84 BLOOD AND CIRCULATION 



Exercise XVI 



Beef or sheep hearts will be available for ex- 

 amination. Identify the four chambers of the 

 heart: the two large ventricles forming the tip, 

 and the two smaller atria which lie above. Find 

 the openings from the veins into the atria, and 

 from the ventricles into the arteries. Examine 

 the valves of the heart. The tricuspid valve is 

 between the right atrium and ventricle, the bi- 

 cuspid (or mitral) between the left atrium and 

 ventricle. Between each ventricle and its artery, 

 a semilunar valve is found. Try to imagine the 

 action of each chamber and valve while the heart 

 is beating and blood is flowing through it. 



CIRCULATION OF BLOOD 



After you understand the organization of the 

 beef or sheep heart, two of you working together 

 should obtain a pithed frog. Draw out the 

 tongue, and pin it across the hole in the frog 

 board, or pin out the webbing of one foot across 

 this opening. (The tongue is usually more satis- 

 factory because it has less pigmentation to ob- 

 scure the capillaries.) Position the whole assem- 

 bly on the stage of a compound microscope. You 

 should be able to see clearly the circulating 

 blood, and identify arteries (or arterioles), veins, 

 and the interconnecting capillaries. Note the 

 elasticity of the red blood cells as they course 

 through the blood vessels. Can you find a 

 leucocyte pushing through a capillary wall? 



Slit open the abdomen of the frog to expose 

 the beating heart. Trace the circulation through 

 the lungs (pulmonary circulation). Find the 

 aorta, the venae cavae, and the other major 

 blood vessels. Take this opportunity to review 

 the organs of the viscera. 



Make a smear preparation of frog's blood on 

 a microscope slide and stain it as you did your 

 own blood. What striking difTerence do you see 

 between human and frog blood cells? 



Examine the prepared slides of an artery and a 

 vein in cross section and of a piece of lung in 

 cross section. Arterial walls are thicker and more 

 rigid than those of veins, though both are com- 

 posed of three layers. You will probably see 

 red blood cells within the blood vessels. 



HEMOGLOBIN 



Most of the oxygen is transported in the blood 

 stream in loose combination with hemoglobin. 

 In vertebrates, the hemoglobin is entirely con- 

 tained within the red blood cells. It is composed 

 of a protein, globin, to which heme is attached 

 as prosthetic group. Heme is a complex of 

 ferrous iron (Fe++) with protoporphyrin. 



The function of hemoglobin depends upon its 

 capacity to combine reversibly with oxygen: 

 Hb + 02^ Hb02. Hemoglobin absorbs oxy- 

 gen in the lungs, where the oxygen concentration 

 is high, and gives it up again in the tissues, where 

 the oxygen concentration is low. 



Hemoglobin possesses a characteristic absorp- 

 tion spectrum, and each of its derivatives has a 

 different spectrum. Much of the chemistry of 

 hemoglobin has been learned by observing these 

 spectra. We shall examine the spectra of hemo- 

 globin and several typical derivatives with a hand 

 spectroscope. 



Begin by orienting yourself in the visible spec- 

 trum. See that it stretches from a wavelength of 

 about 400 TUfi, in the far violet, to 700 m/x, in the 

 far red. The absorption spectra of hemoglobin 

 and its derivatives are visible as shadows in the 

 green and yellow regions. These are called ab- 

 sorption bands, and each is characterized by the 

 wavelength at which the shadow is deepest, the 

 absorption maximum. 



Oxyhemoglobin 



Examine a few milliliters of a diluted prepara- 

 tion of blood (1 ;100) in the hand spectroscope. 

 Note two absorption bands in the green, at 

 about 577 m/z and 539 mju. These are the bands 

 of oxyhemoglobin, Hb02. Any hemoglobin ex- 

 posed to air, as this is, is oxygenated. 



Reduced hemoglobin 



To the preparation you have just examined, 

 add a minute amount of the reducing agent, 

 sodium hydrosulfite (sodium dithionite, 

 Na2S204), and re-examine the spectrum. You 



