Week 14 (Pre-class Assignment)

Please answer these questions in complete sentences.

1.     If there was one aspect of BIO 160 (class, organization, or content, or something else) you especially enjoyed, what was it?

During this fall semester in BIO 160 my favorite part has been doing the labs in class. I like being able to move around and ask questions to fellow students as well as Joanne. I felt this was a good way to get to know other students and it helped me learn a lot.

2.     If there was something especially challenging (class, organization, or content, or something else), what was it?

Surprisingly, I found Chapter 2 (basic Chemistry) to be the most challenging for me. I also had some trouble with the anatomy of a skeletal muscle fiber, that section was tough!

Thirst Mechanism

Thirst Mechanism is the driving force for water intake. An increase in plasma osmolality of only 2-3% excites the hypothalamic thirst center. Thirst is "the physiological urge to drink water". In studies, it is recognized when subjects report the conscious sensation of a desire to drink. Under normal conditions, most water intake is due not to thirst but to social and cultural factors (eg drinking with meals or at work breaks, water in food). Thirst offers a backup to these behavioral factors and to the ADH response. Both the thirst and the ADH mechanisms are regulated in the hypothalamus. Water intake can be considered to consist of two components: a regulatory component (due to thirst) and a non-regulatory component (all other fluid intake).

The 4 major stimuli to thirst are:

1.Hypertonicity: Cellular dehydration acts via an osmoreceptor mechanism in the hypothalamus

2.Hypovolaemia: Low volume is sensed via the low-pressure baroreceptors in the great veins and right atrium

3.Hypotension: The high-pressure baroreceptors in carotid sinus & aorta provide the sensors for this input

4.Angiotensin II: This is produced consequent to the release of renin by the kidney (eg in response to renal hypotension)

Four layers of the Alimentary Canal (GI) Tract

The four layers of the alimentary canal starting with the innermost layer.

1.     The mucosa is formed by epithelium and a small amount of connective tissue, comes into direct contact with food passing through the canal. It secretes mucus that protects and lubricates the lining.

2.     The submucosa is composed of loose connective tissue, blood vessels, and many nerve endings. The blood vessels carry away the nutrients that are absorbed, and the nerve endings stimulate the muscle fibers so that the food is continually moving by peristalsis.

3.     The muscularis externa consists of a circular band and a longitudinal band of visceral muscles; is the thickest of the four layers. Main function: peristalsis.

4.     The serosa is continuous with the mesentery, the connective tissues that attach to the posterior body wall and hold the digestive organs in their proper position.