Lab 7 Sample Solutions

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Sample solutions to Digestive/Urogenital Tables and Discussion Questions: 1. In the shark and rat, the stomach has large internal folds called rugae. Examine the rugae in the shark stomach. What is the function of these rugae? The rugae are longitudinal folds in the wall of the stomach that allow the stomach to expand in size to hold a large meal, without actually stretching the wall of the stomach. As the stomach fills, the rugae unfold to increase the volume of the stomach. 2. a) What are the two main functions of the small intestine in vertebrates? How does the length of the small intestine affect each of these two functions? 1. Digestion (both chemical and bacterial): Chemical and bacterial digestion takes time, and the longer the small intestine, the more time it will take for food to pass through it, so the more time will be available for digestion. 2. Absorption of nutrients: If all other factors are equal, a longer small intestine will have a larger internal surface area. The larger the surface area of the small intestine, the more efficient it will be at absorbing nutrients – increasing surface area increases the absorption rate of nutrients. Nutrients are absorbed through the cell membranes of the epithelial cells that line the intestine, so the larger the surface area of the small intestine, the more epithelial cells are available to absorb nutrients. b) Consider the length of the small intestine in your three animals. Which one has the shortest small intestine (relative to the rest of the digestive system)? The small intestine in this animal has an internal structure that allows it to efficiently perform the functions that you identified in part a), even though it is relatively short. What is the name of this structure, and how does it allow the small intestine to efficiently perform these functions? The small intestine is relatively short in the shark. The spiral valve increases the internal surface area of the ileum of the small intestine for efficient absorption of nutrients, and also increases the distance that food must travel, slowing the passage of food through the ileum to allow more time for digestion and absorption. The structure of the spiral valve (a “Tesla valve”) also propels the contents of the intestinal lumen unidirectionally, even without smooth muscle contraction (peristalsis). 1

3. In the rat and pigeon, the internal surface of the small intestine has small folds called plicae. What is the function of these structures? How are they different from the rugae of the stomach? Plicae increase the surface area available for nutrient absorption. Unlike the rugae of the stomach, plicae do not unfold to increase the volume of the intestine. Rugae are longitudinal folds while plicae are transverse folds. 4. Gently squeeze the proventriculus and the gizzard in the pigeon. Which of these two structures feels harder? Consider the function of each, and explain why one structure is more firm while the other is more soft. The gizzard feels hard while the proventriculus feels relatively soft. The proventriculus secretes digestive enzymes and HCl (hydrochloric acid), while the gizzard grinds food as well as secretes digestive enzymes. To grind food, the gizzard must be extremely muscular - muscle contraction force increases as the cross-sectional area of muscle increases. The proventriculus does not grind food, so it is not as muscular as the gizzard, and the layers of muscle surrounding the proventriculus are much thinner. The thick, muscular walls of the gizzard feel hard when squeezed compared to the relatively thin walls of the proventriculus. 5. Consider the digestive tract in your three dissection animals. Which of these three animals would you predict have cellulose in their diet? Which structure of the digestive tract supports your prediction? Explain. Pigeon and rat. Both of these vertebrates have specialized structures for digestion of cellulose. The rat has a caecum that houses a large population of bacteria and other microorganisms that are capable of digesting cellulose. The caecum is a blind-ended sac so plant matter can remain in the caecum for prolonged periods of time to allow for digestion by these bacteria (hindgut fermentation). The pigeon has colic caeca, two small sacs that contain populations of bacteria that can digest cellulose. The shark has no specialized structures for digestion of cellulose. 6. Observe the various salivary glands of the rat. Some of these produce digestive enzymes while others produce mucus. In fact, in all three of these animals there are glands throughout the entire length of the digestive tract that produce mucus. Why does saliva contain mucus? Why is it important to produce mucus along the entire digestive tract? Mucus lubricates food to help it pass more easily down the esophagus when it is swallowed, and to prevent the food from scratching the lining of the esophagus and stomach. Mucus coats and protects the living epithelial cells that line the digestive 2

tract from being digested by enzymes, and from being damaged by the acidic secretions of the stomach and the basic secretions of the intestines. 7. In the shark, the liver produces large amounts of oil (squalene). What is the main function of this oil in the shark? Squalene helps to increase the buoyancy of the shark - oil is less dense than water. Chondrichthyes do not have air-filled gas bladders to help with buoyancy so large amounts of oil perform this function instead. (Note that even with the squalene, sharks are still negatively buoyant, so if they stop swimming they will slowly sink.) This oil also provides energy storage (like the fatty tissue of the rat) that can be used when food is limited. 8. Sharks are marine fish. They have a specialized rectal gland that helps with osmoregulation by excreting excess salts into the rectum. Would you expect to find a structure with a similar function in a freshwater vertebrate like salamanders (Amphibia)? Explain why or why not. Consider the osmoregulatory challenges faced by marine vertebrates compared to freshwater vertebrates. No. The osmoregulatory challenge faced by marine vertebrates is that the concentrations of ions such as Na + and Cl - in the marine water is higher than the concentrations of these ions in their bodily ﬂuids, such that these ions diffuse into the body, both through the skin and gills, and through the lining of the digestive tract, when they ingest the saltwater. The rectal gland helps to excrete these excess ions to maintain osmotic balance. In many marine vertebrates, because the concentration of ions is higher in the water than in the bodily ﬂuids, water moves by osmosis out of the body into the water; however, sharks retain high levels of urea in the bodily ﬂuids, such that they are hyperosmotic to the marine water, so water actually moves into the shark by osmosis. In freshwater vertebrates like salamanders, the freshwater environment has a lower concentration of ions than their bodily ﬂuids, such that ions diffuse out of the body while water diffuses into the body. Therefore, in salamanders living in freshwater, the challenge is to retain ions and excrete excess water, not to excrete excess ions, therefore we would not expect that salamanders would have a structure like the rectal gland of the shark that is specialized for ion excretion. 9. Consider the path of urine ﬂow in sharks. Describe the differences between males and females in the path of urine as it travels from the kidneys to the opening through which it exits the body. (See Kardong Figure 14.33 for clarification of the shark male urogenital system). In female sharks, urine is drained from the kidneys by the archinephric duct into the cloaca, and exits from the body via the cloacal vent. In male sharks, the archinephric duct is primarily used to transport sperm, not urine. 3

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Most urine is produced by the posterior region of the kidney and is drained by the accessory urinary duct into the cloaca, then exits via the cloacal vent. 10. Many sauropsids, including birds, excrete ammonia as uric acid. In the petri dish on display, observe the uric acid precipitate that was excreted by a snake. It has not been altered in any way since it was excreted. How does this compare to the mammalian process of excreting ammonia in the form of urea (consider your own urine)? What might be an advantage of excreting ammonia as uric acid compared to excreting ammonia as urea? Uric acid is not very soluble in water, so it requires very little water to excrete. In the cloaca, uric acid forms a precipitate with salts, such that almost all of the water can be reabsorbed by the cloaca. This helps sauropsids to excrete nitrogenous wastes while losing almost no water - this is good for water conservation. Mammals excrete nitrogenous wastes as urea, which is very soluble in water. To conserve water, mammals use the loops of Henle in the kidney to reabsorb water from the urine. However, excreting nitrogenous wastes as uric acid as a thick white paste (e.g., as in birds) conserves a higher proportion of water than excreting nitrogenous wastes as urea (e.g., liquid human urine). 11. The population of a shark species can decline drastically if too many individuals of that species are killed during any given fishing season. Based on your knowledge of the Squalus (dogfish shark) reproductive system, explain why shark populations are very susceptible to overfishing. Sharks mature slowly. Dogfish males do not reach sexual maturity until about 11 years of age and females do not reach sexual maturity until about 18 years. Mature females often produce only 6 or 7 young at a time. (Compare this to female salmon (Actinopterygii), which produce thousands of eggs at a time). Dogfish are ovoviviparous, so the embryos develop inside the mother, in the uterus of the oviduct, obtaining nourishment from their large yolks. The gestation period in dogfish is 20 to 24 months! Dogfish are slow to mature, and mature females can only produce a relatively small number of young once every two years, thus, the dogfish population grows slowly. Dogfish are vulnerable to overfishing because if a large number of them are killed, then replacement of those individuals will take many years. (Since dogfish are ovoviviparous with a long gestation period, if a pregnant mother is killed, the young developing within her will also die, whereas in oviparous species, if the mother is killed after she lays her eggs then her young could still survive.) 12. a) Compare the location of the testes in rats to the location of the testes in the shark and the pigeon. In rats, the testes are located in the scrotum, an external pouch. In the other two animals, the testes are located deep within the body cavity. 4

b) Why might the location of the testes differ in rats compared to these other animals? Mammals are endothermic, keeping the internal body temperature at approximately 37 o C, but prolonged exposure to 37 o C can be lethal to mammalian sperm. The temperature in the scrotum is approximately 2 o C lower, which allows for successful sperm production and survival. Birds are also endothermic, with even higher body temperatures than mammals (approximately 40 to 41 o C), but avian testes are able to produce sperm successfully at these temperatures (Beaupré et al , 1997), so in pigeons the testes are located within the body cavity. Sharks are ectothermic and aquatic, so their internal body temperature is unlikely to ever reach levels that would be lethal to sperm. c) What is one disadvantage of the location of the testes in rats? The testes in the scrotum are more vulnerable to mechanical injury than testes located deep within the body cavity, where they would be anatomically well-protected. Table 6.1: Complete this table with the functions of each structure of the digestive system in the rat. Name the structure with an equivalent function in the shark and the pigeon. If there is no equivalent structure, write "absent." Structure Function in Rattus Shark ( Squalus , Mustelus or other) equivalent Columba equivalent Esophagus Transports food from the pharynx to the stomach esophagus esophagus Stomach Secretes HCl, digestive enzymes, mechanical digestion of food stomach proventriculus (secretion) and gizzard (mechanical digestion, some secretion) Pyloric sphincter Controls passage of food from stomach into duodenum Pyloric sphincter Pyloric sphincter Small intestine Chemical and bacterial digestion, nutrient absorption Small intestine Small intestine Large intestine Water reabsorption and ion balance Large intestine (and rectal gland for ion excretion) Large intestine 5

Caecum Houses bacteria for digestion of cellulose absent Colic caeca Anus Elimination of feces (undigested material) cloacal vent cloacal vent Pancreas Secretes digestive enzymes into duodenum, controls blood sugar through secretion of insulin and glucagon pancreas pancreas Gall bladder Absent. Bile (which emulsifies fat) is secreted directly into the small intestine; it is not stored. Gall bladder is present: stores bile (bile emulsifies fat). Absent. Bile is secreted directly into the small intestine; it is not stored. Liver Produces bile, regulates nutrients in blood, detoxifies blood, many more… liver liver Table 6.2: Complete this table with the functions of each structure of the urogenital system in the rat. Name the structure with an equivalent function in the shark and pigeon. If there is no equivalent structure, write "absent." Structure Function in Rattus Squalus , Mustelus equivalent Columba equivalent Kidney Filters wastes from blood, osmoregulation (Metanephric) Opisthonephric kidney and rectal gland (osmoregulation) Metanephric kidney Ureter Drains urine from kidney females: archinephric duct males: accessory urinary duct ureter Urinary bladder Stores urine, some reabsorption of water/ions absent absent (they have a cloaca that receives the urine, and it is expelled from here). Urethra Transports urine from bladder to outside Absent (no bladder) Absent (no bladder) 6

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Testis Produces sperm, male sex hormones testis testis Epididymi s Stores sperm, matures sperm Seminal vesicle, sperm sac epididymis Vas deferens Transports sperm (it is the archinephric duct) Archinephric duct Vas deferens (archinephric duct) Penis Copulatory organ: transports sperm from male to female Claspers have same function Absent (sperm is transferred by touching cloacal openings) Ovary Produces eggs, female sex hormones ovary ovary Oviduct Transports eggs oviduct oviduct Uterus Growth of embryos, site of placental attachment Uterus (growth of embryos, but not placental attachment since embryos are nourished with yolk) Absent (oviduct temporarily stores eggs until laying) Vagina Receives sperm from male copulatory organ cloaca Absent (no copulatory organ in males, sperm is transferred from male to female by touching cloacal openings) 7

Lab 7 Sample Solutions

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