VII. The cardiovascular system
     C. The heart
          7. Effect of electrolytes on the heart rate
               a.   Potassium and sodium
                    An increase in the concentration of potassium and/or sodium in the blood plasma results in an increase in the concentration of sodium and potassium in the tissue fluid surrounding the cardiac muscle cells.  An increase in the concentration of these two positive ions around the muscle cells slows the heart rate.  The reason for the slowing effect on the heart is that the collection of these two positive ions around the cells causes the charge on the outside of the cell to become more positive than normal.  Thus, it becomes harder for the cell to depolarize and it depolarizes (and then contracts) less often.  Cardiac arrest occurs if the potassium level in the blood plasma rises to three times its normal concentration.
               b. Calcium
                    An increase in the concentration of calcium in the blood plasma excites the heart and causes the rate to speed up.

          8. Nervous control of the heart
               You will recall that the heart has three pacemakers that create an intrinsic heart rate of 100 beats/min.  However, a parasympathetic motor nerve called the vagus nerve and a sympathetic motor nerve innervate the heart.  How do these nerves affect the intrinsic heart  rate?
               a. Parasympathetic nerve (vagus)
                    When the vagus nerve stimulates the S-A and A-V nodes (figure 13.17), it causes the heart rate to slow.  Under normal circumstances, the vagus constantly stimulates the heart produces a constant slowing of the heart.  If one experimentally severs the branch of the vagus that innervates the heart, what would you expect to happen to the heart rate?
               b. Sympathetic nerve
                    When the sympathetic nerve stimulates the S-A node, A-V node, Purkinje fibers, and myocardium (figure 13.17), the heart rate speeds up and the heart contracts more forcefully.  Under normal circumstances, there is a steady propagation of impulses along the sympathetic nerve and they produce a steady speeding of the heart.
               c. Coordination of sympathetic and parasympathetic stimulation
                    Since under normal circumstances the sympathetic and parasympathetic nerves both stimulate the heart all of the time, it would appear that the effects of these two nerves would cancel each other out and that the heart would always beat at the intrinsic rate (110x/min).  However, sometimes the sympathetic nerve stimulates the heart faster than the parasympathetic nerve resulting in a speeding of the heart.  And at other times, the parasympathetic nerve stimulates the heart faster than the sympathetic resulting in a slowing of the heart.  So what coordinates the stimulation by these nerves?  There are two centers in the medulla oblongata of the brain that coordinate the stimulation of the heart by the sympathetic and parasympathetic nerves.  These centers are called the cardioaccelerator and cardioinhibitor centers.  What effect on the heart rate does the cardioaccelerator center have?  And what effect on heart rate does the cardioinhibitor center have?  Which nerve, sympathetic or parasympathetic, does the cardioaccelerator center activate?  Which nerve does the cardioinhibitor center activate?  So how do these centers operate?  These centers are controlled by changes in blood pressure, concentration of blood oxygen, concentration of blood carbon dioxide, and blood pH.  Changes in one or more of these factors will tell these centers to stimulate the heart and change its rate.
                    1) Changes in blood pressure
                         There are specialized cells in the carotid artery and aortic arch that are sensitive to changes in blood pressure.  When the blood pressure changes, the change activates these pressoreceptors (baroreceptors).  These receptors then send a nervous message to one of the centers in the medulla oblongata and the center responds by increasing its stimulation of the heart.  Which center would be activated if your blood pressure falls below normal?  If your blood pressure falls, your body is going to react to try to raise it.  How could the body raise the blood pressure?  It could do so by stimulating the heart to speed up.  If the pump works faster and harder, the blood pressure will rise.  So, when your blood pressure drops, this drop is detected by pressoreceptors in the carotid arteries and aortic arch.  These receptors send a message to the cardioaccelerator center.  The cardioaccelerator center then sends a sympathetic message to the heart and the heart rate speeds up.    When the blood pressure gets back to normal, the receptors will sense it and will tell the cardioaccelerator center that it no longer needs to stimulate the heart as much.  How does your body try to lower your blood pressure if it rises above normal?
                    2) Changes in concentration of blood oxygen and carbon dioxide
                         In the carotid arteries and aortic arch, there are chemoreceptors sensitive to the concentration of oxygen and carbon dioxide.  These receptors will be activated if the concentration of oxygen and/or carbon dioxide rise above or fall below normal.  The receptors then activate one of the centers in the medulla oblongata and the center sends a nervous message to the heart causing a change in its rate.  Which center would be activated if the oxygen concentration falls or the carbon dioxide concentration rises?  The body is going to attempt to raise the oxygen concentration and lower the carbon dioxide level back to normal.  To do so, the respiration rate will have to speed up and more blood will have to be pumped to the lungs.  For more blood to be pumped to the lungs, the heart rate will have to speed up.  So, the chemoreceptors in the carotid arteries and aortic arch detect the drop in the concentration of oxygen and the rise in the concentration of carbon dioxide.  They send a message to the cardioaccelerator center and the cardioaccelerator center sends a sympathetic message to the heart resulting in a speeding of the heart.  When the concentrations of oxygen and carbon dioxide are normal once again, the chemoreceptors detect this and tell the cardioaccelerator center that it no longer needs to stimulate the heart as much.
                    3) Change in pH of the blood
                         An increase in the concentration of carbon dioxide in the blood causes the pH of the blood to drop (become more acid). What is the normal pH of the blood?  pH 7.4.  When the blood becomes more acid, the hemoglobin is prevented from binding to oxygen resulting in a drop in the oxygen concentration of the blood.  When the blood pH becomes more acid and concentration of blood oxygen falls, the body will attempt to raise the pH and oxygen concentration back to normal.  It can do this by increasing the respiration rate and pumping more blood to the lungs as in the previous example.  A drop in the blood pH is detected by chemoreceptors in the carotid arteries, aortic arch, and medulla oblongata.  These receptors send a sympathetic message to the cardioaccelerator center and the cardioaccelerator center stimulates the heart rate to speed up.  More blood is pumped to the lungs so that more oxygen can be picked up by the blood and more carbon dioxide can be eliminated from the blood.

     D. The circulation
          1. Layers of vessel wall
               We are going to study five types of blood vessels:
                     arteries
                     arterioles
                     capillaries
                     venules
                     veins

               The arteries and arterioles belong to the arterial circulation; these vessels are responsible for transporting blood away from the heart to the tissues.  The capillaries belong to the microcirculation; they are responsible for transporting the blood through the tissues.  The venules and veins make up the venous circulation; these vessels collect the blood from the capillaries and return it to the heart.

   Arteries, arterioles, venules and veins have three layers of tissue in their walls (figure 13.18).  These layers are the
                 tunica intima (interna)
                 tunica media
                 tunica adventitia (externa)
The tunica intima is the innermost layer (the layer up against the blood).  It is composed of a single layer of flattened epithelial cells with an underlying basement membrane.  The tunica media is composed of an inner layer of elastic fibers and an outer layer of circular smooth muscle.  The tunica adventitia is composed of collagenous fibers.  The thickness of the vessel wall and the diameter of the vessel's lumen vary from one type of vessel to another.

   The wall of a capillary contains only a tunica intima and a very thin outer layer of connective tissue (figure 13.20).

   The walls of arteries are wider than those of arterioles and the walls of arterioles are wider than those of capillaries.  Capillaries are narrower than venules and venules are narrower than veins.  Arterioles are more numerous than arteries and venules are more numerous than veins.  Capillaries are the most numerous type of blood vessel.

          2.  Types of blood vessels
               a. Arteries
                    Arteries and veins are the widest blood vessels.  However, the wall of an artery is thicker and the diameter of the lumen is narrower than those of a vein.  Elastic fibers in the walls of the arteries make them distensible (stretchable) and reboundable.  This is very advantageous when the heart contracts and ejects blood into the aorta.  Because the aorta can distend when the blood is ejected into it, the blood pressure in the aorta will rise only slightly.  What would happen to the blood pressure if the aorta could not distend?  When the heart relaxes, the aorta rebounds.  If it couldn't rebound, what effect would that have on the blood pressure?
               b. Arterioles
                    An arteriole is described as being a narrow artery.  It has a lot of circular smooth muscle in its wall.  This muscle is very important.  When this muscle contracts, the diameter of the arteriole narrows.  When the muscle relaxes, the arteriole dilates.  Constriction and dilation of the arterioles controls blood flow to the capillaries down the line.  What would happen to the volume of blood flowing through the capillaries supplied by an arteriole if the arteriole constricts?  Constriction and dilation of the arterioles also regulates blood pressure in the arteries (arterial blood pressure).  If arterioles constrict, blood backs up into the arteries.  What effect would this back up have on arterial blood pressure?
               c. Capillaries
                    Capillaries are the most numerous, narrowest, and thinnest-walled blood vessels.  The epithelial cells that line the capillaries have gaps in their walls (figures 13.20 and 13.22).  Because of the gaps and the thinness of the vessel wall, substances in the blood that are narrower than the gaps can move out of the blood into the fluid around the body cells.  In addition, substances in the fluid around the body cells can move into the blood.  What controls the movement of materials into and out of the blood?  Diffusion and filtration.  Substances will diffuse into and out of the blood from an area where they are in greater concentration to an area where they are in lesser concentration.  The blood pressure in the capillaries will force materials narrower the gaps out into the tissues (filtration).
               d. Venules
                    Venules are narrow veins.  They collect blood from the capillaries and carry the blood to the veins.
               e. Veins
                    As mentioned previously, the walls of veins are thin and the lumen is wide.  Therefore, veins can hold a lot of blood.  In fact, most of the circulating blood is located in the veins.  Because the walls of veins are less elastic than those of arteries, the blood pressure in veins is very low, and the blood in veins flows slowly, veins tend to expand to accommodate the blood that they contain.  The veins below the level of the heart return blood to the heart against gravity.  To keep the blood flowing toward the heart and counteract the downward pull of the blood by gravity, the veins below the level of the heart have one-way flow valves that open to allow blood to flow toward the heart and close if the blood starts to flow backward.

          3. Blood pressure
               a. Definition
    What is blood pressure?  It is the outward force or pressure that the blood exerts against the wall of a blood vessel.  The blood pressure is greatest in the arteries leaving the heart (aorta and pulmonary trunk) and gradually decreases as the blood flows along the vascular system from these arteries to the veins that return blood to the heart (superior vena cava, inferior vena cava, and pulmonary veins).  Blood flows along this pressure gradient from the arteries where the pressure is high to the veins where the pressure is low.

    The blood in the arteries exists at two pressures: systolic and diastolic.  The systolic pressure is created when the ventricles contract and eject blood into the aorta and pulmonary trunk.  It is the pressure exerted on the walls of the arteries as they expand to accommodate the blood forced into them by the ventricles.  It is the higher of the two values.  The diastolic pressure is created when the ventricles relax. It is the lower of the two values.  During the diastolic pressure, the walls of the arteries rebound.  If they did not rebound, the pressure in the arteries would drop even lower and blood flow would slow down too much.  The arterial blood pressure can be taken in several arteries but it is usually taken in the brachial artery.

               b. Maintenance of arterial blood pressure
                    There are five primary factors that maintain and/or influence arterial blood pressure.  Let's look at them one at a time.
                    1) Rate of the heart beat and force of the contraction
                         The heart is a pump.  Any change in pump activity will affect the pressure of the blood.  The arterial blood pressure rises as the heart rate increases and heart contracts harder. What factors influence the rate of the heart beat and force of the contractions?  Concentration of oxygen in the blood, concentration of carbon dioxide in the blood, change in pH of the blood, and change in blood pressure.  These factors were addressed earlier in the notes under the section "coordination of sympathetic and parasympathetic stimulation of the heart".
                    2) Elasticity of the vessel walls
                        As described earlier, the walls of the arteries are elastic.  When the ventricles contract (systole) and eject blood into the arteries leaving the heart, these arteries expand.  By expanding, the pressure in the arteries does not rise as high as it would if the arteries did not expand.  Thus, expansion keeps the systolic pressure in the arteries from soaring.  When the ventricles relax (diastole), the pressure in the arteries drops and the walls rebound in order to keep the diastolic pressure in the arteries from plummeting.   Expansion and rebounding of the arterial walls prevent wide fluctuations in arterial pressure.  In arteriosclerosis, the walls of the arteries lose their elasticity and the person's arterial blood pressure rises.  Arteriosclerosis is one of the causes of the rise in blood pressure that occurs as we age.
                    3) Peripheral resistance
                        What is peripheral resistance?  It is the resistance to blood flow or the lowering of blood flow in smaller blood vessels, namely the arterioles.  You will recall that arterioles have a lot of circular smooth muscle in the walls.  When the circular smooth muscle contracts, it causes an arteriole to constrict.  As described earlier, constriction of an arteriole will decrease the blood flow to vessels on down the line and create a back up of blood in the arteries feeding blood into the arteriole.  The back up of blood causes the arterial pressure to increase.  Relaxation of the smooth muscle results in dilation of the arterioles, an increase in blood flow to vessels down the line, an elimination of back up of the blood, and a decrease in arterial blood pressure.

Sympathetic motor nerves innervate the smooth muscle of the arterioles and stimulate them to always be contracted to some extent.  When sympathetic stimulation of the muscle increases, muscle contraction increases and the vessel constricts.  When sympathetic stimulation decreases, muscle contraction decreases and the vessel dilates.

What regulates the sympathetic stimulation of the muscles of the arterioles?  It is regulated by the vasomotor center in the medulla oblongata.  What tells the vasomotor center what to do?  The pressoreceptors and chemoreceptors in the carotid arteries and aorta that were described previously relay information to the vasomotor center.  The pressoreceptors are sensitive to changes in blood pressure.  Let's see what happens if a person's blood pressure falls.  The pressoreceptors detect this decline and send a nervous message to the vasomotor center in the medulla oblongata.  The medulla oblongata then increases the sympathetic stimulation of the smooth muscle of the arterioles causing the arterioles to constrict.  What effect will this constriction have on the arterial blood pressure?  It will raise it.  When the blood pressure gets back to normal, what will the pressoreceptors do?  What series of reactions occur if a person's blood pressure rises?  What series of reactions would occur if the concentration of oxygen in the blood falls, the concentration of carbon dioxide in the blood rises, and/or the blood pH becomes more acid?
                    4) Blood volume
                        If the blood volume decreases, as when a person hemorrhages, there
will be less blood in the blood vessels and, therefore, less blood pushing against the walls of the vessels and the arterial blood pressure will decline.  The decline in blood pressure will then be detected by the pressoreceptors mentioned earlier and the body will attempt to raise the blood pressure by increasing the heart rate and constricting the arterioles.
                    5) Blood viscosity
                        You can think of viscosity as referring to thickness.  A more viscous fluid is thicker and flows more slowly than a less viscous fluid.  If the blood became more viscous as might happen in leukemia, what effect would this have on the arterial blood pressure?  The pressure would rise.  Why?  Because the heart would have to pump harder to push the blood through the blood vessels.  As established earlier, the harder the pump works, the higher the blood pressure goes.

            c. Maintenance of venous blood pressure
                As mentioned earlier, the blood pressure in the veins is very low and because of this the blood flows slowly.  The body has to do something to keep the blood flowing under these circumstances so that it will not collect in the veins.  What factors keep the blood flowing and maintain the blood pressure in the veins?
                1) Vasomotor activity
                    The single-unit smooth muscle of the veins contains pacemakers that spontaneously cause the smooth muscle to contract and relax.  These contractions create a pumping action that promotes blood flow and maintains blood pressure.
                2) Massage by adjacent skeletal muscles
                    Many of the veins, particularly those in the legs and arms, are surrounded by skeletal muscles.  As these muscles contract, they squeeze the veins.  This squeezing action promotes blood flow.
                3) Respiratory pump
                    The process of inhaling and exhaling creates a pump that promotes the flow of blood in veins located in the chest.  When one inhales, the flow of the blood in the veins of the chest speeds up.  When one exhales, the flow slows down.  This pumping action keeps the blood flowing in the chest and acts as a suction to pull blood in other veins toward the chest.