The Digestive System - An Overview
Most of us know that food goes in one end of our body and comes out the other end, but where exactly does it travel in between? How does our body transform the whole food that we eat into nutrients? In this Nutrition Basics post, we'll be taking a trip through the digestive system. Later on, I'll go into more detail about the specific components of the digestive system, but this will serve as good basic knowledge to help you understand the organs responsible for digestion & absorption of the food that we eat.
What is digestion?
Digestion is defined as the process of making food absorbable by breaking it down into simpler chemical compounds in the body by mechanical (movement) and chemical (enzymes) means. Digestion is what needs to take place to turn the food that we eat into nutrients that the body uses for energy, growth, and repair. Digestion also results with waste material that is expelled from the body.
What is the digestive tract // gastrointestinal tract?
The digestive/gastrointestinal tract (also abbreviated as GIT) is a long, continuous tube that begins at the mouth and ends with the anus. The GIT is made up of a series of organs lined with various types of endothelial cells, some of which release enzymes and hormones that help break down food into smaller components. Along the entire length of the GIT are muscles that are responsible for coordinating the movement of food from one end to the other. The main organs that make up the GIT include the mouth, oesophagus, stomach, small intestines, colon, rectum, and anus. Accessory organs, which are important for the production and secretion of enzymes and hormones, include the liver, gallbladder, and the pancreas. The digestive tract takes place in six main processes:
Ingestion of food
Secretion of digestive enzymes and fluids
Mixing & motility
Digestion of food into smaller components
Absorption of nutrients into the bloodstream
Excretion of waste material from the body
This is where everything begins. Before you even take the first bite of your food, seeing and smelling your meal triggers the secretion of saliva from your salivary glands. As you place food in your mouth, it is broken down mechanically (through the chewing action of your teeth and the movement of your tongue) and chemically (through the action of enzymes found in your saliva). Chewing is really important for good digestion, so make sure to chew your food more. Swallowing is a reflex that is responsible for moving chewed food from the mouth to the oesophagus via the pharynx.
The oesophagus is responsible for transporting a bolus of food from your mouth to your stomach. This muscular tube pushes food to the stomach by means of peristalsis. At the lower end of the oesophagus, you'll find the lower oesophageal sphincter, which controls the entrance of food into the stomach. Lower oesophageal sphincter function is important to keeping digestive juices and chyme in the stomach.
The stomach is a sac-like organ that has a capacity of ± 1L. This organ has strong muscular walls which allows it to act as a mechanical mixer of the food that we swallow. In addition to the mechanical churning action of the stomach, digestion occurs here by means of stomach acid and digestive enzymes which change food into a more liquid form called chyme. Food moves from the stomach into the small intestine through the pyloric sphincter.
The small intestine is made up of three segments, namely the duodenum, jejunum, and ileum. The small intestine, particularly the duodenum, is the site of the majority of the chemical digestion and absorption of nutrients takes place. Digestive enzymes enter the small intestines from the pancreas, gallbladder, and liver. Mechanical digestion also continues to occur here through the action of peristalsis, which moves the food through the small intestines and mixes it with digestive juices. Once the food you eat is broken into small, simple components it is absorbed through the walls of the small intestine and into the bloodstream. The motility of food through the small intestines is highly dependent on the action of nerve receptors, hormones, and muscles. Anything that is not absorbed in your small intestine moves to the large intestine.
As part of the digestive system, the liver is responsible for producing bile, which is stored and concentrated in the gallbladder. Following their absorption into the bloodstream, nutrients travel to the liver to be processed before they are distributed to the rest of your body.
The gallbladder is responsible for storing and concentrating the bile that is produced in the liver. As food moves through your GIT and enters your small intestine, bile travels from the gallbladder to the duodenum to aid the digestive process. Bile is responsible for:
Emulsifying and absorbing fats consumed in the diet
Carrying waste from the liver that can't be excreted by the kidneys
The main function of the pancreas as part of the digestive system is to produce a number of digestive enzymes that are secreted into the duodenum to break down food components into absorbable nutrients.
Large Intestine // Colon
The colon is a large muscular tube that is made up of the caecum, ascending colon, transverse colon, descending colon, and sigmoid colon. The colon plays a critical role in the processing of waste material so that it can be eliminated from your body. Waste material left over from the digestive process, otherwise known as stool, passes from one end of the colon to the other by means of peristaltic contractions. It starts off in a liquid-like form and ultimately exits your body as a solid mass. This happens thanks to the absorption of water in the colon. Your colon is also the home to a large, diverse colony of microorganisms, which are responsible for many weird and wonderful things. The bacteria found in your colon can synthesise vitamins, process waste products and food particles, protect your body from bad bacteria, and more. There is a huge interest at the moment in the diverse roles that microbes found in your colon play, so stay tuned for more info on this in future :) As the descending colon becomes full, it empties its contents into the rectum to start the process of elimination from the body.
Rectum & Anus
The rectum is a short chamber that connects the colon to the anus. The rectum receives stool as it exits your colon, tells you that you need to get to a toilet ASAP because it there is a stool that needs to be evacuated, and holds the stool until you manage to get to the loo. The rectum has nerve receptors that send signals to your brain, which decides whether or not the rectal contents can be released. The anus is the final part of the GIT. It consists of muscles that line the pelvis, as well as an internal and external sphincter. These muscles work together to stop stool from exiting the body when it's not supposed to and allowing it to exit the body when the time is right. Defecation takes place when the brain tells the anal sphincters to relax and the rectum to contract, allowing the contents to be expelled from the body.