What Is Digestion?
Digestion is the process of transforming food into basic nutrients that can be absorbed and used by the
body. Just as a car needs gas to move, our bodies require fuel to function and perform daily activities.
Once absorbed, these nutrients can be used to fuel cellular growth, fight infections, and provide energy to
our muscles and brain. Without the process of digestion, the nutrients we initially consume in food would
be useless.
Before food even enters the mouth, a cascade of events involving the nervous system and a variety of
hormones are set off based on a variety of stimulus:
• cognition: thoughts about food can start the flow of saliva
• sound: hearing a description of the meal
• odor: smells stimulate a hunger response and influence its taste
• appearance: seeing the food
• taste: begins as the food enters your mouth and also how it feels
Once food enters the mouth, it begins to break down into smaller units in
both mechanicaland chemical processes. The mechanical process begins in the mouth with chewing. From
there, involuntary muscle contractions are used to move food mixtures along the tract, a process referred
to as peristalsis.
However, it should also be noted that chemical aids may be present along the various stages of digestion.
Enzymes, acid, bile, and mucus are all chemical means the body has at its disposal to further aid in
breaking down food. Notably, enzymes are proteins that catalyze (speed up) chemical reactions. Relative
to digestion, enzymes catalyze the chemical reactions required to break down food particles into smaller
parts, which prepares the nutrients for absorption.
Absorption
Nutrient absorption occurs across the wall of the gastrointestinal (GI) tract—a long hollow tube consisting
of several layers of tissue that begins with the mouth and ends at the anus. The mucosa(intestinal wall) is
the inner-most layer, and it is made of absorptive cells and glands. Circular and longitudinal
muscles comprise the outer layers, both of which function to mix and move food along the GI tract.
Notably, circular bands of muscle are most often found where one part of the tract connects to another,
serving as valves to control the flow of the food particles.
As shown in Figure 2.1, there are three processes the body can use to move nutrients from the GI tract
into the blood/lymph system and eventually into the cells:
1. Passive diffusion: Substances move easily in and out of cells without the use of energy. Nutrients move
from high to low concentrations.
2. Facilitated diffusion: No energy is required, but a special protein carrier is required to help substances
cross in or out of the cell. Nutrients move from high to low concentrations.
3. Active transport: Energy is required to move substances in or out of the cell. Nutrients move from low
to high concentrations.
Neither passive nor facilitated diffusion require an initial input of energy to transport nutrients across a
membrane. However, a specialized carrier (or gatekeeper) protein is required for facilitated diffusion. In
, both cases, nutrients naturally move from an area of high concentration to an area of low concentration.
Unlike passive transport, active transport not only requires an input of energy, it also moves nutrients
against the natural gradient, moving from areas of low concentration to areas of high concentration.
Anatomy & Physiology
As shown in Figure 2.3, the GI tract can be divided into six main parts:
1 - mouth
2 - esophagus
3 - stomach
4 - small intestine
5 - large intestine
6 - rectum
Additionally, four main organs produce and secrete substances that aid in digestion, but they are not part
of the GI tract:
1 - salivary glands
2 - liver
3 - gallbladder
4 - pancreas
Mouth (the beginning of the GI tract): The function of the mouth is to alter the food particles to prepare
them to be swallowed. As previously described, both mechanical and chemical digestions take place in the
mouth. Mechanical digestion occurs as the teeth break the food into smaller particles. The salivary glands
are then stimulated to secrete saliva. Saliva is a watery fluid containing (1) a lubricant (mucus) and (2)
enzymes to prepare food for the next step in the GI tract.
Mucus mixes with food, lubricating the particles and making it easier to swallow.
Enzymes released in the mouth have specific functions:
• Salivary amylase breaks down starches.
• Lingual lipase breaks down fats.
As an important component of the mouth, the tongue gives us the ability to taste food. The taste buds on
the tongue provide the pleasure sensation we experience when we eat. There are five flavors that can be
identified:
1 – sweet
2 – salty
3 – sour
4 – bitter
5 – umami (a meaty flavor)
Esophagus (the tube that connects the throat with the stomach): The role of the esophagus is to transport
food to the stomach. During the swallowing process, food lands on a flap of tissue called the epiglottis.
Digestion is the process of transforming food into basic nutrients that can be absorbed and used by the
body. Just as a car needs gas to move, our bodies require fuel to function and perform daily activities.
Once absorbed, these nutrients can be used to fuel cellular growth, fight infections, and provide energy to
our muscles and brain. Without the process of digestion, the nutrients we initially consume in food would
be useless.
Before food even enters the mouth, a cascade of events involving the nervous system and a variety of
hormones are set off based on a variety of stimulus:
• cognition: thoughts about food can start the flow of saliva
• sound: hearing a description of the meal
• odor: smells stimulate a hunger response and influence its taste
• appearance: seeing the food
• taste: begins as the food enters your mouth and also how it feels
Once food enters the mouth, it begins to break down into smaller units in
both mechanicaland chemical processes. The mechanical process begins in the mouth with chewing. From
there, involuntary muscle contractions are used to move food mixtures along the tract, a process referred
to as peristalsis.
However, it should also be noted that chemical aids may be present along the various stages of digestion.
Enzymes, acid, bile, and mucus are all chemical means the body has at its disposal to further aid in
breaking down food. Notably, enzymes are proteins that catalyze (speed up) chemical reactions. Relative
to digestion, enzymes catalyze the chemical reactions required to break down food particles into smaller
parts, which prepares the nutrients for absorption.
Absorption
Nutrient absorption occurs across the wall of the gastrointestinal (GI) tract—a long hollow tube consisting
of several layers of tissue that begins with the mouth and ends at the anus. The mucosa(intestinal wall) is
the inner-most layer, and it is made of absorptive cells and glands. Circular and longitudinal
muscles comprise the outer layers, both of which function to mix and move food along the GI tract.
Notably, circular bands of muscle are most often found where one part of the tract connects to another,
serving as valves to control the flow of the food particles.
As shown in Figure 2.1, there are three processes the body can use to move nutrients from the GI tract
into the blood/lymph system and eventually into the cells:
1. Passive diffusion: Substances move easily in and out of cells without the use of energy. Nutrients move
from high to low concentrations.
2. Facilitated diffusion: No energy is required, but a special protein carrier is required to help substances
cross in or out of the cell. Nutrients move from high to low concentrations.
3. Active transport: Energy is required to move substances in or out of the cell. Nutrients move from low
to high concentrations.
Neither passive nor facilitated diffusion require an initial input of energy to transport nutrients across a
membrane. However, a specialized carrier (or gatekeeper) protein is required for facilitated diffusion. In
, both cases, nutrients naturally move from an area of high concentration to an area of low concentration.
Unlike passive transport, active transport not only requires an input of energy, it also moves nutrients
against the natural gradient, moving from areas of low concentration to areas of high concentration.
Anatomy & Physiology
As shown in Figure 2.3, the GI tract can be divided into six main parts:
1 - mouth
2 - esophagus
3 - stomach
4 - small intestine
5 - large intestine
6 - rectum
Additionally, four main organs produce and secrete substances that aid in digestion, but they are not part
of the GI tract:
1 - salivary glands
2 - liver
3 - gallbladder
4 - pancreas
Mouth (the beginning of the GI tract): The function of the mouth is to alter the food particles to prepare
them to be swallowed. As previously described, both mechanical and chemical digestions take place in the
mouth. Mechanical digestion occurs as the teeth break the food into smaller particles. The salivary glands
are then stimulated to secrete saliva. Saliva is a watery fluid containing (1) a lubricant (mucus) and (2)
enzymes to prepare food for the next step in the GI tract.
Mucus mixes with food, lubricating the particles and making it easier to swallow.
Enzymes released in the mouth have specific functions:
• Salivary amylase breaks down starches.
• Lingual lipase breaks down fats.
As an important component of the mouth, the tongue gives us the ability to taste food. The taste buds on
the tongue provide the pleasure sensation we experience when we eat. There are five flavors that can be
identified:
1 – sweet
2 – salty
3 – sour
4 – bitter
5 – umami (a meaty flavor)
Esophagus (the tube that connects the throat with the stomach): The role of the esophagus is to transport
food to the stomach. During the swallowing process, food lands on a flap of tissue called the epiglottis.
