Digestion and absorption
Digestion - The process by which large food particles are broken down into nutrients
through mechanical and chemical degradation
Absorption - The stage of digestion during which nutrients are transported into the
Alimentary canal and digestive glands :
The digestive tract is a muscular tube, approximately 18-27 feet in length that extends
from the mouth to the anus. It is composed of seven compartments and four accessory
organs. The compartments carry the food as it passes through the digestive tract while
the accessory organs secrete enzymes or produce molecules that aid indigestion. The
accessory organs are connected to the main digestive tract by a series of ducts. The
compartments occur in the following order: mouth, pharynx, esophagus, stomach,
small intestine, large intestine and anus. The accessory organs consist of the paired
salivary glands, the pancreas, the liver, and the biliary system.
GI system breaks down particles of ingested food into molecular forms by enzymes
(digestion) that are then transferred to the internal environment (absorption).
Compartments of alimentary canal:
The process of digestion begins in the mouth. Within the mouth lie the teeth,
tongue and jaws. Through a chewing motion, the food is mechanically broken
down between the teeth and mixed with saliva, which aids in chemical digestion.
Upon stimulation, saliva is produced in the salivary glands and brought into the
mouth. It contains salivary amylase, an enzyme that digests starch. Once the
digestion in the mouth is completed, the first phase of swallowing is initiated. This
stage is voluntary and is characterized by contraction of the muscles of the floor
of the mouth and tongue that propel the food bolus into the pharynx.
The buccal cavity leads into a short pharynx which serves as a common passage
for food and air. The oesophagus and the trachea (wind pipe) open into the
pharynx. A cartilaginous flap called epiglottis prevents the entry of food into the
glottis (opening of wind pipe) during swallowing.
The oesophagus is a thin, long tube which extends posterior passing through the
neck, thorax and diaphragm and leads to a ‗J‘shaped bag like structure called
stomach. A muscular sphincter (gastro-oesophageal) regulates the opening of
oesophagus into the stomach. The food moves through the oesophagus due to
rhythmic contraction or longitudinal and circular muscle. This wave of contraction
and relaxation is called peristalsis. Mucous secreted by epithelial cells in the inner
lining helps in the smooth passage of food.
The stomach is a C-shaped pouch that receives the food bolus from the
esophagus. It aids both in mechanical and chemical digestion. Acting like a
churn, the stomach mixes the food with gastric acid and breaks down the food
into a milky substance known as chyme. The acid reduces the pH of the
stomach, in the process allowing activation of an enzyme called pepsin. This
starts the chemical digestive process.
5. Small Intestine
The majority of digestion occurs in the small intestine. This compartment has
three distinct portions, each of which is highly specialized for different digestive
functions. The first third is the duodenum. This part is responsible for signaling
the distal digestive tract that food is arriving and that the stomach should stop
sending food. Chemical digestion is very active at this stage, and food is broken
down into basic proteins, carbohydrates, and fats. Digestive enzymes are
released from the pancreas to enable the degradation process. Bile is released
from the gall bladder and mixes with the chyme to aid in fat digestion and
The second portion of the small intestine is the jejunum. It is composed of many
folds that increase the surface area for absorption, known as the brush border
As the digestive products move through the small intestine, different parts of the
brush border membrane selectively allow the absorption of different nutrients.
The third portion of the small intestine is the ileum. It is responsible for the very
selective absorption of some significant nutrients, including vitamin B-12 and
vitamin C. By the end of the small intestine, the majority of the nutrients have
been absorbed. The material is now mostly waste products and water, and is
referred to as liquid stool.
6. Large Intestine
It is broader than the small intestine. It consists of caecum, colon and rectum.
Caecum is a small blind sac which hosts some symbiotic micro-organisms. A
narrow finger-like tubular projection, the vermiform appendix which is a vestigial
organ, arises from the caecum. The caecum opens into the colon. The colon is
divided into three parts - an ascending, a transverse and a descending part. The
colon is lined internally by mucosal cells secreting mucous that makes the
passage of undigested material easy. The descending part opens into the rectum,
which is the posterior region of large intestine.
Undigested material called faecal matter is stored in the rectum temporarily before
it is expelled through the anus.
Rectum opens to the outside by the opening called anus which is guarded by a
sphincter. It removes undigested matter outside by the process known as
defecation or egestion.
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