Nutrition Science and Everyday Application - beta v 0.1

THE DIGESTIVE SYSTEM 147
powerful peristaltic contractions help mash, pulverize, and churn it into chyme. Chyme is a
semiliquid mass of partially digested food along with gastric juices secreted by cells in the
stomach. These gastric juices contain hydrochloric acid, which lowers the pH of the chyme
in the stomach. This acidic environment kills many bacteria or other germs that may have
been present in the food, and it causes the three-dimensional structure of dietary proteins to
unfold. Gastric juices also contain the enzyme pepsin, which begins the chemical breakdown
of proteins in the peanut butter and bread. Gastric lipase continues the breakdown of fat
from the peanut butter.
Small Intestine
Chyme released from the stomach enters the small intestine, where most digestion and
absorption occurs. The small intestine is divided into three parts, all part of one continuous
tube: the duodenum, the jejunum, and the ileum.
Once the chyme enters the duodenum (the first segment of the small intestine), the
pancreas and gallbladder are stimulated to release juices that aid in digestion. The pancreas
(located behind the stomach) produces and secretes pancreatic juices which consist mostly
of water, but also contain bicarbonate that neutralizes the acidity of the stomach-derived
chyme and enzymes that further break down proteins, carbohydrates, and lipids. The small
intestine’s absorptive cells also synthesize digestive enzymes that aid in the breakdown of
sugars and proteins.
The gallbladder (a small sac located behind the liver) stores, concentrates, and secretes a
fluid called bile that helps to digest fats. Bile is made in the liver and stored in the gallbladder.
Bile is an emulsifier; it acts similar to a detergent (that would remove grease from a frying
pan) by breaking large fat droplets into smaller fat droplets so they can mix with the watery
digestive juices.
Peristalsis and segmentation control the movement and mixing of chyme through the
small intestine. As in the esophagus and stomach, peristalsis consists of circular waves of
smooth muscle contractions that propel food forward. Segmentation helps to mix food with
digestive juices and facilitates absorption.
Nutrient absorption takes place mainly in the latter part of the small intestine, the ileum.
The small intestine is perfectly structured for maximizing nutrient absorption. Its surface
area is greater than 200 square meters—about the size of a tennis court! The large surface
area is due to the multiple levels of folding, villi, and microvilli that cover the internal tissue
of the small intestine. Villi are tiny finger-like projections that are covered with enterocytes or
absorptive cells. The absorptive cell membrane is made of even smaller projections, called
microvilli (Figure 3.12). These microvilli are referred to collectively as the brush border since
their appearance resembles the bristles on a brush.