What makes a sponge so absorptive????

In your base groups of three: The Reader is the oldest, the Writer is the youngest and the Clarifier is the age in the middle. Your job is to understand the structure and function of the small intestine in order to analyze a real live case study. Each of you will turn in a written summary of four key questions.

Absorption in the Small Intestine

The intestines are the parts of the digestive system responsible for the absorption of nutrients and water. Two anatomic regions exist, the small intestine and the large intestine. Both of these are further subdivided into anatomically discernible subdivisions. The small intestine has three parts: the duodenum, the jejunum, and the ileum; the large intestine is subdivided into the colon, cecum, rectum, and is continuous with the anus, the last portion of the alimentary canal.

Adapted from Dr. Thomas Caceci http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/Lab19.htm

This diagram lays out in schematic form some of the salient features of the two major subdivisions of the intestine, and highlights their similarities and differences.

The most visible and significant difference is the presence of villi in the small intestine, and their absence in the large intestine. The small intestine is a place in which nutrients are absorbed, and it has a huge surface area to fulfill this role (see below). Material moving through here is in liquid form. The villi are a means to enhance the absorptive surface, and the contents can flow around and over them efficiently. Between the protruding villi are deep intestinal crypts where new epithelial cells are generated by mitosis.

Adapted from Dr. Thomas Caceci http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/Lab19.htm

The large intestine primarily absorbs water, and compacts and dries out the fecal bolus: villi would be a hindrance to movement of the semi-solid fecal mass, and would likely be injured by its passage. Hence, there are no villi in the large intestine, and in addition, there are numerous goblet cells whose secretions act as lubrication for the moving material.

The Need For Surface Area

Actual digestion or breakdown of foodstuffs in the gut is extracellular; i.e., it takes place in the lumen of the tube, with the resulting breakdown products being absorbed across the plasma membrane of the lining cells. This process represents a nice application of a general rule: if you want to improve the efficiency of a process which is associated with or limited by a transmembrane phenomenon, the way to do it is increase the surface area.

Consider a hypothetical small intestine with a length of 5 meters and a diameter of 25 mm. This tube, without modification, would have a volume of a little less than a liter, and a surface area of 3.9x106 square millimeters, approximately the size of a small windowpane. Increasing the total surface area available produces an enormous increase in the efficiency of the absorptive process, since the rate of absorption per unit area is limited.

If an average villus is 1.0 mm long by 0.5 mm in diameter, and if villi are distributed at a uniform 25 per square millimeter, the increase in surface area compared to a simple tube is substantial. With villi present at this density, the available surface area for absorption from that volume would be more on the order of a very large room. Since the villi are not the only (or even the principal) adaptation to increase the surface area, the actual increase is vastly greater: the presence of microvilli on the cells and crypts between villi increases the surface area available for absorption to several acres. This enormous Adapted from Dr. Thomas Caceci http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/Lab19.htm

reserve capacity is essential. Loss of some of it to injury, scarring, surgical excision, etc., leaves plenty of functional tissue to allow for normal nutritional intake.

Need more absorption capability? Then make more absorptive surface. This design solution is found over and over in the "engineering" of living organisms.

Small Intestine: Duodenum

Now it is time to be a clinical veterinarian.

Click on the link below

http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/CASES/SWINE%20TGE/SWINE%20TGE.htm

Adapted from Dr. Thomas Caceci http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/Lab19.htm

Answer the four questions in paragraph form by reading and discussing the case study with your group.

1. What cells and tissues are affected by this virus? What, specifically, is happening in the piglets that's causing the diarrhea? What structural changes would you expect to see in the gut?

2. Why is the mortality so high in 4-7-day old piglets but not in weaned ones?

3. Why are the surviving 14-month-old pigs not doing well? What would be revealed by histological examination in these animals?

4. What triggered the outbreak?

Adapted from Dr. Thomas Caceci http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/Lab19.htm