We humans eat a variety of food stuff which needs to be broken down into simpler substances in order for our body to utilize the nutrients present in the food we ate. We humans follow heterotrophic mode of nutrition. Nutrition, as we all know, is the biochemical and physiological process by which an organism uses food to support its life. "Hetero" in the word heterotrophic literally means "other" and "trophic" literally means "nourishing". Therefore, the word heterotrophic means "other-nourishing". Heterotrophic organisms obtain food from sources other than photosynthesis. They can not synthesize their own food from simple inorganic substances. Instead, they directly consume the organic substances.
Holozoic Nutrition
Heterotrophic nutrition is further divided into Saprophytic, holozoic and parasitic mode of nutrition. Here we will focus on holozoic mode of nutrition as we humans follow the holozoic mode of heterotrophic nutrition. Now, what is holozoic mode of nutrition? Well, it is a mode of nutrition in which organisms take in food and break it "inside" their body. Examples: Humans, animals, amoeba etc.
Holozoic word can be divided into the words "holo" meaning "whole" and "zoikos" meaning "animal". So holozoic literally means animals which eat their food whole.
Steps Of Holozoic Nutrition
There are certain steps involved in holozoic nutrition.
1) Ingestion: Complex food is taken into the digestive system through a mouth or opening.
2) Digestion: The food particles are broken down into simpler forms.
3) Absorption: The vital nutrients are absorbed by the body.
4) Assimilation: Digested nutrients are used to make material which are transported into the cells of the body.
5) Egestion: The undigested portions are eliminated from the body.
Let us now understand these processes in detail in the human digestive system.
The Human Digestive System
1) Mouth: The human digestive system starts from the mouth which opens in the buccal cavity. The mouth has an upper jaw called the maxilla and a lower jaw called the mandible. Our mouth has 8 incisors which are used for cutting, 4 canines for tearing, 8 premolars and 12 molars for grinding and crushing the food. Thus our teeth are dissimilar. Hence, we are called heterodonts.
Tongue is a thick muscular sensory organ on the floor of the buccal cavity. The salivary glands present near our tongue secret saliva onto the floor of the mouth. These glands help in chewing by lubricating the food in our mouth. The secreted saliva contains the enzyme ptyalin (also called salivary amylase) which acts on food substances like starch, and breaks them down into simpler molecules. The food breaks down into smaller particles and gets mixed with the saliva to form bolus.
The oral cavity then leads through the pharynx to two different pipes leading below. One pipe serves as the wind pipe or the trachea. The other pipe is the food pipe or the esophagus. At the tracheal opening, there is a small flap like structure called the epiglottis. The epiglottis guards the entrance of the trachea so that food does not enter there.
2) Esophagus: The bolus that was formed then enters the esophagus by peristaltic movement. Peristalsis is a series of wave like muscular contractions that moves food in the digestive tract. The esophagus then opens into the stomach through a ring of muscle called the esophageal sphincter. The food drops into the stomach from the esophagus.
3) Stomach: The stomach is a large J-shaped sack located in the upper portion of the abdominal cavity. It has three distinct regions:
⋅ The upper part, known as the fundus.
⋅ The middle part, known as the body.
⋅ The lower part, known as the pylorus.
The pylorus is guarded by the sphincter muscles which opens into the upper section of the small intestine.
Now, when the food enters the stomach, the walls of the stomach, at the same time, release the inactive enzyme pepsinogen and concentrated hydrochloric acid (HCl). Each day, around 2 liters of HCl is secreted by the stomach. HCl helps in killing bacteria and aids in digestion.
Hydrochloric acid activates the enzyme leading to the formation of pepsin. Pepsin helps in the breakdown of proteins. The walls of the stomach also secrete a thick mucus layer which protects the stomach wall from the action of concentrated hydrochloric acid.
The muscular walls of the stomach help in mixing and churning the food which help in the breakdown of bolus into chyme. The chyme passes into the duodenum.
4) Small Intestine: The small intestine has an upper section known as the duodenum. This is U shaped. Here, the food mixes with the juices secreted by the liver and the pancreas.
The duodenum is followed by jejunum which is coiled and tubular. The last part of the small intestine is called the ileum.
5) Accessory Digestive Organs:
a) Liver: Produces bile juice
b) Gall Bladder: Stores and releases bile juice
c) Pancreas: Releases pancreatic juices consisting of trypsin, amylase and lipase.
The chyme, which comes from the stomach, is acidic in nature, due to which the enzymes in the pancreatic juice may become ineffective. So the bile juice, stored in the gall bladder, turns the chyme into an alkaline substance suitable for the enzymes to function. The bile juice also helps in emulsification of fats.
When the chyme gets converted into an alkaline substance, the pancreatic enzymes come in action.
a) Trypsin digests remaining proteins.
b) Amylase digests remaining carbohydrates.
c) Lipase digests remaining fats.
The digested food particles then move into the lower part of the small intestine. The small intestine has numerous hair or finger like projections on its inner surface. These are called villi. The small intestine of a human body is 22 feet or 7 meters long and an inch or 2.5 centimeters wide. Due to the presence of villi, the surface area of the small intestine is 250m².
The villi, which are present on the walls of the small intestine, help in increasing the surface area of small intestine which aids in efficient absorption of nutrition. A single villus itself contains thousands of microvilli consisting of blood vessels and lacteal.
The useful molecules get absorbed into the blood and the lacteal. The nutrients thus absorbed are transported to various body parts. The remaining waste material and water molecules move into the large intestine.
6) Large Intestine: The small intestine leads to a broader but much shorter large intestine or colon. At the juncture where the small intestine opens into the colon, a small sack is present which is known as caecum. A small finger like projection extends from the caecum known as the appendix. Appendix acts as a safe house for good bacteria which help in digestion of food in case of food poisoning or infection. After an infection, it helps in the regeneration of good bacteria in the gut.
The large intestine ascends from the caecum as the ascending colon. The remaining particles reach the ascending colon and then travel horizontally through the transverse colon to the descending colon and then to the rectum.
Water molecules are absorbed by the walls of the colon and rectum. From the rectum, all the waste material is excreted out through the anus.
Thus, the process of digestion is completed. I hope you understood the human digestive system. Thank you for reading.
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