The majority of digestion takes place in the small intestine. The walls of the small intestine contain enzymes, known as digestive juices. They aid in breaking down the nutrients, in the food we consume, into smaller molecules.
Once broken down, the molecules will be small enough to pass through the wall of the small intestine and into the bloodstream. Then they can be used by the body for energy and other functions.
For the body to fully digest and absorb nutrients from the food we eat, it needs to have a large enough surface area in contact with those nutrients. The small intestine is extremely long. This means it provides a large surface area for the absorption of the digested food molecules. Also, the inner wall of the small intestine has special adaptations that allow it to absorb substances more efficiently.
These adaptations include:
The diagram below shows how the villi in the small intestine help increase the surface area for absorption. Remember that each individual finger-like structure is called a villus.
The small intestine has a thin wall, which is just one cell thick. This thin wall allows for a shorter distance for substances to travel across, known as the diffusion path. This shorter diffusion path makes it easier for nutrients to pass through the walls of the small intestine and into the bloodstream.
If the inner wall of the small intestine was thick, it would be more difficult for nutrients to pass through and be absorbed, which would reduce the efficiency of the digestion process.
The interior wall of the small intestine is covered in millions of finger-like structures called villi. These villi help increase the surface area of the small intestine, making it easier for nutrients to be absorbed.
There is a network of blood vessels, called capillaries, running through the villi. These capillaries carry away the absorbed nutrients, transporting them throughout the body via the bloodstream.
The removal of the products of digestion by the bloodstream helps to increase the concentration gradient (the difference in concentration between two areas). This increase in the concentration gradient helps to drive the absorption of nutrients across the walls of the small intestine and into the bloodstream, making the process more efficient.
The large intestine is a long, tube-like organ in the digestive system that is responsible for the absorption of water from undigested food. As the undigested food moves through the digestive system, it comes into contact with digestive juices that add water to it. This water is then absorbed back into the bloodstream through the walls of the large intestine.
The undigested food that is left over, after the absorption of water, is passed out of the body through the anus as faeces. This is a process known as egestion. It is important to note that egestion is not the same as excretion, which is the process by which the body removes waste products and other substances that it no longer needs. The large intestine is mainly responsible for egestion, while the kidneys and other organs are responsible for excretion.