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| ‹ Starfish Development › |
The egg begins development after fertilization and becomes an embryo. The embryo then undergoes successive cleavage divisions. This means that the single celled embryo then cleaves, or splits, and forms two. Then these two eventually become four; then 8 and 16. At this point, the ball of cells is solid and very dense. It is called a morula. As cleaving continues, the solid ball of cells eventually becomes hollow. It is now called a blastula. Even though this structure is considered hollow, its inside cavity, the blastocoel, is actually filled with liquid. Eventually, some of the cells fold inward and form a gastrula. |
| When viewing the blastula stage, it is important to be able to identify the animal pole vs. the vegetal pole. The cells on the animal pole are smaller and more compacted, while the cells on the vegetal pole are larger and seem farther apart. |
| ‹ Intro to the Fetal Pig › |
| The Digestive System |
| Stomach |
| This organ is very important to the digestive system. It can serve as a part-time storage reservoir, which allows the person (or a different type of animal) to eat a large meal at one time and deal with it over an extended period of time. This organ also produces gastric juices, which can break down food and can eliminate many unwanted pathogens that may have been living on the food. The stomach also contains muscles that allow the food in it to become mixed and liquefied, which is necessary for entry into the small intestine. The stomach also slowly releases the food into the small intestine for further digestion via the pyloric sphincter. |
| Small intestine |
| This part of the body is virtually solely responsible for all absorption of nutrients into the blood. Once food enters the small intestine (it is now called chyme), most the food has been broken down, thanks to the stomach. Once within the small intestine, the chyme is exposed to pancreatic enzymes and bile, which breaks it down even further, enabling it to be absorbed. (The small intestine does NOT produce pancreatic enzymes or bile. They are produced by other organs, which will be discussed later.) Remember that the portion that connects this portal to the stomach is called the duodenum. The duodenum joins the jejunum, which leads to the ileum. This portion of the small intestine is sometimes called the jejunum-ileum since the jejunum and the ileum cannot be macroscopically differentiated. |
| Pancreas |
| This organ lies within the loop of the duodenum. The pancreas plays two major roles in digestion. Since food is broken down in the stomach, it is very acidic (pH in the stomach is approximately 3.5). This means that the acid must be neutralized quickly before it damages the lining of the duodenum. The pancreas secretes an enzyme that neutralizes this acid. Also, in order to digest the food (now chyme, actually), it must be broken down even further. The pancreas secretes another enzyme that passes through the duodenum to do this. |
| The pancreas also secretes insulin and glucagon, which play extremely vital roles in carbohydrate and lipid metabolism. They are absolutely necessary in maintaining normal blood concentrations of glucose. This means that the pancreas is a digestive AND an endocrine organ. |
| Liver |
| The liver is the largest gland in the body (it is considered a gland since it produces a secretion) and performs an amazing number of tasks that affects many parts of the body. One consequence of this is that any type of hepatic (liver) disease can have widespread effects on virtually all other organ systems. For purposes of this discussion, we will focus on the liver’s function in the digestive system. The liver produces an enzyme called bile, which is critical for digestion and absorption of fats in the small intestine. Many waste products can also be eliminated from the body by secretion into bile and elimination in feces. |
| Secretion into bile is also a major route for eliminating cholesterol. Fats and cholesterols are insoluble in water and other aqueous solutions, but they are made soluble in bile. Gallstones, which are composed mainly of cholesterol, result from processes that allow cholesterol to precipitate out of solution in bile. |
| Gall Bladder |
| The gall bladder is closely related to the liver. They are close together inside the body and they also share an enzyme. Since the liver continuously produces bile, the gall bladder is there to help regulate its use. The liver secretes bile and then gets stored in the gall bladder until needed. When needed (mostly when fat is eaten), the gall bladder releases bile through the cystic duct, then through the common bile duct and into the duodenum. The gall bladder also makes the bile more concentrated by removing water through its wall, making fat digestion more efficient. |
| Jejuno-ileum |
| This very important portion of the small intestine is lined with microscopic villi and microvilla, which increases its surface area. This increase in surface area greatly enhances its absorbing capacity. The jejuno-ileum will be covered more closely in Lab 9. |
| Colon (large intestine) |
| The colon carries out three major functions. The first is water and electrolyte absorption. Most water has already been absorbed by the time it reaches the colon, but a considerable amount of water and electrolytes like sodium and chloride remain and must be recovered. The second function includes the formation and storage of feces. As the food mass passes through the colon, it is dehydrated, mixed with bacteria and mucous and formed into feces. The third function includes microbial (bacterial mostly) fermentation. The colon of all species is riddled with microbial life. These bacteria produce enzymes that are capable of digesting molecules that is otherwise indigestible. Cellulose is an example of such a molecule. |
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updated 3-12-06
