Physiology 2 Chapter 9

When ventricular contraction occurs, the pressure in the left ventricle is initially below that of the pressure in the aorta. The ventricle must build up enough pressure to exceed aortic pressure before the semilunar aortic valve is forced open and blood is ejected into the aorta. The period before the valve opens is called isovolumetric contraction, during which time the ventricle contracts but blood does not flow out of the ventricle into the aorta. While blood is flowing through the aorta, during ventricular ejection, the high resistance of the systemic circuit combined with the elasticity of the aorta leads to the storage of mechanical energy in the walls of the aorta. This mechanical energy is released when the ventricle relaxes, which results in the maintenance of a high aortic pressure, allowing blood to continue flowing throughout the diastolic period, despite the drop in ventricular pressure. This maintenance of a high aortic pressure by the storage of mechanical energy is the force that closes the semilunar valve and causes the isovolumetric contraction when the next ventricular systole occurs.

Aortic blood flow starts to increase only some time after the initiation of ventricular contraction. Similarly, aortic blood flow continues at a relatively high level well into the diastolic period. Explain why.

All three vertebrates have closed circulatory systems, with a heart that pumps blood, arteries that send blood to tissues, capillaries that allow the diffusion of materials between blood and tissues, and veins that bring blood back to the heart. They differ in the structure of their circuits, which is related to differences in respiratory structures. Water-breathing fish have a single-circuit system in which the heart pumps blood, in series, through the gills and tissues. Tetrapods, a group that includes amphibians and mammals, have a double-circuit system, in which the right side of the heart pumps blood through the pulmonary circuit, while the left side of the heart pumps blood through the systemic circuit. These circuits are in series, so that blood leaving the pulmonary circuit enters the systemic circuit, and vice versa. The left and right sides of the heart are joined as a single organ, but can differ in their degree of separation depending on the animal group. In mammals, the two sides of the heart are completely separated, which allows the pulmonary and systemic circuits to have very different hydrostatic pressures: The low pressure in the pulmonary circuit prevents fluid loss from the thin pulmonary capillaries, while the high pressure of the systemic circuit is essential to propel blood to all organs of the body. In amphibians, there is incomplete separation of the two circuits: Although there are two separate atria, a common ventricle pumps blood to the systemic and pulmocutaneous circulatory circuits. The pulmocutaneous circuit brings blood from the heart to the lungs and skin for oxygenation. Oxygenated blood from the lungs goes directly to the left atrium of the heart, while oxygenated blood from the skin mixes with deoxygenated blood from the systemic circulation before returning to the right atrium.

Compare and contrast the circulatory systems of fish, amphibians, and mammals.

Look to graph

Compare and contrast the molecular events of the action potential in the pacemaker cells of the sinoatrial node to those in a ventricular contractile cardiomyocyte.

A fish heart is composed of four chambers arranged in series that pump blood through the gills and then through the body. A mammalian heart is the equivalent of two pumps, each with two chambers: a left heart that pumps blood to the body, which then returns to the right heart, which pumps blood to the lungs. However, in mammals these two pumps are located immediately next to each other in the body, and are incorporated into a single organ. They pump together, and their pumping is coordinated by a single site (the sinoatrial node) located near the right atrium. In fish, contraction is coordinated by the sinus venosus. A fish heart contains both spongy and compact myocardium, although the relative proportions of the two types of tissue vary depending on the species. A mammalian heart contains almost exclusively compact myocardium. A mammalian heart always has coronary arteries. The presence of coronary arteries varies among species of fish.

Compare and contrast the structure of a fish heart with the structure of the mammalian heart.

Cardiac output is the rate of fluid pumping, or the volume of blood pumped per unit of time. It depends on heart rate, the number of contractions per unit of time, and on the stroke volume, the amount of blood pumped per contraction. Changes in either heart rate or stroke volume will alter cardiac output.

Define heart rate, stroke volume, and cardiac output.

Blood - circulates in a closed system. Complex tissue w/ multiple components and does not have direct contact w/ tissues Lymph - formed from blood by ultrafiltration, lacks cells, travels through lymphatic system Hemolymph - circulating fluid of open systems. Direct contact w/ tissues & interstitial fluid. Contains cells, proteins & fluid

Distinguish between blood, lymph and hemolymph

No. Eg. Sponges, cnidarians, and flatworms lack system that transports an internal fluid, but can move external fluids past cells

Do all animals have a circulatory system?

Most but not all, some polychaetes have open systems

Do all annelids have a closed circulatory system?

he valves open and close in response to pressure differences. The AV valves open when pressure in the atria first exceeds pressure in the ventricles. The AV valves close when pressure in the ventricles first exceeds pressure in the atria. The semilunar (pulmonary or aortic) valves open when pressure in the ventricle exceeds pressure in the arteries. The semilunar valves close when pressure in the arteries first exceeds pressure in the ventricles.

Draw a sketch equivalent to the graphs in Figure 9.30 and indicate the points at which the various valves open. Justify your choices.

Beta-blockers should reduce or prevent the increase in heart rate and cardiac output that normally occurs during exercise. Normally, norepinephrine and epinephrine released as a result of sympathetic stimuli such as exercise can bind to β adrenergic receptors on the pacemaker cells and increase heart rate. Beta-blockers prevent this action and thus reduce or prevent the increase in heart rate seen with exercise. Norepinephrine and epinephrine also normally increase stroke volume by increasing contractility through phosphorylation of L-type Ca2+ channels, phosphorylation of proteins in the SR that cause it to release more Ca2+, phosphorylation of myosin that increases the rate of cross-bridge cycling, and enhancing Ca2+ reuptake by the SR. Beta-blockers block this normal increase in contractility, and thus reduce stroke volume. Cardiac output is the product of heart rate and stroke volume, so blocking the expected increase in heart rate and stroke volume with exercise will cause beta-blockers to block the expected increase in cardiac output.

Drugs called beta-blockers inhibit the actions of the sympathetic nervous system. Predict what taking beta-blockers would do to heart rate and cardiac output during exercise.

I would expect resting heart rate to increase. Although mammals have a myogenic heart, with pacemaker cells that initiate cardiac contractions, the rate and force of cardiac contraction are modulated by the sympathetic and parasympathetic nervous systems. The parasympathetic system tends to reduce heart rate, while the sympathetic system tends to increase heart rate and the force of contraction. Both the sympathetic and parasympathetic nervous systems are active at all times, and it is the balance between the activities of these two systems that sets heart rate. Parasympathetic nerves release acetylcholine as a neurotransmitter, which reduces heart rate by triggering the opening of K+, which hyperpolarizes the cells by decreasing Ca2+ permeability, which slows down the rate of depolarization. Acetylcholine only has minor effects on the contractility of the mammalian heart, so the major effect of parasympathetic stimulation is a reduction in heart rate, with only modest effects of the strength of contraction. If you abolish parasympathetic nerve transmission with atropine, the balance between the sympathetic and parasympathetic stimulation shifts in favor of sympathetic stimulation. By removing the parasympathetic signals that reduce heart rate, the net effect is to cause resting heart rate to increase Because atropine does not affect the sympathetic nervous systems, I would not expect an effect on the capacity of the heart to increase its force and rate of contraction (for example, during exercise.

During an experiment, dogs were given the drug atropine, which abolishes parasympathetic nerve transmission. What effects would you expect on the heart and why?

Norepinephrine from sympathetic neurons and epinephrine from the adrenal medulla can increase heart rate and stroke volume. Heart rate increases as a result of the opening of Na+ (funny) and Ca2+ channels that increase the rate of depolarization of pacemaker cells, and in an increase in the speed of depolarization along the conduction pathway. The rate and the force of contraction increase as a result of four mechanisms triggered by these chemical signals: (1) increased membrane permeability to Ca2+ during depolarization; (2) increase in the release of Ca2+ by the sarcoplasmic reticulum in response to action potentials; (3) increase in ATPase activity of myosin heads; and (4) increase in the Ca2+ ATPase activity, which increases the rate of relaxation between contractions. Acetylcholine from parasympathetic neurons can decrease heart rate and stroke volume. Heart rate decreases as a result of the opening of K+ channels that lead to a hyperpolarization of pacemaker cells. The stroke volume decreases as a result of a reduction in the intracellular Ca2+ signal.

Explain how changes in heart rate or stroke volume affect cardiac output.

The elastic expansion of the aorta and large arteries during systole and the subsequent contraction during diastole reduces the rise in pressure during the former and the fall during the latter, resulting in a more even blood flow across the cardiac cycle.

Explain how the large arteries (such as the aorta) dampen pressure fluctuations and even out blood flow across the cardiac cycle.

Blood, like any fluid, moves from areas of high pressure to low pressure. Accordingly, the highest pressure must be at the start of the circuits (in the ventricles of the heart) while the lowest must be at the end of the circuits (in atria of the heart). For the systemic circuit, the highest pressure occurs in the left ventricle, where pressure is generated through ventricular contractions. The pressure in the left ventricle is also highly variable, from near zero during diastole to high higer levels (e.g. 100-120 mm Hg in humans) during systole. The aorta and subsequent large systemic arteries that receive blood from the left ventricle dampen these variations. This dampening of pressure changes occurs as a result of the storage of mechanical energy in the elastic tissues of the arteries. As blood reaches arterioles, there is a large drop in pressure resulting from the high resistance of these vessels (because of their relative small sizes and numbers). Blood continues to travel through capillaries, venules, and veins, and pressure gradually drops until it reaches its lowest level, at the end of the circuit (the right atrium).

Explain the changes in blood pressure as blood flows through the mammalian circulatory system.

Move paracellulary or transcellularly. Lipid-soluble substances move through membrane by diffusion. Vessicles transport water-soluble proteins by transcytosis

How can substances move across capillaries?

Norepinephrine and epinephrine released as a result of sympathetic stimuli can bind with β adrenergic receptors on the pacemaker cells, stimulating a pathway that opens funny and Ca2+ channels and increases the rate of depolarization and thus increases heart rate. Acetylcholine released as a result of parasympathetic stimuli can bind with muscarinic cholinergic receptors, stimulating a pathway that increases K+ permeability and reduces Ca2+ permeability. This lowers the starting potential of the pacemaker cells and slows the rate of depolarization and thus slows heart rate.

How does the nervous system modulate heart rate?

Resistance is greater when vessels are in series because vessels in series is equal to sum of their resistances, while in parallel is 1 over the sum of the inverse of resistance.

Imagine 3 identical blood vessels arranged in series and in parallel. In which will resistance be greater and why?

The diastolic filling time is directly dependent on heart rate, as a faster heart rate reduces the time between contractions. The systolic ejection time depends mostly on how long it takes for ventricles to build up enough pressure to open the valves and eject blood in arteries, which depends on arterial pressure and cardiac contractility, but not on heart rate.

Increased heart rate can greatly reduce diastolic filling time, but has less impact on systolic ejection time. Why?

Arteries: large diameter, thick walls Capillaries: small diameter, thin walls Venules: diameter similar to arterioles, thin walls, lack smooth muscle Veins: Large diameter and thick walls, but not as thick as arteries

List major types of blood vessels and compare diameter and thickness.

Closed circulatory systems evolved w/ increased metabolic rates and high O2 demand. Insects have high metabolic rates but simple open system because their circulatory system does not play major role in O2 transport

Major factor in evolution of closed systems? Do all animals fit this rule?

The depolarization in the sinoatrial node spreads through the atrium via gap junctions, triggering contraction of the atrial cells. It also spreads through the atrium via conducting pathways to the atrioventricular (AV) node that delays the conduction of the signal slightly. The only way for the depolarization to pass to the ventricle is via the AV node. The depolarization then spreads via the bundles of His to the apex of the heart and then upward via Purkinje fibers and gap junctions, causing the ventricle to contract from the apex toward the atria.

Outline the steps of electrical conduction through the mammalian heart.

Drug blocking β receptors (also known as a beta-blocker) would prevent the increase in the force and rate of contraction caused by epinephrine and norepinephrine. Reducing blood pressure by decreasing the force or the rate of cardiac contractions.

Tom suffers from high blood pressure. What may help him deal with this problem and why?

A drop of deoxygenated blood from the systemic circulation comes from the inferior or posterior vena cava, depending on whether it came from the lower or upper body. In enters the right atrium, crosses the right atrioventricular (or tricuspid) valve, and enters the right ventricle. From the right ventricle, it will be pumped through the pulmonary semilunar valve and enter the pulmonary artery, which will bring the drop of blood to the lungs. The drop of blood will travel through pulmonary capillaries, where gas exchange will occur, and come back toward the heart in one of the pulmonary veins. This oxygenated blood will enter the left atrium, cross the left atrioventricular (or bicuspid) valve, and enter the left ventricle. From there, it will be pumped through the aortic semilunar valve into the aorta, and will be delivered to capillaries in various parts of the body. Deoxygenated blood from the systemic circulation will enter veins that connect to the inferior or posterior vena cava, and the cycle starts again.

Trace the movement of a drop of blood through the human circulatory system, listing all of the structures it passes (including all of the parts of the heart).

Vasoconstriction and vasodilation are increases and decreases (respectively) in the radius of the arterioles. Poiseuille's equation Q = ΔPr4/8Lη indicates that changes in radius have a very large effect on flow because flow is proportional to radius to the fourth power and thus small changes in radius can greatly change flow. Thus it is very efficient for the circulatory system to regulate flow to tissues by changes in radius (i.e., by vasoconstriction and vasodilation).

Use Poiseuille's equation to explain why the circulatory system regulates the distribution of blood to tissues by vasoconstricting or vasodilating arterioles.

Advantage is that the can develop dif. pressures. Allows blood to be driven long distances, and protects capillaries of lungs. Disadvantage is that it is an inflexible system and lungs must be perfused even if animal isnt breathing

What are some possible advantages and disadvantages of having completely separated pulmonary and systemic circuits.

Double circulation systems allow having very different fluid pressures in each circuit. This is important for air-breathing vertebrates, because a large pressure in the systemic circuit is essential to insure proper blood flow to all tissues. A low pressure in the pulmonary circuit prevents fluid leakage through the thin pulmonary capillaries, which would lead to reduced gas exchange efficiency.

What are some possible advantages of a double circulation over a single-circuit circulation?

Length Radius Viscosity of fluid Radius is most important

What are the major factors that determine the resistance of a tube such as a blood vessel?

1. Hearts 2. Contractile blood vessels 3. External muscular structures eg. skeletal muscles

What are the three main types of pumping structures in animal circulatory systems?

A heart or other pumping structure (such as contractile blood vessels or external structures such as skeletal muscles), a fluid that can be pumped (such as blood or hemolymph) and a series of tubes or channels through which the fluid can flow.

What are the three primary components that are found in all animal circulatory systems?

They close passively as pressure in the ventricles drops below pressure in the arteries.

What causes semilunar valves to close?

At the onset of exercise, heart rate and stroke volume increase and thus cardiac output increases (because CO = HR × SV). However, mean arterial pressure remains fairly constant (it may increase a bit, but not nearly as much as might be expected based on the increase in cardiac output). Thus, total peripheral resistance must fall, because CO = MAP/TPR. The patterns of blood flow also change greatly, with much more blood flow going to the skeletal muscles and much less blood flow going to the visceral organs.

What happens to heart rate, stroke volume, cardiac output, mean arterial pressure, and patterns of blood flow at the onset of exercise in humans?

Pressure in the left ventricle increases slightly during atrial systole as additional blood is forced into the ventricle as a result of atrial contraction.

What happens to pressure in the left ventricle during left atrial systole?

A pathways that allows blood to move between the systemic and pulmonary circuits. Allows animals to bypass the pulmonary circuit when the animal is not breathing.

What is a cardiac shunt, and what are some possible benefits of this process in reptiles?

The initial phase of ventricular contraction. AV and semilunar valves are closed while ventricle is contracting. Volume of ventricular blood does not change, but pressure increases.

What is isovolumetric (isovolumic) contraction?

In a closed system, the fluid remains within the blood vessels at all points. In an open circulatory system, the fluid enters a sinus and comes in direct contact with the tissues.

What is the difference between an open circulatory system and a closed circulatory system?

Velocity is the speed at which a drop of blood travels, in units of distance per time, while the rate of blood flow is the volume of blood passing through a region per unit of time. The relationship between the two is that velocity is equal to flow rate divided by the surface area of the vessel. As a consequence, everything else being equal, increasing velocity in a vessel will increase its flow rate. However, a vessel with a high blood velocity can have a lower flow rate than a vessel with a lower velocity if its surface is smaller. Similarly, everything else being equal, increasing the flow rate in a given vessel will increase its blood velocity, but a vessel with a high flow rate can have a lower velocity than a vessel with lower flow rate if its surface area is larger.

What is the difference between the velocity of the blood and the rate of blood flow? How are these two concepts related?

The skeletal muscle pump increases venous return to the heart. The contraction of skeletal muscle squeezes the veins, and increases the pressure in the local area, which acts as a driving force for the movement of blood. Because the veins contain one-way valves that open in the direction towards the heart, the the valves that are farthest from the heart will close and those that are closer to the heart will open , which causes blood to flow towards the heart.

What is the influence of the skeletal muscle pump on venous return to the heart?

The right ventricle generates much lower pressure than the left ventricle. The right ventricle pumps blood to the lungs, and the low pressure helps to protect the delicate lung capillaries. The left ventricle pumps blood to the body, and the high pressure helps to drive blood through the high resistance associated with this circuit.

What is the physiological importance of the difference in pressure generated by the right and left ventricles of the mammalian heart?

Many cardiomyocytes have pacemaker capacity—the ability to undergo spontaneous and rhythmic depolarizations. If the normal conducting pathway was blocked, the result could be cardiac arrhythmia: The atria may depolarize "normally" as a result of action potentials initiated by the sinoatrial node, while the ventricles may depolarize independently, at their own rhythm, because of action potentials initiated by other cells. The result would be highly inefficient cardiac contractions, because the capacity of the heart to pump blood depends on the coordinated depolarization/contraction events.

What would happen if the connection between the AV node and the bundle of His were blocked (in a way that didn't directly affect any other parts of the heart)?

The specialized conducting pathways allow the ventricle to contract from the apex (the bottom) of the heart toward the top, squeezing blood out into the arteries. In addition, the delay in conduction at the AV node allows atrial contraction to be completed before ventricular contraction begins, maximizing the entry of blood into the ventricle.

Why do mammalian hearts have specialized conducting pathways?

Mean arterial pressure is homeostatically regulated via the baroreceptor reflex, so although cardiac output increases, adjustments in total peripheral resistance allow flow to be maintained with only slight changes in blood pressure. This can easily be seen in the law of bulk flow, which can be written as CO = MAP/TPR. If CO increases and MAP stays the same, then TPR must drop.

Why does blood pressure only change slightly during exercise despite the large increase in cardiac output?

The unstable resting membrane potential of pacemaker cells confers the myogenic rhythm of the heart. This unstable resting membrane potential slowly brings the cell toward the threshold potential for the generation of an action potential, without the need for input from the nervous system.

Why is the unstable resting membrane potential of pacemaker cells critical to their function?