Exercise 8 – COMPARATIVE VERTEBRATE STRUCTURE

Student Learning Outcomes

At the completion of this exercise you should:

(1) Know the functions of the digestive system and how the digestive system relates to biomolecules and cellular respiration

(2) Know the functions of the major organs and structures in the abdominal and thoracic cavities

(3) Be able to identify and locate the major organs and structures in the abdominal and thoracic cavities on a dissected rat

(4) Know the structures/organs that make up the gastrointestinal and respiratory tracts

Here are the videos that are used to supplement the lab exercise:

Video #1: An Introduction to the Digestive System

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Video #2: The Digestive System Part 1

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Video #3: The Digestive System Part 2

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Video #4: The Digestive System Part 3

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Video #5: The Rat Dissection

This lab covers the basics of the mammalian digestive system and ties together Module 4 (on biomolecules) and Module 7 (Cellular Respiration). I recommend completing this lab prior to beginning Module 7. In this lab, you will learn about the various organs that are involved in digestion–an important event that must occur before individual cells can break down small molecules (like glucose), for the production of ATP. If you’ve forgotten about the major types of biomolecules, it’d be helpful to review Module 4.

The Digestive System

All vertebrates have a complete, tubular digestive tract with a mouth for the entrance/consumption of food and an anus for the exit of waste. The details may slightly vary from species to species, but the basic needs and, therefore, the basic plan is largely universal to vertebrates. The digestive system is involved in the enzymatic and mechanical breakdown of large biomolecules (polymers and fats) into their smaller constituent monomers, and their subsequent absorption into the bloodstream.

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Question 1. Refer to the introductory presentation: What are the two primary functions of the digestive system?

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Question 2. After watching video #1 and using the lab guide, answer the following: Describe the path that food takes from the mouth to the bloodstream. You only need to state the structures/organs that food passes through on this journey through the gastrointestinal tract.

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Question 3. After watching video #1 and using the lab guide, answer the following: Polymers and fats are enzymatically digested within the small intestine. With the help of enzymes (sent from the pancreas) and bile (sent from the liver), fats are broken down into two types of smaller molecules called:

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Question 4. After watching video #1 and using the lab guide: Polymers and fats are enzymatically digested within the small intestine. With the help of enzymes (sent from the pancreas) and bile (sent from the liver), proteins are broken down into smaller monomer molecules called:

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Question 5. After watching video #1 and using the lab guide: Polymers and fats are enzymatically digested within the small intestine. With the help of enzymes (sent from the pancreas) and bile (sent from the liver), large complex carbohydrates (polysaccharides) are broken down into smaller monomer molecules called:

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Question 6. After watching video #1: What is the advantage of the inner lining of the small intestine having millions of tiny, finger-like projections called villi?

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Question 7. Refer to the introductory presentation: What are the two primary reasons that the cells of the body require the small biomolecules (monosaccharides like glucose, amino acids, fatty acids, etc.) that result from digestion?

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Question 8. After watching video #2: What are enzymes? Use the definition provided in the video.

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Question 9. After watching video #2: What is the role of enzymes in digestion?

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Question 10. After watching video #3: The mechanical breakdown of food (using your teeth) begins in the mouth. The enzymatic breakdown of food also begins in the mouth. Various salivary glands secrete an enzyme called salivary amylase. Amylase breaks down _________________ (a polysaccharide) into ________________ (a monosaccharide).

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Question 11. After watching video #3: Ingested food travels from the esophagus to which organ?

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Question 12. After watching video #3: Hydrochloric acid (HCl) secreted by cells of the stomach helps to break down any pathogens (bacteria or viruses) that were ingested. What does the HCl do to the proteins in food?

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Question 13. Use the lab guide to answer the following question: The enzyme pepsin and hydrochloric acid secreted by the stomach help to unravel (denature) and break down which type of biomolecule polymer?

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Question 14. Use the lab guide to answer the following question: How does the stomach mechanically break down food?

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Question 15. Use the lab guide to answer the following: Describe the major functions of the small intestine.

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Question 16. Use the lab guide to answer the following question: Within which organ does the majority of enzymatic digestion occur?

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Question 17. After watching video #4: The majority of enzymatic digestion occurs within a region of the small intestine called the ____________________.

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Question 18. After watching video #4: What role does bile play in the digestion that occurs within the small intestine?

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Question 19. After watching video #4 and using the lab guide: Bile is transported into the small intestine via the gall bladder and the bile duct. Which organ produces bile?

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Question 20. After watching video #4 and using the lab guide: Which organ produces and secretes a huge variety of digestive enzymes into the small intestine? It secretes enzymes that break down polysaccharides into monosaccharides, enzymes that break down fats into fatty acids and glycerol, and enzymes that break down proteins into amino acids.

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Question 21. After watching video #4: Largely indigestible food (feces) leaves the small intestine and enters the ____________________________. Hint: This is an organ and part of the gastrointestinal tract.

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Question 22. After watching video #4 and using the lab guide: Describe the major functions of the large intestine

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Question 23. After watching video #4: Explain why a lactose-intolerant (they are intolerant due to the lack of a genetic mutation that would allow for lactase production as an adult) individual experiences bloating, gas, and diarrhea after eating large amounts of dairy products (like a plate of cheese-covered nachos).

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Question 24. After watching video #4 and using the lab guide: Which organ absorbs the small, digested biomolecules that result from digestion?

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Question 25. After watching video #4 and using the lab guide: Which organ contains symbiotic bacteria that help break down otherwise indigestible molecules and generates important vitamins?

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Question 26. Use the lab guide to answer the following question: Describe the major functions of the kidneys.

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Question 27. Use the lab guide and the introductory presentation to answer the following question: Describe the major functions of the lungs. The lungs are the major organ of the respiratory system. Make sure to mention which gas is inhaled by the lungs and what this inhaled gas is a reactant for. Also be sure to mention the gas that is exhaled from the lungs?

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Question 28. Use the lab guide to answer the following question: What is the muscular partition that separates the abdominal cavity from the thoracic (chest) cavity called?

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Question 29. Use the introductory presentation to complete the following: Starting with the mouth/nasal cavity, list the structures and organs that make up the respiratory tract (in order).

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Rat Dissection

The remainder of the lab describes the organs and structures that are encountered during the rat dissection. Under normal circumstances, you would be completing a dissection of a rat with your lab group in person. Since we are online, you will instead use this information (below), along with the provided rat dissection video (Video #5 at the beginning of this document), the lab guide, and the introductory PowerPoint slides to learn the material.

In this activity you will examine the structures that make up the major organs of mammals, using a model organism, the rat. The rat’s general internal anatomy is very similar to human anatomy (and all other mammals) with two exceptions: 1) they lack a gallbladder, and 2) they have an enlarged cecum compared to the vestigial appendix of humans. This anatomical similarity, as well as ease of care and their rapid reproductive rate, has made them the most common animal used in medical experiments.

Head and Neck Region

The regions of the nose and mouth function as parts of the respiratory and digestive systems, in some cases, both. *The roof of the mouth is called the palate. The palate separates the mouth cavity from the nasal cavity.* At the back of the mouth are openings of the esophagus, where food goes when it is swallowed, and the trachea, which takes air to the lungs. The opening of the trachea, called the glottis, is covered with a flap of tissue, the epiglottis, when food is swallowed to prevent it from passing into the lungs.

Opening the Body Cavities (Completed for you in the Dissection Video)

The following steps will be completed for you in the dissection video: Pinch the skin in the area of the belly button and make a small horizontal cut with the scissors. The skin is thick and tough and covered in fur. If you cut carefully, you can separate the skin from the abdominal body wall muscles and peel it back to observe these. Eventually you will need to cut all the way through these muscles and the peritoneum–the shiny membrane lining the inside of the body wall.

Lift the skin and make a cut anteriorly to the throat starting at the anus. (If you have a male, cut to one side of the midline to avoid the penis.) Make lateral cuts to the inner thighs, across the bottom of the rib cage, to the fore arm pits and up toward the ears from this central cut. (Shown in diagram above.) Pour out any fluid from the body cavity and pin back the flaps of skin to your dissecting pan.

The empty space around the internal organs is the coelom or body cavity. It is divided into two sections, the thoracic and abdominal cavities, by a large sheet of muscle in a dome shape, called the diaphragm. Contraction of the diaphragm muscle creates negative pressure in the thoracic cavity and causes inhalation of air into the lungs. *Organs are suspended in the body cavity and attached to each other and the internal body wall by thin bands of tissue called mesenteries. These also carry the blood vessels to the organs.*

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Watch the following video until 10:40: Video #5: The Rat Dissection

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Thoracic (aka Chest) Cavity (Covered in Dissection Video)

The thoracic cavity contains the heart and lungs, which are protected inside the rib cage. You will have to cut through the sternum and separate the rib cage to see the organs.

Respiratory System (Covered in Dissection Video)

Follow the trachea from the neck to where they branch into the two bronchi, leading into the lungs. Notice that the lungs have several lobes.

Circulatory System (Covered in Dissection Video)

The heart is in the center of the thoracic cavity. *It is encased in a thin membrane, the pericardium.* Tear the pericardium away and identify the 2 atria and 2 ventricles of the heart (this is not done in video–don’t worry about this). *The remainder of the circulatory system consists of the blood vessels and blood. The largest blood vessels are near the heart, and that they sequentially branch and split, getting smaller and smaller to supply blood to every part of the body. Large vessels also lead directly to the kidneys and liver and connect these organs to the intestine.*

Abdominal Cavity (Covered in Dissection Video)

The abdominal cavity contains the digestive organs and the *urogenital system, which consists of the excretory and reproductive organs.*

Digestive system (Covered in Dissection Video)

The most obvious organ in the abdominal cavity is the dark, multi-lobed liver just posterior to the diaphragm. The liver secretes bile into the small intestine for the emulsification of lipids (including fats) and also has many other functions, including converting toxic nitrogen-containing molecules in the blood into urea for excretion. Underneath the liver and to the rat’s left is the stomach, for food storage and the digestion of proteins. The esophagus connects the mouth to the stomach but is obscured by the organs of the thoracic cavity. The spleen appears as a flat, oval flap connected to the left side of the stomach (from the rat’s perspective). The spleen functions in the production and destruction of blood cells, including white blood cells that are part of the immune system. As such, it is properly part of both the circulatory system and the immune system. The small intestine appears to be a bundle of tubes filling the middle of the abdominal cavity and is the primary site of the enzymatic digestion of carbohydrates, proteins, and lipids and absorption of the resulting small biomolecules. The pancreas is a region of whitish tissue at the junction of the stomach and small intestine, which secretes a huge variety of digestive enzymes into the small intestine. The cecum is a large pouch off of the lower right side of the small intestine, which contains bacteria that help the animal digest plant matter. The large intestine descends from the small intestine to the rectum, where feces are temporarily stored and compacted before defecation via the anus. The large intestine is where water is reabsorbed from the undigested food before it is defecated.

Excretory System (Covered in Dissection Video)

The main organ of the excretory system is the kidney. To find the kidneys, located in the lower abdominal cavity near the spine, move the intestines to one side and gently cut through the membrane lining the body cavity (or remove all of the organs like the guy did in the video). The kidneys are actually outside the abdominal body cavity. Trace their connection to the bladder, via the ureter.

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Acknowledgements

This lab activity was based on material in:

Hickman, C. P., Jr. & L. B. Kats (2003) Laboratory Studies in Animal Diversity, 3rd ed. McGraw Hill, NY.

Krogh, Richardson & Richardson (2005) A Laboratory Guide to the Natural World, 3rd ed. Benjamin Cummings.

Smith, D. G. (2002) Exercises for the Zoology Laboratory, 2nd ed. Morton Publishing Co., Englewood Colorado.