Digestion of Fat

The presence of fat in the small intestine produces hormones which stimulate the release of lipase from the pancreas and bile from the gall bladder. The lipase breaks down the fat into monoglycerides and fatty acids. The bile emulsifies the fatty acids so they may be easily absorbed.

Within the villi, the chylomicron enters a lymphatic capillary called a lacteal, which merges into larger lymphatic vessels. It is transported via the lymphatic system and the thoracic duct up to a location near the heart . The thoracic duct empties the chylomicrons into the bloodstream via the left subclavian vein. At this point the chylomicrons can transport the triglycerides to where they are needed.

The 4 agents that assists digestion

These are the four hormones that aid and regulate the digestive system:

Gastrin - is in the stomach and stimulates the Gastric Glands to secrete Pepsinogen and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH.

Secretin- is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.

Cholecystokinin - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.

Gastric inhibitor peptide- is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Another function is to induce insulin secretion.

The Large Intestines

After the food has been passed through the small intestine, the food enters the large intestines. The large intestine is roughly 1.5 meters long, with three parts: the cecum at the junction with the small intestines, the colon, and the rectum. The colon itself has four parts: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The large intestine absorbs water from the bolus and stores feces until it can be egested. Food products that cannot go through the villi, such as cellulose, are mixed with other waste products from the body and become hard and concentrated feces. The feces is stored in the rectum for a certain period and then the stored feces is egested due to the contraction and relaxation through the anus. The exit of this waste material is regulated by the anal sphincter.

The small intestines.

After being processed in the stomach, food is passed to the small intestines via the Pyloric sphincter. The majority of digestion and absorption that occurs here as chyme enters the duodenu. Here it is further mixed with three different liquids:

1. Bile, which emulsifies fats to allow absorption, neutralises the chyme, and is used to excrete waste products such as bilin and bile acids. It is not an enzyme, however. The bile juice is stored in a small organ called the gall bladder.
2. Pancreatic Juices made by the pancreas.
3. Intestinal enzymes of the alkaline mucosal membranes. The enzymes include: maltase, lactase and sucrase, to process sugars; trypsin and chymotrypsin are also added in the small intestine.

The Stomach, What happens in there?

In the stomach, food is further broken apart, and thoroughly mixed with a gastric acid and digestive enzymes that break down proteins. The acid itself does not break down food molecules, rather, the acid provides an optimum pH for the reaction of the enzyme pepsin. The parietial cells of the stomach also secrete a glycoprotein called intrinsic factor which enables the absorption of vitamin B-12. Other small molecules such as alcohol are absorbed in the stomach as well by passing through the membrane of the stomach and entering the circulatory system directly.

The Throat, what does it do?

The Esophagus, also known as throat, a narrow, muscular tube about 30 centimeters (12 inches) long, starts at the pharynx, passes through the larynx and diaphragm, and ends at the cardiac orifice of the stomach. The wall of the Esophagus is made up of two layers of smooth muscles, which form a continuous layer from the Esophagus to the rectum and contract slowly, over long periods of time. The inner layer of muscles is arranged circularly in a series of descending rings, while the outer layer is arranged longitudinally. At the top of the Esophagus, is a flap of tissue called the epiglottis that closes during swallowing to prevent food from entering the trachea (windpipe). The chewed food is pushed down the Esophagus to the stomach through peristaltic contraction of these muscles. It takes only seconds for food to pass through the Esophagus, and little digestion actually takes place.

The Digestion Phases

The Cephalic Phase- This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point.

Gastric phase - This phase takes 3 to 4 hours. It is stimulated by distention of the stomach, presence of food in stomach and increase in pH. Distention activates long and myentric reflexes. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach to around pH 6. Inhibition of gastrin and HCl secretion is lifted.