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STUDENT OBJECTIVE

Students dissect a fetal pig to observe the mammalian circulatory system and study the pumping mechanism of the heart. The closed circulatory system of the fetal pig is compared to the open circulatory system of an arthropod, the crayfish. Microcirculation of lower vertebrates is observed in a living fish.

EQUIPMENT	AMOUNT
	(Class of 24 with 8 groups)
Compound microscope Dissecting microscope Aquarium	1/student 1/student 2/lab
MATERIALS
Demonstration Pig Heart Dissection (CBS#P20575) Fetal pig (from previous exercise) Dissection equipment Storage material for fetal pigs (see Lab Topic 25) Prepared slides Mammalian artery and vein, cross section (CBS31-4088)* Net Petri dishes Cotton batting Coverslips, #1 medium square glass Crayfish, live Guppies or tadpoles, live Blood smear, Wright stain	1/lab 1/student 1/student 1/lab 1/student 50 cm2/lab box/lab 8/lab 1/student

*Please refer to the Appendix for name and address of supplier.

SOLUTIONS

Invertebrate Ringer’s solution
0.5% chloretone, optional

PREPARATION

Several Weeks before Lab

Fetal pigs should be used from previous exercises. All other livestock should be ordered to arrive the week prior to lab.

One Week before Lab

1. Separate aquaria should be prepared for guppies or tadpoles and crayfish upon their arrival.
2. All dissecting equipment should be inspected and any missing or dull instruments replaced.
3. Crayfish Ringer’s solution preparation:

   0.4 g KCl
   1.5 g CaCl₂
   11.9 g NaCl
   0.2 g NaHCO₃
   0.5 g MgCl₂

   Dissolve salts in 750 ml water and bring to a final volume of 1 liter. Store in dropper bottles.

4. 0.5% chloretone preparation (optional):

NOTES

Since the mortality rate should be relatively low (25%) for live guppies and crayfish, they can be reused in the lab or in later classes.

CLASSROOM SUGGESTIONS

1. Minimum homework would be to have students hand in a flowchart tracing blood flow from one part of the body to another.
2. An excellent cardiovascular simulation program is available from Sunburst Education Software. More information can be obtained on the WWW at http://www.nysunburst.com. Another program called Isolated Heart Laboratories is available from BioQuest. Direct E-mail to asdg@umdd.umd.edu.
3. Check out the links for this lab topic at http://auth.mhhe.com/biosci/genbio/dolphin/ You will find useful materials for developing your lab introduction or summary, and in some cases, you may want to tell students to connect to a particular site for further information.

ANSWERS TO CRITICAL THINKING QUESTIONS

1. Blood entering the capillaries is under hydrostatic pressure from the pumping of the heart. This hydrostatic pressure is greater than the osmotic pressure within the capillary, therefore resulting in a flow of water, O₂ and nutrients out of the capillaries and into the interstitial spaces. At the venous end of the capillary, the hydrostatic pressure within the capillary is lower and the osmotic pressure is higher leading to interstitial fluid moving back into the capillary. However, this inward pressure at the venous end of the capillary is not sufficient for returning all fluid to the circulation.

   The blood within the capillaries is returned to the heart through the venous system. This is a series of thin-walled venules and veins that rely on skeletal muscle contraction during exercise and breathing to move the blood. Veins are equipped with one-way valves that prevent the back flow of blood.

   The interstitial fluid remaining in the tissue spaces is returned to the circulatory system via lymphatic vessels. The smallest of these are lymphatic capillaries that "dead-end" in the tissues. Their walls are very porous, and increasing hydrostatic pressure within the tissues forces interstitial fluid into them. The interstitial fluid, now called lymph, moves from the lymph capillaries into lymphatic vessels in much the same way that blood moves through the venous system. Movement, itself, is facilitated by the muscle contractions associated with movement and breathing. As wells, the lymph vessels have one way valves that prevent back flow. The vessels merge to form larger lymphatic trunks which join to form either the thoracic duct, which empties into the left subclavian vein, or the right lymphatic duct, that empties into the right subclavian vein.

2. — The heart is large and thick muscled.

   — The blood pressure is two to three times greater than blood pressure in human.

   — The giraffe has specialized valves in the jugular veins and a network of carotid arteries called rete mirabilis caroticum that enable it to withstand the surges of blood that occur when the head is lowered and raised. This system allows the blood pressure to the brain to remain constant.

3. Several circulatory adaptations relate to heat exchange. The flippers and tails of aquatic mammals and the feet of birds have rete mirabile in them where arteries and veins run next to each other and exchange heat in a counter-current fashion. Capillaries beneath our skin constrict and dilate in response to environmental temperature to conserve or dispel heat. Other adaptations relate to gas exchange. Many amphibians have highly vascularized mouth lining and skin because gas exchange occurs primarily by these routes. Diving mammals have physiological adaptations of the circulatory system that slows heart rate and suppresses blood flow to muscle while preserving flow to the brain.

SUPPLEMENTAL MATERIALS