CARDIOVASCULAR STRUCTURE AND FUNCTION

Purpose:

      1.  To study the structure and pumping action of the mammalian heart by examination of the dissected pig heart and construction of a mechanical model of the heart.  
      2.  To observe the electrical activity of the heart by electrocardiogram recording.

Materials:

Introduction:

      The cardiovascular or circulatory system consists of the heart and blood vessels.  Its primary function is to maintain a constant flow of blood to the tissues.  In 1628, the English physician, William Hardy, showed that the mammalian circulatory system is a closed system; that is, blood flows in vessels at all points in the system.  By careful observation and experimentation, Harvey concluded that the blood circulated, leaving the heart in arteries and returning to the heart in veins.  Harvey further postulated the presence of pores in the tissues, which served to connect the arteries to the veins.  Of course, we now know that microscopic vessels called capillaries are the connection between the arteries and veins, and that the capillaries are the only sites of exchange between the blood and the tissues.  In today’s lab you will study various aspects of the circulatory system in order to better understand how this important system functions.

A.  Dissection of the pig heart

      The heart of the pig is convenient for study since it is approximately the same size as the human heart and is easily obtained.  The heart is composed primarily of cardiac muscle cells and connective tissue.  Each side of the heart has two chambers, a receiving chamber called the atrium, and a pumping chamber called the ventricle.  Since the heart acts as a double pump we often say the heart consists of a right heart and a left heart.  Right and left sides of the heart can be determined by palpating the ventricular musculature.  The left ventricle appears firm and muscular, while the right ventricle feels soft and flabby.  The difference in musculature of the right and left ventricles reflects the difference in the amount of work required by the two ventricles to pump the blood.  The auricles appear as ear-like projections above the ventricles and the cavity within each auricle is referred to as an atrium.

      The right heart receives blood, which is returning from the tissues in the two large veins, the superior and inferior vena cavae.  This blood is relatively low in oxygen (deoxygenated) and is pumped by the right ventricle to the lungs via the pulmonary arteries.  In the lungs the blood circulates in the pulmonary capillaries in close proximity to the alveoli where it gives up carbon dioxide and picks up oxygen.  The blood returns to the left atrium via the pulmonary veins.  It enters the left ventricle and is pumped from there to the rest of the body.  We can see that the left ventricle must be more muscular than the right ventricle since it pumps blood to the whole body (systemic circuit) while the right ventricle pumps only to the lungs (pulmonary circuit).  The left heart handles only arterial blood which is relatively high in oxygen and low in carbon dioxide, while the right heart handles only venous blood which is relatively low in oxygen but high in carbon dioxide.  You should understand that the pulmonary veins (from the lungs to the heart) are the only veins to carry oxygenated blood, and that the pulmonary arteries (from the heart to the lungs) are the only arteries to carry blood low in oxygen.

      You should observe the following structures on the pig heart.

a.  Right Heart.  The pulmonary artery leaves the right ventricle directly between the ear-like projections of the two auricles.  Entering the right auricle from above is the superior vena cava, which drains the upper part of the body (head, limbs and chest).  The wall of the superior vena cava is much thinner than the walls of the pulmonary artery or aorta.  The inferior vena cava, which drains the lower part of the body, enters the right atrium laterally and can best be observed on the dissected specimen.

      On the interior of the right atrium you should note the regular bands of muscular tissue called the pectinate muscle.  You should also observe the flaps of membranous tissue, which project down into the ventricle where they attach by white cords.  These flaps make up the tricuspid valve or atrio-ventricular (AV) valve between the right atrium and the right ventricle.  The tricuspid valve functions to prevent blood from being forced into the atrium when the ventricle contracts.  Under the flaps of the tricuspid valve is the outlet of the right ventricle into the pulmonary artery.  Cutting open the artery will reveal the pulmonary semilunar valve, which is made up of three pocket-like structures surrounding the outlet of the ventricle.  Blood is pumped from the ventricle by its forceful contraction into the pulmonary artery and the semilunar valves are forced against the walls of the artery.  When the ventricle relaxes the blood in the pulmonary artery immediately starts to backflow and this fills the pockets of the semilunar valve, which then close to prevent blood from re-entering the ventricle.

b.  Left Heart.  There should be four pulmonary veins entering the left atrium, however, when the hearts are removed from the pigs these vessels may have been cut too close to the atrium and may not be present.  Between the left atrium and the left ventricle can be found the second atrioventricular (AV) valve, the mitral or bicuspid valve.  Like the tricuspid valve, the bicuspid valve prevents backflow of blood into the atrium during ventricular contraction.  The outlet of the left ventricle is the aorta.  The aorta is the thick, muscular artery, which carries blood, via its branches, to all tissues (systemic circuit).  Between the left ventricle and aorta is the aortic semilunar valve, which prevents backflow from the aorta into the ventricle during ventricular relaxation.  The openings of the two coronary arteries can be found just above the aortic valve and within its pockets.  The coronary arteries are the only vessels that supply oxygenated blood to the heart musculature itself.  Blockage of either of the main coronary arteries or their smaller branches by atherosclerotic plaques, for example, can ultimately lead to a heart attack where a portion of the heart muscle dies due to the lack of sufficient blood supply.

 

B.  Do-It-Yourself Heart

      Mechanically, the heart is nothing more than two pumps and a series of valves.  By constructing a mechanical model of this pump (one side of the heart) one can get a feeling for the mechanical nature of heart function.  The parts supplied and the diagram below (Fig.1), show how to construct a do-it yourself heart.  Care must be taken to get the one-way valves in the right places and facing in the right direction.  The valves have small arrows on them to indicate which direction they allow flow.

 

            Figure 1.  Diagram of the do-it yourself heart.

            The flow of blood through the cardiovascular system depends upon the maintenance of adequate arterial blood pressure at all times.  It has been shown that arterial blood pressure (BP) is equal to the product of two factors:  the cardiac output and the total peripheral resistance.  The cardiac output is the volume of blood pumped by the heart per minute (liters/min).  Cardiac output itself is the product of two factors:  the heart rate (HR) expressed as the number of beats/min, and the stroke volume (SV) which is the volume of blood pumped per beat, expressed as ml/beat.

            As an example, at rest one might normally find a heart rate of 70 beats/minute and a stroke volume of 70 ml/beat.  Since cardiac output = HR  X  SV, the cardiac output would be 4900 ml/ minute or 4.9 liters/minute.

            The second factor making up the arterial blood pressure is the total peripheral resistance.  This factor is the frictional resistance to flow of the vessels through which the blood must pass.  As a result of this frictional resistance, the pressure imparted to the blood by the heart is dissipated as blood flows through the circulatory system.  Taking into account both factors the arterial blood pressure may be expressed as

            Arterial blood pressure  =  Cardiac output  X  Total peripheral resistance.

            Using the mechanical heart model you will investigate the influence of various factors on the maintenance of blood pressure.  Blood pressure may be qualitatively interpreted as the degree of distension of the balloon aorta or else the apparent force with which fluid is ejected from the peripheral resistance (syringe needle).

            1.  Influence of heart rate and stroke volume on blood pressure and blood flow.

            Establish a heart rate of about 10 beats per minute and a stroke volume of about 15 ml/beat.  Notice the degree of distention of the elastic aorta (balloon) and the force of the water flow from the needle.  By increasing either the heart rate or stroke volume (one at a time) describe the influence of heart rate and stroke volume on the blood pressure and blood flow.

            2.  Influence of an elastic aorta on blood pressure.

            What is the role of the elastic aorta on the blood pressure when the heart contracts (ventricular systole)?  When the heart relaxes (ventricular diastole)?  As mentioned previously, a common type of artrerial disease is atherosclerosis, which results in hardening of the arteries.  Hardening of the arteries results in a loss of their elasticity.  You should note the relative blood pressure for a given cardiac output and peripheral resistance with the elastic aorta in place.  Now substitute a section of inelastic plastic or rubber tubing for the elastic aorta.  At the same cardiac output as used previously, what is the effect of the inelastic aorta on blood pressure?

            3.  Influence of peripheral resistance on blood pressure.

            Peripheral resistance is the frictional resistance to flow found in the cardiovascular system, which results from the branching of large arteries to smaller and smaller arteries.  The smallest arteries are the arterioles, which have extremely small diameters and are responsible for most of the peripheral resistance in the system.  The diameter of the arterioles can be controlled such that an increase in arteriolar diameter (arteriolar dilation) results in decreased peripheral resistance and a decrease in arteriolar diameter (arteriolar constriction) leads to an increased peripheral resistance.  Why do you suppose this is so?  With the 18-gauge needle in place you should note the blood pressure at a constant cardiac output.  Remove the 18-gauge needle and substitute a 23-gauge needle in its place.  The 23-gauge needle has a narrower opening than the 18-gauge needle and represents a constricted arteriole (increased peripheral resistance).  Using the same cardiac output as you used earlier, what is the effect of increased peripheral resistance on blood pressure?

            4.  Role of the atrioventricular (AV) and aortic valves in heart function.

            You should design experiments that will illustrate the roles of the AV and aortic semilunar valves in heart function.  Describe the roles of these valves and the results which occur when these valves malfunction or are absent.

C.  The Electrocardiagram (ECG or EKG)

            Whenever muscles contract, waves of electrical activity pass through the muscle tissue.  Since heart muscle normally contracts in a stereotypical fashion with every beat, the heart generates characteristic electrical activity during each cycle of contraction.  This activity is conducted through the body fluids and may be recorded from electrodes placed at certain spots on the body surface.  This recording is called an electrocardiogram (ECG or EKG).  The figure shows a normal ECG in which several waves may be distinguished.





           Figure 2.  A normal ECG with characteristic wave patterns.  Atrial excitation refers to the P-wave, Ventricular excitation to the QRS wave and Ventricular repolarization to the T-wave.

            The characteristic P wave results from the electrical activity passing over both atria, which precedes their contraction.  The QRS complex results from electrical activity around the ventricles preceding their contraction.  The T wave is associated with the recovery or relaxation of the ventricles. 

            The electrocardiogram has become a potent diagnostic tool since many cardiac abnormalities give rise to abnormal ECG’s.  For example, a condition such as a heart attack, which results in damage to the heart muscle, may lead to changes in the electrical conduction properties of that part of the heart.  These changes often become evident on the ECG.  Although not foolproof, the ECG can alert the physician that problems within the heart do exist.

D.  Capillary Circulation Demostration

            The function of the capillaries is to bring the blood into close contact with all body cells so that gas, nutrient and waste exchange may take place between the cells and the blood.  Capillaries are microscopic and consist of only a single layer of flattened epithelial cells.  It is quite easy to observe the microcirculation (arterioles, capillaries and venules) of blood through the capillaries of the frog.  Capillaries may be observed in the tail of a fish.  A demonstration of capillary circulation has been set up for you.  You should observe the demonstration by locating some capillaries and their associated venules and arterioles.  What criteria are used to distinguish between these vessels?  What is the relative speed of blood flow in the various vessels?

            Examine a bed of capillaries using higher magnification.  Compare the internal diameter of several capillaries with those of the red blood cells passing through them.  What is the relation between the diameters of the red blood cells and the capillaries?

E.  Heart Beat, Pulse Rate and Systemic Blood Pressure

            1.  With your third and fourth fingers palpate the radial artery of your lab partner at his/her wrist and count the pulse for 30 seconds.  Calculate the pulse rate (beats/minute).

 NOTE:  STUDENTS WITH A HISTORY OF CARDIOVASCULAR DISEASE (high blood pressure, heart damage, etc) SHOULD NOT ENGAGE IN THE FOLLOWING EXERCISE.

            2.  Vigorously jog in place for one minute and determine your pulse rate again.  Has it changed?  Why?  After sitting at rest for five minutes, again check your pulse rate.  Has it dropped to its normal level?

            Locate the apex beat of the heart by the palpatory method and place the bell of the stethoscope over the ventricle about an inch above the apex.  Listen to the sounds that occur during the cardiac cycle. 

            LUBB – caused by the closure of the atrioventricular valves
            DUBB – caused by the closure of the semilunar valves

            Determine the number of cycles occurring in 30 seconds, then calculate the heart rate.  Is it the same as the pulse rate?  (NOTE – CLEAN THE EARPLUGS WITH ALCOHOL BEFORE AND AFTER USE OF THE STETHOSCOPE.)


            Arterial blood pressure is determined by placing an inflatable rubber cuff, attached to a pressure gauge (sphygmomanometer) around the arm, inflating it to collapse the underlying artery, and listening to the vessel below the cuff with a stethoscope.  When the cuff pressure exceeds systolic arterial pressure the artery is collapsed and blood flow through it ceases; as cuff pressure is reduced, flow of blood through the artery occurs when cuff pressure falls just below systolic arterial pressure.  At this point a sharp tapping sound may be heard.  The cuff pressure at this point is an approximation of systolic pressure.  As cuff pressure is further reduced, the sounds increase in intensity and then suddenly become muffled at the level of diastolic pressure.  This point is an approximation of that pressure. 

            By convention, blood pressure determined by this indirect method is expressed as a ratio:

                                    systolic pressure    
                                    diastolic pressure

            Obtain a sphygmomanometer/stethoscope unit and perform the following sequence to determine systemic arterial blood pressure.

     1.  Wrap the deflated cuff snugly about the arm placing the listening bell over the brachial artery.

      2.  Close the valve on the inflating bulb and pump up the pressure in the cuff to 160 mm.

     3.  Open the valve slightly allowing the cuff to deflate at 2-3 mm/second while listening for sound.  Record the reading of the manometer when sound is first heard (systolic pressure).

     4.  Continue to deflate the cuff at the same rate while listening to the sound.  Record the reading of the manometer when the sounds become dull and muffled and when the sounds reappear.  The latter reading should be regarded as the diastolic pressure. 

     5.  Rapidly deflate the cuff and remove it from the subject.  Record the systolic/diastolic pressure (i.e.  120/75).

            Measure the systolic and diastolic blood pressure under the following conditions:

                        A.  While the subject is at rest.

                        B.  While the subject is seated, immediately after jogging in place for one minute
                                    or running down to the first floor and back up.

            Record the data and calculate the mean arterial blood pressure according to the following formula:

                        Mean Arterial Pressure = (Systolic pressure ) +  2(Diastolic pressure)
                                    `                                                           3

            Mean arterial pressure may also be determined by calculating the average between the systolic and diastolic pressures (i.e. systolic pressure plus diastolic pressure divided by 2).  However, the above calculation is more accurate because it takes into consideration the length of time for systolic action compared to diastolic action.  Thus systolic activity is approximate half as long as diastolic activity.

To observe circulation in capillaries use a small fish from an aquarium.  Wrap the fish in dripping wet cotton, being careful not to cover the head or tail.  Lay the fish in an open petri dish.  If the fish is not immobilized, place the fish in a container of chloroform to anesthetize it.  As soon as it turns belly up, remove it, rinse it, rewrap it in wet cotton and place it in the petri dish.  Place a few drops of water on the tail and add a coverslip over the tail.  Place the dish on a compound microscope stage and observe the tail under scanning power.  Sketch your observations and answer the following questions. 

            Can you identify capillaries, arteries and venules?

            Is blood flow faster in certain vessels comparted to others?

            Is blood flow continuous in all vessels?  What might control this?