The digestion system is the engine of the body; it is where food enters, is broken down and distributed to areas within the body that can process the constituent nutrients.
Without a properly functioning digestive system, the body will not have access to the nutrients it needs. This will result in nutritional deficiencies.
In order to keep the digestive system healthy and efficient, you need to understand how our organs aid digestion.
As a nutritionist, you have to address issues with your clients digestion and provide them with solutions to their nutritional short-falls.
Similarly, understanding the processes involved will help you achieve the necessary balance when designing diet plans for specific individuals.
The Anatomy of the Digestion System
The digestive system facilities:
- Intake and break down of food
- Absorption of nutrients
- Excretion of waste
The organ groups that accomplish this are the gastrointestinal tract and the accessory digestive system.
The Gastrointestinal Tract
The gastrointestinal tract, or the G.I tract as it is often called, is essentially a long tube that runs from the oral cavity (the mouth) to the anus. Lets take a look a the process.
1. The G.I tract begins with the oral cavity, which contains the tongue, salivary glands and teeth.
2. Beyond the oral cavity is the pharynx, or throat, which houses the epiglottis, a functional flap of skin that covers the entryway to the windpipe when food is being swallowed to prevent food being deposited in the lungs.
As the epiglottis blocks entry to the windpipe, it also serves to channel food into the esophagus; the muscular tube that deliveries food into the stomach.
3. Once the partially digested food reaches the stomach, it goes through a more intensive break-down process and then moves into the small intestine through the pyloric sphincter.
A sphincter is a muscular valve that controls the inflow of material at the junction between organs. There are numerous sphincters throughout the G.I tract.
The small intestine is so named because of its narrow diameter, not its length; in actual fact it is more than six meters long, and constitutes the longest section of the G.I tract.
4. The first 25cm of the small intestine is known as the duodenum, and is the site of intense exposure to digestive juices which break-down food at the microscopic level.
Major nutrient absorption also takes place in the small intestine, as the internal walls have large folds covered with hair-like projections called villi, which increase the surface are of the small intestine, and facilitate nutrient adsorption.
5. After the digested food has made its way through the small intestine, it enters the colon (large intestine) via another sphincter.
The digested material then loses any water that was not absorbed in the small intestine, and is compressed to form faeces.
The terminal end of the colon is the rectum, which houses solid waste once is has solidified into faeces.
The anus, a sphincter that allows faeces to exit the body, responds to pressure from the rectum when it is full, but only open when intentionally relaxed, allowing for control of defecation.
The Accessory Digestive System
With the exception of the salivary glands, the accessory digestive organs are located outside the G.I tract.
- Gall Bladder
- Salivary Glands
All of these organs aid digestion. They achieve this through the production of chemical substances that:
- Break down carbohydrates and protein
- Emulsify fat
- Maintain an ideal pH balance within the G.I tract
The salivary glands are located inside the oral cavity, and release saliva into the mouth via ducts.
Similarly, the gall bladder has ducts located om both the liver and the duodenum, which facilitate the transport of bile from the liver to the duodenum.
The pancreas has ducts, which reach into the duodenum as well, allowing it to deliver pancreatic juice to the small intestine for the purpose of maintaining intestinal pH and breaking down protein and carbohydrate.
The Function of the Oral Cavity
The oral cavity is the initial site of food digestion, and is composed of the cheeks, hard and soft palates, and the tongue.
Within the oral cavity are the salivary glands and teeth, which provide the initial means of chemical and mechanical digestion, respectively.
The Hard Palate and The Soft Palate
The hard and soft palates form the roof of the oral cavity.
1. The hard palate is the bony section located immediately above the tongue, and serves to divide the oral and nasal cavities.
2. The soft palate is located directly behind the hard palate; if you run your tongue from the front to the roof of your mouth to the rear, you will feel the transition from hard palate to soft palate.
3. The soft palate terminates with a hanging ball of tissue at the back of the throat, known as the uvula.
Together, the uvula and soft palate seal off the entrance to the nasal passage during the swallowing reflex to prevent food and liquid from entering.
If you’ve ever breathed in while eating or drinking and felt as though the food or drink went up your nose, it’s because it did. This is because the soft palate only contracts when you are swallowing.
It remains open when you breathe in to allow air to travel down the windpipe (trachea) and through the nasal passage.
The tongue is a muscular organ covered with a mucous membrane that forms the floor of the oral cavity.
The tongue aids in mechanical digestion (the physical breaking up of food) It achieves this by:
- Moving food around in the mouth
- Making it accessible to the teeth
- Forming it into a ball (a bolus)
- Pushing it to the rear of the oral cavity to be swallowed
The tongue is covered with small projections called papillae, which allow the tongue to grip food and move it around the oral cavity efficiently.
There are four types of papillae located on the tongue;
- Two types that contain taste buds (fungiform papillae and vallate papillae)
- One type that contains taste buds during childhood only (foliate papillae)
- One type that contains touch receptors (fusiform papillae)
The Salivary Glands
The salivary glands are located within oral cavity, and secrete saliva through ducts located in the:
- Oral mucosa
Saliva serves to cleanse the mouth and teeth, as well as aid in the chemical breakdown of food.
Note: The term chemical digestion refers to the breaking apart of food molecules into their smaller constituent parts.
It occurs on a microscopic scale, through the action of enzymes biological molecules that break apart other biological molecules.
Saliva contains enzymes that destroy bacteria, and break down starches and lipids (fats) that enter the oral cavity.
Saliva production is stimulated by smell, sight, sound and thoughts of food, which warn the oral cavity to prepare for ingestion of food in advance of it arriving.
Similarly, saliva is produced when there is food within the oral cavity in order to maintain the digestion process, and cleanse the mouth once the food is swallowed.
Teeth are responsible for the initial mechanical or ‘macroscopic’ (can be seen with the naked eye) break-down of food.
Teeth are composed of dentin, a calcified connective tissue that provides strength and stability.
Dentin is denser than bone, as it contains 70% more calcium, which provides strength for tearing and grinding food.
Teeth work closely in conjunction with the tongue. As the teeth grind food, the tongue forms it into a bolus, which is pushed to the rear of the oral cavity to be swallowed.
The Function Of The Stomach
The stomach is a dual purpose organ; it functions through mechanical and chemical digestion. Plus it acts as storage when too much food is consumed at one time for the small intestine to process.
This is how it works:
1. The interior of the stomach, known as the lumen, is composed of folds that form deep indentations, known as rugae.
2. The base of the rugae, known as gastric pits, are lined with gastric glands containing cells that secrete mucous, enzymes and acid that all combine to form gastric juice.
3. The acid secreted by the gastric glands is hydrochloric acid, and functions to kill bacteria entering the stomach, prepare minerals for absorption and denatures proteins; allowing enzymes to break the protein down.
4. The hormone gastrin is also produced within the gastric glands; gastrin enters the bloodstream, and stimulates the growth of gastric glands, strengthens the sphincter between the esophagus and the stomach, and relaxes the pyloric sphincter.
5. When food enters the lumen, mechanical digestion begins, as the rugae contract in a motion known as peristalsis.
The motion churns food within the stomach, breaking it thoroughly apart. Peristalsis also aids in chemical digestion, as it combines the food with the gastric juices, producing a material known as chyme.
The enzymes of the gastric juice then act upon the chyme, initiating the breakdown of proteins; which up to this point have not been acted upon by enzymes, as saliva only contains enzymes that act upon carbohydrates and fats.
6. Peristaltic waves force the chyme toward the pyloric sphincter, the muscular valve that separates the stomach and the duodenum. Due to the quantity of food that is ingested at one time, the stomach does not simply force all of the chyme into the small intestine.
Rather, every peristaltic waves forces a few milliliters of chyme into the duodenum.
The process of emptying the contents of the stomach into the small intestine after a meal typically lasts 2 – 4 hours.
The Function Of The Liver, Gall Bladder And Pancreas
The liver and gall bladder work in conjunction to provide the small intestine with bile to process lipids.
The liver is the second largest organ in the body, second only to the skin.
It is composed of two lobes; the right lobe, which is the larger of the two, and the left lobe.
The lobes of the liver are further subdivided into lobules. These are functional units that contain liver cells called hepatocytes.
Blood passes through the liver via spaces known as sinusoids which also house cells that are responsible for destroying work out cells, and bacteria that come from digestive tract.
One of the liver’s key functions is the production of bile. This is a substance that contains bile salts which are responsible for the break-down and absorption of lipids (fats).
Bile also delivers some of the byproducts of the liver’s other functions (breaking down of old cells, detoxification, excretion of bilirubin, and protein and carbohydrate metabolism) to the G.I tract, where it is transported out of the body.
In the course of one day, the liver excretes approximately 1 litre of bile.
The Gall Bladder
The gall bladder is located slightly to the rear of the liver (posterior). It connects the liver to the small intestine via the common hepatic duct.
The purpose of this connection is to transport bile from the liver to the small intestine. When bile is not needed in the small intestine, the gall bladder also serves to concentrate and store bile.
The pancreas is located below the liver, and is in direct contact with the small intestine.
Like the gall bladder. the pancreas is connected to the duodenum via ducts.
Pancreatic juice is central to the process of digestion. This is because it maintains a safe pH balance within the small intestine, allowing enzymes to function ideally and preventing the acid that is contained in chyme from damaging the lining of the small intestine.
Pancreatic juice also contains enzymes that function in breaking down protein, carbohydrates, and lipids.
The Function of the Small Intestine
We have already discussed the small intestine quite regularly.
This is due to the fact that the small intestine is the main location for nutrient break-down and water absorption in the digestive system.
Chyme is transported through the small intestine in two ways:
This occurs when a large portion of chyme stretches the intestinal wall.
This causes a contraction that mixes the chyme with intestinal juices (composed of bile, pancreatic juice and other intestinal secretions) and forces it forward through the small intestine.
Peristalsis of the small intestine is similar to transport through the stomach.
It creates a wave of muscle contraction along the length of the small intestine whenever new chyme is delivered to the duodenum from the stomach.
The duodenum, the first and shortest part of the small intestine, is where all remaining nutrients are broken down.
The bile and pancreatic juice excretions that we have already discussed in previous sections arrive in the duodenum via ducts. And are combined with the water and mucous secreted by the duodenum.
The combination of mucous, bile, water and pancreatic juice functions to fully break down any renaming carbohydrate, protein and lipid.
The jejunum is the second small intestine section. It functions primarily to absorb nutrients and water.
The entire interior of the small intestine is covered with finger-like projections called villi. These aid transport and increase surface area to aid in nutrient absorption.
In addition to villi, the jejunum has folds in the interior walls that further increase surface area, and facilitate water and nutrient absorption.
The final segment of the small intestine, the ileum, is responsible for absorbing nutrients that were left behind by the jejunum and products of digestion that can be reused by the body, such as bile salts.
The ileum also functions to pass the remaining undigested material and other material is unwanted by the body to the large intestine.
The Function Of The Colon
The colon (large intestine) is approximately 1.8 meters long, but has a much larger diameter than the small intestine. which is why it is designated as the large intestine.
The material that enters the large intestine via the ileum is essentially minerals that have not yet been absorbed, some water and waste material that the body no longer needs.
The main functions of the colon are:
- Bacteria fermentation
- Faecal production
- Some water and electrolyte absorption
As such, the internal anatomy of the large intestine is vastly different from that of the small intestine.
Very little nutrient absorption takes place in the large intestine, which results in intestinal walls that are smooth and have less surface area than the walls of the small intestine.
The large intestine houses numerous forms of bacteria which act upon any remaining carbohydrate in the waste material through the process of fermentation.
- Carbon Dioxide
- Methane Gases
These are the source of flatulence; an accumulation of gas in the large intestine.
Bacteria also break down waste products; bilirubin from the liver is broken down into pigments that give faeces a brown appearance.
Other byproducts of bacterial metabolism are vitamin K, and some B vitamins, which are absorbed by the large intestine.
Production of Faeces
After the waste material has entered the large intestine, most of the remaining water and electrolytes (minerals that are dissolved in water) are absorbed through the intestinal walls.
By the time absorption is complete the waste has been in the large intestine for 3 – 10 hours and has solidified into what we refer to as faeces.
Faecel matter is composed of:
- Undigested Material
- Byproducts of metabolism that the body no longer needs
- Indigestible material (such as fibre)
Faecel matter is housed within the terminal portion of the colon, the rectum, and expelled through the voluntary contraction of abdominal muscles which push the faeces through the rectum and anus.
This process is referred to as defecation.