- Allimentary Canal
The startThe buccal cavity(mouth) is the place where food enters. It is broken down to increase its surface area and decrease its size by chewing(mastication). It is moistened by saliva secreted from the salivary glands. Saliva is a watery mixture of mucus, mineral salts and the enzyme - salivary amylase(eats starch). Saliva also moistens and lubricates the food, before it is sent down the oesophagus by contrating the tongue. As it goes down the oesophagus, the epglotis closes the opening into the larynx(the glotis), so food doesn’t enter the lungs.
Going down the oesophagus, peristalsis occurs. This is a muscular contraction wave-like movement of the circular muscles in the oesophagus. Now it is in the stomach, where it is acted on by gastric juices, secreted from gastric glands in the stomach wall. The gastric juice contains hydrochloric acid and pepsin(an enzyme for breaking proteins into shorter polypeptides chains). Pepsin is secreted in an inactive state, Pepsinogen, this is so that it doesn’t begin destroying the gastric gland(autodigestion). When it reaches the lumen it is activated by the hydrochloric acid. Cells in the wall secrete a mucus, which prevents it from eating away at the wall. HCL provides an optimum pH for the enzyme, and it kills micro-organisms.
Gastic juice is brought on by mental stimulus, and others such as sight, sound, smell and touch. It is also induced by the food actually reaching the stomach. Just before the duodenum are special cells which produce thehormone gastrin into the bloodstream. Gastrin then stimulates the release of pepsin,HCL and muscular stomach movement. This muscular contrations pounds the food into chyme. So the secretion of digestive enzymes are controlled by the nervous system and by the hormone gastrin.
Entry by food from the stomach is controlled by a ring muscle called the pyloric sphincter. Opening and closing, it lets through small amounts of chyme into the duodenum. The duodenum is the first part of the small intestine. Villi cover its surface, between which are glands called crypts of Lieberkuhn. The agents for digestion come from:
Liver - Pancreas - Wall of small intestine.
Pancreas à Produces pancreatic juice, which froms from the pancreas,
down the pancreatic duct, into the duodenum. It contains various enzymes.
Pancreatic amylase – starch à maltose(disacc.)
Pancreatic lipase – triglycerides à fatty acids.
Pancreatic trypsin, elastase – break down proteins.
The protease are secreted as inactive, to prevent autodigestion. Trypsinogen is made active my enterokinase, secreted by the small intestine.
The Intestine Wall à Enzymes secreted by the wall, digest carbohydates:
Maltase – maltose à glucose
Sucrase – sucrose à glucose and fructose
Lactase – lactose à glucose and galactrose.
The end products are all monosaccarides.
Epithelial cells absorb tri&dipeptides, breaking them down to amino acids. Brunner’s glands are found in the wall too, they secrete an alkaline mucus which neutralises the acid from stomach, and protects the wall from autodigestion. The lining is replaced by enzymes in the crypts of Lieberkuhn. These cells constantly divide, and grow up the villi, when they reach the tip, they shed, and are replaced.
Control of secretions in intestine
The release of these fluids and enzymes is partly due to nervous reflexes, stimulated by sight, smell..etc. But also by enzymes. It is the presence of acidic chyme in the duodenum that signals cells to secrete hormones into the bloodstream – secretin and CCK-PZ. Secretin acts on the liver and pancreas – the liver produces bile, and the pancreate releases the non-enzymatic parts of its pancreatic juice. CCK-PZ acts on the pancreas to secrete its enzymes, and acts on the bile duct to secrete bile.
The small intestine is where the products of digestion are absorbed. The villi perform this task. Near the villi wall is an artery, into which food absorbs. The food is absorbed, and goes to the hepatic portal vein, and then to the liver. Is isn’t this straight forward to fats though. In the center of villi, is a lymph vessel(lacteal). This aborbs the fats, and eventually leads into the circulatory system – distributing it around the body. The villi greatly increase the surface area, and to increase it more, each villi have microvilli. Assimilation is the built up of material to form complex materials. In the large intestine, most of the water is removed.