Digestion, Absorption and Transport

1
Chapter 3

• Digestion, Absorption and Transport

2
The process of digestion transforms all kinds of
foods into nutrients.
3
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.
4
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.
5
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.
6
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.
7
The LOWER ESOPHAGEAL SPHINCTER closes to prevent
the BOLUS from re-entering the ESOPHAGUS.
8
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.
9
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.
10
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.
11
The SMALL INTESTINE is about 3 meters long. It is
composed of 3 segments Duodenum Jejunum Ileum
The SMALL INTESTINE does not have the protective
lining or GOBLET CELLS to protect it. It is very
susceptible to damage.
12

• Therefore, as the CHYME enters the DUODENUM,
  several things happen
• 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.

13
The pH Scale Comparison of Digestive Juices
14
The enzymes produced by the PANCREAS act
to further breakdown ALL of the major energy
nutrients CARBOHYDRATES PROTEINS 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.
15
Simple Diffusion
Facilitated Diffusion
Active Transport
There are three ways that these small nutrient
molecules can move into cells. Simple Diffusion,
Facilitated Diffusion, Active Transport
16
Villi
Muscles
17
Capillaries in the Vascular System (red - blue)
Lymphatic System (yellow)
18
Electron Micrograph of Cells
Microvilli
19
(No Transcript)
20
(No Transcript)