.
General Functions of GIT

1. Digestion

2. Endocrine

3. Expulsion

4. Protection (HCl, IgA, Peyer’s patches)

5. Motility

6. Absorption

7. Secretion

8. Storage
Assignment 1:

How does the gastrointestinal system contributes to body homeostasis?
THANKS