Nutrition and Health
1.1 Introduction to the Digestive System
The digestive system is a set of tissues and organs that, together, helps the human body
“break down food in order to absorb its nutrients” (Tresca, 2012). The digestive system itself
can be categorised into two sections, the gastrointestinal tract and the accessory organs.
The gastrointestinal tract is a series of hollow organs comprising of (in order of their
function) the mouth, oesophagus, stomach, small intestine, large intestine, rectum and the
anus. The gastrointestinal tract can also be broken into the upper portion including any
organs laying above the large intestine and the lower portion including all the organs after
and including the large intestine. Alongside the gastrointestinal tract are the accessory
organs that also aid the function and process of the digestive system. The accessory organs
consist of the liver, the gallbladder and the pancreas. Figure 1.1 depicts a labelled diagram
of the digestive system in its entirety.
Mouth
Palate
Uvula Salivary
Tongue Glands
Teeth Parotid
Pharynx Sublingual
Submandibular
Oesophagus
Liver
Stomach
Gallbladder
Pancreas
Small
Intestine
Duodenum Large Intestine
Jejunum Transverse Colon
Ileum Ascending Colon
Cecum
Descending Colon
Sigmoid Colon
Appendix
Rectum
Anus
Figure 1.1 (Stankiewicz, 2019) – Labelled Diagram of the Digestive System
, 1.2 Structure of the Digestive System
Mouth
Food molecules begin to pass through the gastrointestinal tract when they are placed into
the mouth, or oral cavity. Within the mouth larger food molecules are broken down into
more easily digestible sizes through a process called mastication. Mastication, or chewing, is
the first mechanical process that prepares the food for transportation through the
gastrointestinal tract; this lump of masticated food is called a ‘bolus’. The bolus is shaped
when pressed against the cheeks and palate before being pushed towards the pharynx.
Aiding this process is the salivary glands. The secretion of the digestive enzyme amylase
found in abundance within the saliva “initiates the process of enzymatic hydrolysis” (Keeton,
2021). Amylase breaks down starch into the molecules of the double sugar maltose and
dextrin. Additionally, the saliva lubricates the food making it easier to swallow, maintains
tooth enamel and reduces the levels of bacteria found within the mouth.
Oesophagus
The oesophagus is a connective, tubular and hollow organ used to transport the bolus from
the pharynx (located at the back of the oral cavity and beginning of the oesophagus) directly
into the stomach. The oesophagus is approximately 25cm long and lies behind the trachea
and heart but in front of the spinal column. The bolus is transported down the oesophagus
through a series of muscular contractions and relaxations called peristalsis. Peristalsis wave
contractions originate from the pharynx and create a pressure gradient that forces the bolus
to descend down the oesophagus. The upper third of the oesophagus pushes the bolus from
the pharynx downwards through voluntary contractions due to a lining of striated muscles.
The middle third is comprised of a combination of striated and smooth muscles and the
lower third contains only smooth muscles, rendering the remaining two thirds as an
involuntary movement. At the base of the oesophagus, there is a sphincter that prevents
gastric contents from rising and causing acid reflux into the oesophagus. This sphincter is
constantly in tension and only releases when the bolus reaches the sphincter. It then relaxes
and allows the bolus to enter the stomach.
Stomach
The stomach is a hollow organ that receives food from the lower third of the oesophagus.
Located on the upper left region of the abdomen, the stomach acts as a reservoir to allow
for the ingested food to be churned and combined with gastric juices. These gastric juices
are a mixture of hydrochloric acids, digestive enzymes and other organic substances that
allow for chemical digestion. Gastric chief cells or zymogenic cells are located within the
stomach and release the enzyme pepsinogen which is then activated and turned into the
digestive enzyme pepsin. Pepsin is vital for the breaking down of proteins within the food
molecules and turning them into amino acids or smaller peptides. When they reach to the
small intestine the amino acids are absorbed and transported into the bloodstream for use
around the body. Additionally, alongside the acids and enzymes that break down the food
molecules, mechanical digestion occurs through muscle contractions that churn the food.
The walls of the stomach are lined with three layers of “smooth muscle arranged in
longitudinal, circular, and oblique (diagonal) rows” (ADAM, 2012). The combination of
muscles creates multiple forms of contractions and relaxations that allows the bolus to be
churned and thoroughly mixed with the gastric juices. The stomach can also begin to absorb
1.1 Introduction to the Digestive System
The digestive system is a set of tissues and organs that, together, helps the human body
“break down food in order to absorb its nutrients” (Tresca, 2012). The digestive system itself
can be categorised into two sections, the gastrointestinal tract and the accessory organs.
The gastrointestinal tract is a series of hollow organs comprising of (in order of their
function) the mouth, oesophagus, stomach, small intestine, large intestine, rectum and the
anus. The gastrointestinal tract can also be broken into the upper portion including any
organs laying above the large intestine and the lower portion including all the organs after
and including the large intestine. Alongside the gastrointestinal tract are the accessory
organs that also aid the function and process of the digestive system. The accessory organs
consist of the liver, the gallbladder and the pancreas. Figure 1.1 depicts a labelled diagram
of the digestive system in its entirety.
Mouth
Palate
Uvula Salivary
Tongue Glands
Teeth Parotid
Pharynx Sublingual
Submandibular
Oesophagus
Liver
Stomach
Gallbladder
Pancreas
Small
Intestine
Duodenum Large Intestine
Jejunum Transverse Colon
Ileum Ascending Colon
Cecum
Descending Colon
Sigmoid Colon
Appendix
Rectum
Anus
Figure 1.1 (Stankiewicz, 2019) – Labelled Diagram of the Digestive System
, 1.2 Structure of the Digestive System
Mouth
Food molecules begin to pass through the gastrointestinal tract when they are placed into
the mouth, or oral cavity. Within the mouth larger food molecules are broken down into
more easily digestible sizes through a process called mastication. Mastication, or chewing, is
the first mechanical process that prepares the food for transportation through the
gastrointestinal tract; this lump of masticated food is called a ‘bolus’. The bolus is shaped
when pressed against the cheeks and palate before being pushed towards the pharynx.
Aiding this process is the salivary glands. The secretion of the digestive enzyme amylase
found in abundance within the saliva “initiates the process of enzymatic hydrolysis” (Keeton,
2021). Amylase breaks down starch into the molecules of the double sugar maltose and
dextrin. Additionally, the saliva lubricates the food making it easier to swallow, maintains
tooth enamel and reduces the levels of bacteria found within the mouth.
Oesophagus
The oesophagus is a connective, tubular and hollow organ used to transport the bolus from
the pharynx (located at the back of the oral cavity and beginning of the oesophagus) directly
into the stomach. The oesophagus is approximately 25cm long and lies behind the trachea
and heart but in front of the spinal column. The bolus is transported down the oesophagus
through a series of muscular contractions and relaxations called peristalsis. Peristalsis wave
contractions originate from the pharynx and create a pressure gradient that forces the bolus
to descend down the oesophagus. The upper third of the oesophagus pushes the bolus from
the pharynx downwards through voluntary contractions due to a lining of striated muscles.
The middle third is comprised of a combination of striated and smooth muscles and the
lower third contains only smooth muscles, rendering the remaining two thirds as an
involuntary movement. At the base of the oesophagus, there is a sphincter that prevents
gastric contents from rising and causing acid reflux into the oesophagus. This sphincter is
constantly in tension and only releases when the bolus reaches the sphincter. It then relaxes
and allows the bolus to enter the stomach.
Stomach
The stomach is a hollow organ that receives food from the lower third of the oesophagus.
Located on the upper left region of the abdomen, the stomach acts as a reservoir to allow
for the ingested food to be churned and combined with gastric juices. These gastric juices
are a mixture of hydrochloric acids, digestive enzymes and other organic substances that
allow for chemical digestion. Gastric chief cells or zymogenic cells are located within the
stomach and release the enzyme pepsinogen which is then activated and turned into the
digestive enzyme pepsin. Pepsin is vital for the breaking down of proteins within the food
molecules and turning them into amino acids or smaller peptides. When they reach to the
small intestine the amino acids are absorbed and transported into the bloodstream for use
around the body. Additionally, alongside the acids and enzymes that break down the food
molecules, mechanical digestion occurs through muscle contractions that churn the food.
The walls of the stomach are lined with three layers of “smooth muscle arranged in
longitudinal, circular, and oblique (diagonal) rows” (ADAM, 2012). The combination of
muscles creates multiple forms of contractions and relaxations that allows the bolus to be
churned and thoroughly mixed with the gastric juices. The stomach can also begin to absorb
