Food in the Small Intestine

As the contents of the stomach become thoroughly liquefied, they pass into the duodenum, the first segment (about 10 of the small intestine. Food typically takes 4-5 hours to pass through the stomach into the duodenum, the first part of the small intestine. After being churned and mixed with digestive juices in the stomach, food chyme moves slowly into the folds of the small intestine through the pyloric sphincter or valve. The small intestine (or small bowel) is the longest section of the digestive tract (approx 17 feet) and is divided into three segments: the duodenum, jejunum and ileum, each of which performs different digestive functions. Chyme from the stomach is propelled through the small intestine by peristalsis.

Functions of the Small Intestine

The small intestine is where most chemical digestion takes place; peptides (complex chains of protein molecules) are broken down into amino acids; lipids (fats) are broken down into fatty acids and glycerol; and carbohydrates are broken down into simple sugars like glucose. To accomplish this, chyme is mixed with additional digestive juices including bile from the liver and pancreatic juice and amylase from the pancreas, as well as other intestinal enzymes such as maltase, lactase and sucrase to break down the chyme and assist in nutrient absorption. Absorbed nutrients flow in the bloodstream to the liver where they are further metabolized and then either stored or sent to cells in other parts of the body. In total, food typically takes 4-5 hours to transit all three sections of the small intestine. Along the way its consistency changes from porridge (chyme) to a thin watery mixture.

Do You Know?

Since the small intestine (about 17 feet) is much longer than the large intestine (about 5 feet) people often wonder why it is referred to as "small". The answer lies in its diameter (3-4 cm),which is about 3 times narrower than the "large" intestine.

Digestive Function of the Duodenum

The duodenum continues the process of food breakdown. Its name stems from the Latin "duodenum digitorum", meaning, twelve fingers or inches. It is roughly horse-shoe-shaped.Anatomically, it is is sub-divided into four segments: the superior,descending, horizontal and ascending duodenum. Inside the duodenal tube, chyme is mixed with fluids from the gallbladder (bile and pancreas (pancreatic juice). Bile breaks down fat particles into smaller droplets, while pancreatic juice contains enzymes that convert fats into fatty acids and glycerol, and sodium bicarbonate to neutralize stomach acid.

Digestive Function of the Jejunum

Roughly 4-7 feet in length, the jejunum is where chemical breakdown of the food chyme is completed. Pancreatic enzymes, along with enzymes produced by the jejunum wall, finalize the food digestion process. The term jejunum stems from the Latin jejunus, meaning empty. In some gastric bypass operations, part of the jejunum is also "bypassed" to reduce calorie intake.

Digestive Function of the lleum

Roughly 7-5 feet in length, the ileum is the final section of the small intestine,linked to the large intestine by the ileocecal valve. The main function of the ileum is to absorb nutrients. Bile is also absorbed here and returns to the liver through blood vessels in the intestinal walls. The unabsorbed watery remains of the food chyme now pass into the large intestine for water-removal and final processing, before being expelled from the body.

Absorption of Nutrients in the Small Intestine

It is inside the small intestine that we absorb most of the nutrients in our food.Although the small intestine has a relatively small diameter, the intestinal walls are covered in wrinkles called rugae, which are themselves covered in millions of finger-like projections called villi, which are themselves studded with millions of smaller projections called microvilli. This provides a surface area of about the size of a tennis court for nutrient absorption. Inside each villus is a series of lymph vessels (lacteals) and blood vessels (capillaries). The lacteal lymph vessel absorbs digested fat into the lymphatic system which eventually drains into the bloodstream. The blood vessels receive other nutrients and transport them via the hepatic portal vein to the liver. Here the blood is filtered, toxins are removed and the nutrients are processed. An important provide sufficient energy for the body. Excess glucose is converted in the liver to task performed by the liver in this context is the regulation of blood glucose levels to begin to drop, (eg. between meals), the glycogen is re-converted to glucose in glycogen in response to the hormone insulin, and stored. When blood glucose levels response to messages conveyed by the hormone glucagon.

Digestion in the Large Intestine

After all nutrients have been absorbed from ingested food during its passage through the small intestine, the watery waste passes into the large intestine. It is the final

section of the gastrointestinal tract and its main function is to remove water (plus any
remaining minerals) from the food waste and compress it into a form for easy
expulsion from the body. As the chyme passes through the large intestine, the water is removed and the chyme is combined with mucus and bacteria (gut flora), and is
converted into feces.

How Food Waste Moves Through The Large Intestine

As in the esophagus and small intestine, undigested food is propelled through
the large intestine by waves of muscular contraction and expansion, called peristalsis.
However, unlike in the small intestine where these waves occur at irregular intervals,
peristalsis in the large intestine is ntinuous. In addition, 2-3 times a day, a more
vigorous type of movement (gastrocolic reflex) occurs which propels material
towards the rectum and anus. As waste matter is pushed into the rectum, it triggers a desire to defecate.

Structure of the Large Intestine

The large intestinc (also referred to as the large bowel, or the lower gastrointestinal tract) is a thick tube of about 5 feet in length which gets progressively narrower in diameter. It consists of 4 regions: the ceacum, colon, rectum, and anal canal. (The term "colon" is sometimes used to describe the entire large intestine). The cecum (or caecum) is a short pouch into which food enters from the ileum (via the ileocecal valve) and exits into the ascending colon of the large intestine. The colon is the longest segment of the large intestine. It is sub-divided into 4 sections, named after
their position in the pelvis: the ascending colon, transverse colon, descending colon
and sigmoid colon. The rectum is the final part of the large intestine. Faeces formed in
the colon collect in the rectum before being cxcreted via the anus. After the rectum
comes the anal canal, a short passage about 15 inches long, terminating in two muscular rings: the internal and cxternal sphincters. As waste products from the rectum pass into the anal canal, nerves in the rectum cause the internal sphincter to relax and open. Then the external sphincter also relaxes, permitting fecal discharge. Defecation may be involuntary or under voluntary control. Young children learn voluntary control through the process of toilet training. Once trained, loss of control causing fecal inconvenience may be caused by physical injury (such as damage to the anal sphincter, intense fright, inflammatory bowel disease, impaired water absorption in the colon and psychological or neurological factors

Liver

The liver is one of the most important and largest organ in the human body. It is
located in a central position of the abdomen, and is closely involved in almost every aspect of the body's physiological activities. Because of its central role, liver disease can be extremely life-threatening. The liver has a multitude of important and complex functions. Some of these functions are:

a. Carbohydrate metabolism

Glucose is a vital energy source for cells and its levels in Liver the blood stream must remain constant. The liver helps maintain blood glucose levels in response to the pancreatic hormones insulin and glucagon.

After a meal, glucose enters the liver and levels of blood glucose rise. This excess glucose is dealt with by glycogenesis in which the liver converts glucose into

glycogen for storage. The glucose that is not stored is used to produce energy by a

process called glycolysis. This occurs in every cell in the body, In between meals or during starvation, blood glucose levels fall. The hepatocytes (bile secreting liver cells) detect this change, and restore glucose levels by either glycogenolysis which converts glycogen back to glucose, or gluconeogenesis

in which non-sugars such as amino-acids are converted to glucose. cholesterol and phospholipids essential for many body functions. If fat is in excess, the liver prepares for storage. Lipogenesis is the metabolic process in which fats, The liver is involved in fat metabolism and synthesizes lipoproteins, composed of fatty acids and glycerol, are converted for storage in subcutaneous tissue. If energy and glucose levels are low, stored fat is converted back into glycerol

b. Fat metabolism

and other storage depots.

and fatty acids by a process called lipolysis. This occur in adipose cells, but the fatty

acids and glycerol are transported to the liver for use as an alternative energy supply.

c. Protein metabolism

Amino acids are transported to the liver during digestion and most of the

body's protein is synthesized here. If protein is in excess, amino acids can be converted

into fat and stored in fat depots, or if required, made into glucose for energy by

gluconeogenesis which has already been mentioned. However, before amino acids

can be utilized in these ways, the first step is to remove the nitrogen-containing amino

group NH,. This very important metabolic process is called deamination. In the

hepatocytes, NH, (the amino group) quickly changes into ammonia NH3, which is

highly toxic to the body. The liver acts fast to convert ammonia into urea that then can

be excreted in the urine and eliminated from the body.

d. Detoxification

The liver plays a vital role in detoxification and destruction of endogenous and

exogenous harmful substances. The liver's own phagocytes which reside within the

lobules, known as Kupffer cells, digest and destroy cellular debris and any invading

bacteria. Other exogenous substances such as drugs and alcohol are detoxified by the

liver. Amino acids are deaminated, some hormones are inactivated, and bilirubin, a

product of the breakdown of old red blood cells, is also detoxified and rendered

harmless by liver metabolism.

e. Storage

The liver plays an important role as a storage facility. The hepatocytes take up

many types of vitamins and minerals from the blood and store them. These include

vitamins A, B, D, E, K and minerals like iron and copper. Glycogen which is formed

from excess glucose is also stored by the liver, although muscle tissue can also store

glycogen

f,. Bile

The liver synthesizes bile which is important for fat digestion and is also a route of excretion from the body. Bile consists of water, bile salts, cholesterol, phospholipids, electrolytes and bile pigments which give it its typical yellowish-green

colour. Bile is stored and concentrated in the gall bladder. The presence of fats in the

gut during meals stimulates the gall bladder to empty. Bile enters the duodenum

lipase enzymes. Metabolic wastes and drug products may form part of the bile which

by can then be excreted from the body through the digestive tract in the faeces. Bilirubin, the toxic end product of haemoglobin breakdown, is excreted from the body in this way.

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