What is the digestive system?
The digestive system is made up of the gastrointestinal tract, the liver, pancreas and
gallbladder. The gastrointestinal tract is made up of hollow organs (mouth, esophagus,
stomach, small intestine, large intestine, and anus), and the liver, pancreas and gallbladder
are the solid organs. The digestive system works to break nutrients that we consume down
into small enough parts for our body to absorb them (NIDDK, 2023).
Roles and anatomies of the units in the digestive system
Mouth
The mouth is an oval shaped cavity inside the skull which mainly functions to break down
food to make it easier to swallow and digest, and to speak. The different parts of the mouth
are the lips, vestibule, gums, teeth, hard and soft palate, tongue and salivary glands (Better
Health Channel, 2012). At the roof of the mouth is the palate which is made of bone and
muscle and separates the mouth from the nasal cavity. There is a lining all over the inside of
our mouth called the oral mucosa, and we have gums made of tissue which keep our teeth
in place. We have a tongue which is a muscle that helps us speak and moves food around
our mouth, and 32 teeth which crush and tear down food to aid with digestion. In our mouth
are three pairs of salivary glands which produce saliva, a mixture of enzymes, proteins,
mucus and electrolytes which all serve to pick up food debris and bacteria from around the
mouth, and begin the process of breaking down food as it is about to be swallowed. Our lips
are composed of a surface epidermis (skin), connective tissue and a layer of muscle. The
outer skin layer contains hair, sweat and sebaceous glands, and the interior surface is lined
with a moist mucous membrane (Lips | anatomy, 2019).
Pharynx
Commonly referred to as the throat, the pharynx is the passageway that connects the mouth
and nose to the breathing passages (trachea and lungs) and the oseophagus. It is split into
three parts: the nasopharynx, the oropharynx, and the laryngopharynx. It is a ring-like tube
made of muscle that serves both the respiratory and digestive systems as it receives air from
the nasal cavity and air food and water from the oral cavity (Philadelphia, 2014)(National
Cancer Institute, n.d.).
Oesophagus
The oesophagus is a tube made of muscle which connects our mouth to our stomach,
contracting and carrying food and liquids from the pharynx. Its walls are made of four layers
of tissue: the mucosa, the submucosa, the muscularis propria, and either the serosa or
adventitia. The mucosa is a thin layer of cells which line the inside and has glands which
secrete a mucus to keep lining moist and protect it. The muscularis propria is made of
smooth muscle tissue which contracts in order to move food and liquid through the
oesophagus. (https://www.osmosis.org/learn/Esophagus_histology)
Stomach
The stomach is an organ which has several functions in the digestive system in order to aid
the digestion of food and absorption of nutrients and minerals from what we intake. It has
muscles which churn the food and contains digestive juices (acids and enzymes) which all
break down the food in order to make it more digestible (kidshealth.org, n.d.). The four main
layers of the stomach wall include the mucosa, submucosa, muscularis externa, and the
serosa. The mucosa (innermost layer) is very important as it is covered by epithelial tissue
and composed of gastric glands which secrete gastric juices - these help digest the food
contained within the stomach.
, Small intestine (duodenum, jejunum, ileum)
The small intestine has three parts that all help to further digest food that comes from the
stomach. It absorbs nutrients (proteins, carbohydrates, vitamins, minerals and fats) and
water from the food so that they can be used by the body. The organ itself is about 20 feet
long and connects the stomach to the large intestine; it folds many times over in order to fit
inside the abdomen (National Cancer Institute, 2011). The duodenum is the initial part of the
small intestine which is split into four parts (superior, descending, horizontal, and ascending)
and is where absorption begins. Enzymes from the pancreas enter the duodenum to break
down food, bicarbonate is secreted into it in order to neutralise stomach acid, and the liver
introduces bile into it in order to further break down and absorb lipids from food. The
jejunum’s primary function is to absorb sugars, amino acids, and fatty acids, and the ileum
absorbs any remaining nutrients that were not previously absorbed, particularly vitamin B12
and bile which is recycled. The small intestine is made of a collection of blood vessels,
nerves, and muscles which all work together in order to perform its functions; its layers
include the serosa (outside layer), the muscularis (a thin outer layer which shortens and
elongates the gut, and a thick inner layer which causes constriction; these muscle layers
work together to move and churn food with enzymes down the GI tract), the submucosa
(connective tissue containing blood vessels and nerves) and the mucosa (innermost layer
with a large surface area for maximum absorption) (Collins, Nguyen and Badireddy, 2020).
Large intestine
The large intestine is composed of the colon, rectum and anus; it’s one long tube which
continues from the small intestine and turns food waste into stool, passing it from the body
when you go to the toilet. When food reaches the large intestine, most of its nutrients have
been absorbed, and therefore its job is to dehydrate what's left by absorbing water and
electrolytes as it passes through the system until it becomes stool. The colon secretes
mucus to bind and lubricate the food waste in order to help it pass through smoothly, and
contains bacteria which break down remaining carbohydrates in order to produce vitamins K
and B which are absorbed through the mucosa. When the colon’ muscle movements deliver
the food to the rectum, the stool is made of indigestible matter and dead cells from our
intestines. When the stool enters the rectum, our body’s urge to excrete it is triggered, and it
reaches the anal canal which it will travel through when it leaves our body. The anus is the
canal itself, and it is shut on each side by a muscle sphincter. The internal sphincter opens
automatically to let the waste through, and the external sphincter is the one that we control
to allow it out when we’re ready. When the stool in the rectrum triggers our body’s urge to
secrete it, nerves send signals to the internal sphincter to cause it to relax. The walls of the
large intestine are made up of muscles and tissues, and there are separate layers of circular
and longitudinal muscles which allow it to contract in a number of ways. Its mucous lining
blood supply, nerve endings and glands (Cleveland clinic, 2021).
Pancreas
The pancreas is an organ and gland located in the back of our abdomen. It performs two
main functions:
1. The exocrine function, where it produces enzymes to aid with digestion
2. And the endocrine function, where it sends out hormones to control the amount of
sugar in our bloodstream.
In the context of digestion, the exocrine system is the most relevant, and it consists of
lacrimal (tear) glands, mammary glands, mucous membranes, the prostate, salivary glands,
sebaceous (oil) glands, and sweat glands. They all produce substances which travel through
a duct which all aid with the digestion of food. The pancreas itself releases the enzymes
