Chapter 3Chapter 3

Digestion, Absorption and Digestion, Absorption and TransportTransport

The process of digestion transforms all kinds of foods into nutrients.

By the end of this chapter you should be able to follow the ingestion of food from the mouth all the way to the the end of the gastrointestinal (GI) tract.

You will need to describe the mechanical, biochemical, and regulatory processes along the way.

As food is passed into the mouth, it is MASTICATED by the teeth and tongue (process referred to as mastication).

Simultaneously, the salivary glands excrete SALIVA. Saliva contains starch-digesting enzymes which begin to break down the starch to sugars.

As you swallow, the food is guided by the PHARYNX into the ESOPHAGUS.

A flap of tissue called the EPIGLOTTIS prevents the food (usually) from entering the TRACHEA which leds to the lungs.

As the food/saliva mixture enters the ESOPHAGUS, it is now called a BOLUS.

The ESOPHAGUS has SPHINCTER muscles at each end (Upper and Lower).

The UPPER ESOPHAGEAL SPHINCTER opens to allow the food to enter the ESOPHAGUS.

As the BOLUS moves downwards, the ESOPHAGUS passes through an opening in the DIAPHRAGM towards the STOMACH.

The LOWER ESOPHAGEAL SPHINCTER opens to allow food to enter into the STOMACH.

The LOWER ESOPHAGEAL SPHINCTER closes to prevent the BOLUS from re-entering the ESOPHAGUS.

As the BOLUS enters the STOMACH, GASTRIC JUICES are secreted by cells of the STOMACH.

These JUICES contain water, enzymes and HCl (acid).

This acid lowers the pH of the STOMACH to about 1.5 – 2.

The BOLUS / JUICE mixture is now referred to as CHYME. The acid kills bacteria in the BOLUS and activates the enzymes that digest PROTEIN.

The entire GI tract is lined with multi-directional muscles that move the food along (referred to as SEGMENTATION)

The STOMACH MUSCLES flex and relax to grind the CHYME until it no longer appears as food.

The STOMACH cells would also be digested under these conditions.

However, there is a resistant lining of cells in the STOMACH and special GOBLET CELLS produce MUCOUS that coats the lining protecting the STOMACH from digestion

At the bottom of the STOMACH, the PYLORIC SPHINCTER opens to allow the CHYME into the SMALL INTESTINE.

The SMALL INTESTINE is about 3 meters long. It is composed of 3 segments –

DuodenumJejunum

Ileum

The SMALL INTESTINE does not have the protective lining or GOBLET CELLS to protect it.

It is very susceptible to damage.

Therefore, as the CHYME enters the DUODENUM, several things happen:

1. PANCREAS produces PANCEATIC JUICE (NaHCO3 (bicarbonate) and ENZYMES) through the same DUCT.

2. BILE produced in the LIVER, stored in the GALLBLADDER is added through the BILE DUCT.

The pH Scale:

Comparison of Digestive Juices

The enzymes produced by the PANCREAS act to further breakdown ALL of the major energy nutrients:

CARBOHYDRATESPROTEINS

FAT

as the food moves along the small intestine.

Now ADSORPTION also begins to take place. Small molecules are transported into cells lining the small intestine.

There are three ways that these small nutrient molecules can move into cells.

Simple Diffusion, Facilitated Diffusion, Active Transport

Simple Diffusion Facilitated Diffusion Active Transport

Villi

Muscles

Lymphatic System(yellow)

Capillaries in theVascular System

(red - blue)

MicrovilliElectron Micrograph of

Cells