Locomotion and Digestion

The bones are a structural material of the vertebrate skeleton. The bone is a special form of connective tissue in which an organic extracellular matrix containing collagen fibers is imbedded with small, needle-shaped crystals of hydroxyapatite (which is formed from calcium phosphate and calcium hydroxide). The collagen fibers run in various directions, but the hydoxyapatite crystals are aligned parallel to the long axes and the curved ends of bones. Hydroxyapatite is brittle but rigid, thus giving bones a lot of strength; the compo

The stomach is a sac-like portion of the digestive tract. Its inner surface is highly convoluted, enabling it to fold up when empty and open out like an expanding balloon as it fills with food. While the human stomach has a volume of only about 50 millimeters when empty, it may expand to contain 2-4 liters of food when full. Carnivores that engage in sporadic gorging as an important survival strategy have stomachs that are able to distend much more than that. The stomach contains and extra later of smooth muscle for churning food and mixing it with the secretions of the gastric glands of the mucosa. These exocrine glands contain 2 kinds of secretory cells, parietal cells (which secrete hydrochloric acid, HCl) and chief cells (which secrete pepsinogen, a weak protease, which is a protein-digesting enzyme, that requires a very low pH to be active). The low pH needed to activate pepsinogen is provided by the HCl. Activated pepsinogen molecules then cleave each other at specific sites, producing molecules of a much more active protease, pepsin. This process of secreting a relatively inactive enzyme (zymogen) is then converted into a more active enzyme outside the cell prevents the chief cells from digesting themselves.

The activities of the gastrointestinal tract are coordinated by the nervous system and the endocrine system. For example, the nervous system stimulates salivary and gastric secretions in response to the sight and smell of food. When good arrives in the stomach, proteins in the food stimulate the secretion of a stomach hormone, gastrin, which stimulates the secretion of pepsinogen and HCl in the stomach. In a negative feedback loop, the lowering of stomach pH caused by HCl reduces the secretion of gastrin, decreasing HCl production. In this way, the secretion of gastric acid is normally kept under tight control. The passage of chyme from the stomach into the duodenum inhibits the contractions of the stomach, with the result that no additional chyme will enter the duodenum until the previous amount has been processed. This inhibition is mediated by neural impulses and by a hormone known as gastric inhibitory peptide (GIP), which is released by the duodenum. The secretion of GIP is stimulated most strongly by the presence of fat in the chyme. Fatty meals therefore take longer to pass through the gastrointestinal tract than meals low in fat. The duodenum secretes 2 more hormones. One is cholecystokinin (CCK), like GIP, is secreted in response to the presence of fat in the chyme. CCK stimulates the contraction of the gallbladder that are responsible for injecting bile into the duodenum. Therefore, CCK triggers a process that allows fat to be emulsified and digested more efficiently. CCK also stimulates the secretion of digestive enzymes by the pancreas. The other duodenal hormone is secretin. Release in response to the acidity of the chyme, secretin stimulates the pancreas to release bicarbonate, which then neutralizes some of the acidity. Secretin has the distinction of being the 1st hormone ever discovered.

Every vertebrate¡¦s food is taken in through the mouth. The methods of capturing food are widely carried, and the teeth (when present) are specialized for each method. Carnivorous mammals have pointed teeth that lack flat grinding surfaces. Carnivores often tear off pieces of their prey but have little need to chew them, since digestive enzymes can act directly on animal cells. Herbivores must pulverize the cellulose cell walls of plant tissue before digesting it; the teeth are large and flat, with complex ridges that are well suited for grinding. Humans are omnivores, eating both plant and animal good regularly, and human teeth are specialized for both tasks. Humans are carnivores in the front of the mouth and herbivores in the back. The 4 front teeth in the upper and lower jaws are sharp, chisel-shaped incisors used for biting. On each side of the incisors are sharp, point

Some topics in this essay:
HCl Activated, ATP Oxygen, ADP Pi, , CCK GIP, Heart Association, BMR Exercise, muscle fibers, muscle fiber, smooth muscle, skeletal muscle, muscle cells, amino acids, nervous system, motor neuron, gastrointestinal tract, digestive tract, smooth muscle cells, products digestion absorbed, dense connective tissue, lost ability synthesize, thick thin filaments,