CVS Hemodynamics

Pulmonary Circuit

Properties of Pulmonary Circuit

The heart pumps a volume of blood into the lungs which after getting oxygenated is transported back to heart. This oxygenated blood is then pumped out of the heart and into the aorta and subsequently into the systemic circulation. As a consequence, at any given time the lungs (pulmonary vasculature) contain around 500ml of blood, thereby allowing them to function as a reservoir of the blood. This reservoir volume is increased by another 500 ml when the person is in the supine or lying down position. This is because in supine position there’s an increased venous return (due to the effect of gravity) to the right heart from the peripheries and therefore more blood accumulates in the central parts of body. Upon standing up, this extra 500ml of blood gets redistributed under the effect of gravity, to the now more dependent parts of the body which includes the peripheral tissues and the lower extremities.

The division of vasculature in the pulmonary circuit is somewhat different from that in the systemic circulation. The arteries in the pulmonary circuit divide in a binary fashion, thereby following the pattern of division of the airways. The veins too exhibit a pattern similar to the arterioles and the bronchioles, thereby finally converging into forming one large pulmonary vein. These pulmonary veins transport the oxygenated blood back to the heart.

The primary function of lungs to deliver oxygenated blood to the heart.

Apart from the primary function of lungs to deliver oxygenated blood to the heart, it has following secondary functions.


Pulmonary vascular bed acts as a filter for the blood received by the right side of the heart and before this blood leaves the left side of the heart to enter the systemic circulation. Blood arriving from the right side of the heart to the lungs may contain an embolus or a thrombus or any foreign particle that may result in obstruction of the vessels. The pulmonary artery divides into numerous small capillaries with diameters too small to allow passage of any dislodged particles into the pulmonary artery and subsequently in to the left side of the heart. Therefore, the pulmonary vascular bed acts as a filtration barrier that prevents introduction of any particles into the systemic circulation.

Metabolic Functions

The pulmonary vasculature also has some metabolic functions:

  1. The endothelium of the pulmonary vessels is lined by fibrinolytic enzymes that cause lysis of the fibrin clots which get stuck in the pulmonary vasculature.
  2. The pulmonary capillary endothelium is lined by another enzyme as well. This enzyme is called the Angiotensin Converting Enzyme (ACE). The function of ACE is to covert Angiotensin I to Angiotensin II. ACE is also present in the endothelium of other vascular beds and the plasma, however, the amount present in the pulmonary vascular bed is the highest. ACE within the pulmonary vascular bed is responsible for more than 70% of the total conversion of Angiotensin I to Angiotensin II in the body.
CVS Hemodynamics

Control of the Coronary Blood Flow

Coronary Circulation

The coronary vasculature supplies blood to the muscles of the heart (myocardium). These coronary arteries are the first branches (right & left coronary arteries) that arise from the aorta, and they run on the surface of the heart’s muscle. Branches from the arteries penetrate the muscle in order to supply blood to deeper myocardial tissues. When the heart contracts during systole, the small arteries that are embedded deeply within the heart musculature are also squeezed along with the heart. This mechanical compression of coronary arteries causes a brief period of occlusion of coronary blood flow during systole. The whole body receives oxygenated blood during systole except for the heart itself. The heart will be supplied with the oxygenated blood when the atria and ventricles are in a relaxed state which happens during diastole. At that time, the coronary vasculature is not compressed and the perfusion is at a maximum. The right heart muscles contract with lesser force compared to the left heart. Hence, a relatively small amount of perfusion is maintained in the right heart even during systole.

Even under normal resting conditions, the heart tissue extracts 80% of the total oxygen content from the blood that is supplied to it.

This high extraction of oxygen is necessary to meet the demands of highly active muscles of the heart. The demand for oxygen is increased substantially when there is an increase in the heart rate, as happens during exercise.

Compared to the cardiac muscle, the skeletal muscle extracts only 20% of oxygen from the blood at normal resting conditions.

In other tissues, when oxygen demand increases, vasodilatory metabolites such as H+ ions and carbon dioxide cause a rightward shift of the oxygen dissociation curve. This allows greater dissociation of oxygen from the hemoglobin. However, this rightward shift of the hemoglobin oxygen dissociation curve is not valid for the blood in the coronary circulation. This is because the extraction of oxygen by heart tissue is already at maximum level even under normal resting conditions. Therefore, the increased demand for oxygen during exercise can only be compensated by increasing the blood flow rate of the coronary circulation.


Auto-regulation of Coronary circulation

The flow of coronary vasculature is under the influence of both intrinsic and extrinsic mechanisms. The extrinsic mechanisms include the sympathetic and parasympathetic nervous systems.


Parasympathetic nervous system

The parasympathetic nervous system input is relayed by the right and left vagus nerves. Right vagus nerve innervates the Sinoatrial node, while the left vagus nerve innervates the AV node. The parasympathetic nervous system regulates the blood flow indirectly by controlling the heart rate (chronotropy) and the force of contraction (inotropy).


 Sympathetic nervous system

The sympathetic nervous system innervates the SA node & AV node, the myocardium and the vascular smooth muscle of coronary vessels. Hence, sympathetic nervous system regulates the heart rate, force of contraction of ventricular myocardium and the flow into the coronary vasculature. However, the extrinsic mechanisms involved in vascular diameter changes are ineffective during physiological conditions. Although there is sympathetic stimulation on the vascular smooth muscles, the effects are negated by the constant release of nitric oxide from the endothelium. If this nitric oxide release is blocked by an atherosclerotic plaque, the unchecked sympathetic stimulation causes vasoconstriction leading to the pain of angina. Atherosclerosis decreases coronary perfusion by reducing the coronary artery lumen and also by blocking the release of NO from the coronary artery endothelium.


Under physiological conditions, the metabolism induced changes in vascular caliber are more prominent. The metabolites produced by the heart tissue responsible for vasodilation are:

  • H+ ions
  • K+ ions
  • Adenosine
  • Bradykinin and Prostaglandins

Volume – Work Relationship

There is a direct relationship between the volume of blood entering the heart (preload) and the work done by the heart to pump it out. The pressure remains the same as there is no change in myocardial contractility or afterload.

This can be observed when a person is exercising during which the volume of blood returning to the heart increases.

Pressure – Work Relationship

There is a direct relationship between the pressure developed in the peripheral vasculature and the work done by the heart against it. The demand for oxygen in pressure overload situations is higher than in volume overload.

Patients with atherosclerosis show this kind of pressure – work relationship in their cardiovascular system.

CVS Hemodynamics

Blood Pressure Control By Baroreceptors

Mean Arterial Pressure

The mean arterial pressure (MAP), also considered as the perfusion pressure, is taken as the pressure difference between the arteries and the veins. The regulation of blood pressure is done in order to maintain the MAP.

The MAP dictates the amount of oxygen and nutrients that is supplied by the blood vessels and the waste that is carried away from the tissues.

Regulation Of Blood Pressure

The body has the ability to counteract long term as well as short term changes in blood pressure. The long term pressure changes cause the body to respond through the activation of renin-angiotensin system.

Rapid/short term changes in blood pressure compel the body to activate the following receptors:

  • Baroreceptors are present on the arch of aorta and carotid sinus
  • Chemoreceptors are present in the carotid sinuses, arch of aorta and medulla oblongata
  • Atrial receptors are present on the wall of right atrium


The baroreceptors are the pressure sensing bodies. They are also called stretch receptors.They are modified nerve endings attached to the cytoskeleton present within the nerve endings. The receptors  are sensitive to rapid offsets in blood pressure. The baroreceptors are densely situated on the walls of the arch of aorta and the carotid sinus. The carotid sinus is present on the base of internal carotid artery at the level of bifurcation of the common carotid artery. The sinus area is slightly dilated as the tunica media which is normally comprised of muscles, is relatively thin. The tunica adventitia, on the other hand, is thicker than usual. This is the layer of the blood vessels where the nerve receptors are situated. Same is true for the location of baroreceptors on the arch of aorta.

Rapid offsets in pressure can occur, for example, in a previously standing person who suddenly sits down. During the process, a large volume of blood is shifted from the peripheral to the central regions of the body. Consequently, a large volume of blood enters the heart and this volume overload or increased preload causes the heart to increase its cardiac output. A simultaneous increase in blood pressure will also be observed with increase in cardiac output. The increase in blood pressure is registered by the baroreceptors.

Similarly, a drop in blood pressure is registered by the baroreceptors when the person stands up suddenly from a sitting position. High blood pressure in the blood vessels causes stretch of these receptors which results in movement of sodium ions into the nerve endings, thereby, initiating an action potential.

These baroreceptors have a baseline firing pattern. That means they have an intrinsic potential to generate action potentials at a particular frequency at all times. This frequency is increased when the baroreceptors receive a stretch stimulus secondary to increase in blood pressure. The carotid sinuses increase their rate of impulse generation when the pressure in them builds up to values greater than 50 mm Hg. Below this threshold pressure, the carotid baroreceptors fail to initiate an action potential. On the other hand, the arch of aorta can record drops in blood pressure up to 30 mm Hg. The upper limit for blood pressure, after which the frequency of action potential stops increasing, is 175 mm Hg. The normal MAP is calculated to be 93 mm Hg. At this pressure, the baroreceptors are believed to be the most sensitive and even slight changes in pressure will result in rapid firing of action potentials.

At blood pressures lower than 30 mm Hg, the chemoreceptors come into play. The chemoreceptors function by sensing the arterial concentration of carbon dioxide, oxygen, Ph and other metabolites . They do not detect changes in blood pressure.

Baroreceptor Reflex

The baroreceptor reflex like other reflex arcs is comprised of three units:

  • Afferent nerve carrying impulses from the receptors
  • Central processing unit
  • An efferent nerve that innervates the effector


Afferent impulses from the carotid sinus are carried by the Herring nerve, a branch of Glossopharyngeal nerve (CN-9). In the case of baroreceptors present on the arch of aorta, the Vagus nerve (CN-10) is the afferent nerve that carries impulses to the spinal cord. Both, the Vagus nerve and the Glossopharyngeal nerve, feed impulses from the baroreceptors into the nucleus of tractus solitarius. These nuclei are situated in the medulla of the spinal cord and their job is to process the incoming afferent impulses. Also within the Medulla and lower 1/3rd of the Pons, there are vasoconstricting center, the vasodilatory center and the cardio-inhibitory center. These centers receive processed impulses from the nucleus of tractus solitarius and from here efferent impulses in the form of sympathetic and parasympathetic nerves arise. Impulses are carried to the heart via the parasympathetic Vagus nerve. Sympathetic impulses travel down the intermedio-lateral segment of the spinal cord and give rise to efferent motor spinal nerves which enter the sympathetic ganglion running parallel to the spinal cord. Postganglionic sympathetic nerves ultimately supply the heart and the peripheral vasculature. Another preganglionic sympathetic nerve also supplies the adrenal medulla which results in the release of epinephrine and norepinephrine, which further contribute in enhancing the sympathetic activity. The end result is either an increase or decrease in the blood pressure, thereby correcting the disturbance in hemodynamics of the body. This phenomenon is also referred to as the buffering effect, since the change in pressure is buffered back to normal. The Vagus and Glossopharyngeal nerves, because of the same reason, are therefore known as the buffering nerves.

Factors Responsible For Change in Mean Arterial Pressure

MAP = Heart Rate x Cardiac Output

Whereas, CO = SV (stroke volume) x TPR (total peripheral resistance)

Therefore, MAP = HR x SV x TPR

The stroke volume is altered by altering the force of contractility of the heart muscles. The sympathetic nerves supplying the heart muscles affect the stroke volume. The parasympathetic nerves supplying the SA and AV node are responsible for producing changes in heart rate. The TPR can be increased or decreased by changing the diameter of peripheral vasculature which is under the control of the sympathetic nervous system.

Effects of Baroreceptors in Various Conditions

Due To Changes In Blood Pressure

  • Reduced Blood Pressure: Reduction in blood pressure will result in a decrease in the number of afferent impulses from the baroreceptors. The sympathetic activity will increase and as a result, the TPR, HR and the stroke volume will all increase. At the same time, the parasympathetic input will taper down. All these changes will result in increasing the blood pressure back to normal.
  • Increased Blood Pressure: This happens in situations like exercise or stress. Increased blood pressure will result in stretching of the stretch receptors. This increases the frequency of afferent impulses. Sympathetic supply will decrease and the parasympathetic system will take over. Finally, the blood pressure is decreased back to normal.

 Due To Changes In Cardiac Output

  •  Decreased Cardiac Output: Occurs in situations of vomiting, diarrhea, hemorrhage etc. As a result of these, both the volume, and therefore pressure of the blood decreases. Afferent impulse firing of the baroreceptors decreases. As a consequence, there’s a sympathetic overflow which causes an increase in HR, TPR and SV. Due to an increase in these parameters, the blood pressure is raised back to normal.
  • Increased Cardiac Output: There’s an increased impulse generation from the baroreceptors due stretch caused by increased volume of blood. This increased afferent input from the baroreceptors results in activation of the PANS. Once activated, the parasympathetic nervous system decreases the blood pressure back to normal.

Massaging The Carotid Sinus

Massaging the carotid sinuses physically increases the pressure on the baroreceptors present there. The carotid baroreceptors respond by increasing the rate of afferent impulse firing. The sympathetic system will be shut down and the parasympathetic system is activated. This results in decrease in blood pressure of the body.

Carotid massage by activating parasympathetic nervous system increases AV nodal refractory period, thereby decreasing AV node conduction and finally decreasing Heart Rate. This is the reason Carotid sinus massage is the initial menuever used in the treatment of paroxysmal supra-ventricular tachycardia.

Stenosis Of The Carotids

Stenosis of carotids proximal to the sinus or obstruction of the carotids due to atherosclerosis will cause the baroreceptors to register a decrease in pressure. Therefore, sympathetic system activation follows. Increased sympathetic activity causes a resultant increase in blood pressure. This increase in blood pressure may cause hypertension in an otherwise normal person.

Baroreceptor responses are summarized in the table below


It’s important to understand that control of BP by baroreceptor is a short term regulation of blood pressure. Any short term derangements are dealt via the baroreceptor response, whereas long term control of the BP is controlled via the RAAS (Renin Angiotensin Aldosterone System).

The baroreceptors also have the ability to adapt to chronic changes in blood pressure. If the mean pressure is changed over time to a new value, the baroreceptors will start using that MAP as the baseline. Any subsequent blood pressure changes will then be rectified keeping in view the new baseline value of MAP.

CVS Hemodynamics

General Principles of Hemodynamics

Organization of Cardiovascular System

The human heart has 4 chambers which are the two atria and the two ventricles. These 4 chambers are divided into 2 functional units referred to as the left heart and the right heart. These atria and ventricle are single functional unit, separated by the atrioventricular valves. These AV valves are one way valves and allow blood flow in the forward direction only.

Right heart is formed by the right atrium and the right ventricle, and it forms one functional unit. The right atrium receives the venous deoxygenated blood from three following sources

  1. Superior vena cava: brings deoxygenated blood from the head, neck and upper limb region.
  2.  Inferior vena cava: brings deoxygenated blood from the lower extremities, the abdominal region and the rest of the body except the heart itself.
  3.  Coronary sinus: brings deoxygenated blood from the veins of the heart itself.

During diastole when the atria contract, this deoxygenated blood is pumped into the right ventricle. During systole, the right ventricle pumps this deoxygenated blood out of the heart and into the pulmonary circuit via the pulmonary artery.

The right heart plus the pulmonary arteries, capillaries and veins together form the pulmonary circulation.

The right side of the heart deals with deoxygenated blood only and it functions to send this deoxygenated blood to the pulmonary circulation to get oxygenated.

Left heart forms another functional unit and consists of the left atrium and left ventricle. The left atrium receives oxygenated blood from the pulmonary circuit via the  four pulmonary veins, two from each lung. When the atria contract, the left atrium pumps this oxygenated blood into the left ventricle. During systole, this oxygenated blood is pumped out of the heart via aorta, when the left ventricle contracts. The aorta then carries this oxygenated blood into the systemic circulation.

The left heart plus the systemic arteries (starting at the aorta), capillaries and veins together form the systemic circulation.

The left heart deals with the oxygenated blood and sends it to the systemic circulation via the aorta.


Arteries conduct blood away from the heart towards the tissues. Arteries normally carry oxygenated blood away from the heart, but an exception to this rule are the pulmonary arteries and the umbilical arteries(during fetal life only) which carry deoxygenated blood away from the heart and toward the lungs & placenta respectively. Veins normally carry deoxygenated blood, except the pulmonary veins in adults and the umbilical vein (during fetal life only) which bring back oxygenated blood to the heart from the lungs and the placenta respectively.


Systemic Tissue

As part of the systemic perfusion, the oxygenated blood in the aorta is eventually transported to the following six major systemic tissues. These systemic tissues receive blood via a parallel system of arteries which originate at various levels from the aorta itself.

  1. Cerebral: The CNS plus the head & neck region. 15% of the cardiac output enters the cerebral arteries.
  2. Coronary: The myocardium itself which receives oxygenated blood during diastole in contrast to the rest of the body which receives oxygenated blood as part of systole. 5% of the total cardiac output is designated for the myocardial perfusion via the kidneys.
  3.  Splanchnic: The gastrointestinal system and its accessory organs such as the liver, spleen, pancreas and the biliary system. 25% of the total cardiac output reaches the GIT system via the splanchnic arteries.
  4. Renal: The kidneys and the genitourinary system. Kidneys, as part of the renal system, receive 25% of the total cardiac output.
  5. Skeletal: Roughly 25% of the total cardiac output is reaches the skeletal system. Exercise increases the percentage of cardiac output which is made available for the skeletal system. Bones and the musculature of the body form part of this system.
  6.  Cutaneous: The skin and its associated structures (sebaceous glands, hair follicles). Around 5% of the total cardiac output reaches the cutaneous circulation.


Direction of Blood Flow and chemical composition of Blood

There are four pulmonary veins which bring back oxygenated blood from the lungs to left atrium. This blood is rich in oxygen (PaO2=100 mm Hg) and low in carbon dioxide (PaCO2=40 mm Hg). The mitral valve which forms the left atrioventricular valve, allows passage of blood from the left atrium into the left ventricle during the diastole phase. When the left ventricle begins to contract and its pressure rises more than the left atrial pressure, the mitral valve closes to prevent backflow of the blood. This ensures anterograde flow of blood in to the aorta i.e., the forward direction of blood flow. Backflow from the aorta back into the left atrium is prevented by the semilunar aortic valve. It’s important to remember that all the valves of the heart are tricuspid i.e., having three cusps, except the mitral valve which is bicuspid i.e., having two cusps. However, only the right atrioventricular valve is referred to as the tricuspid valve.

From the aorta, the blood is transported to the systemic tissues which are mentioned above. The aorta divides into large and medium sized arteries, which eventually give arise the arterioles. The arterioles continue to form capillaries, and these capillaries merge together to form venules at their venous ends. The venules eventually end up forming the veins. The veins are low pressure vessels which return the deoxygenated blood back to the right heart via the three above mentioned sources of venous return to the heart. This deoxygenated blood is low in oxygen (40 mm Hg) and rich in carbon dioxide (47 mm Hg). The right atrioventricular valve, which is also referred to as the tricuspid valve, allows this deoxygenated blood to flow from the right atrium into the right ventricle. During ventricular systole when the left ventricle contracts, this deoxygenated blood is pumped out of the right side of the heart via the pulmonary artery. The backflow of this deoxygenated blood into the right side of the heart during ventricular diastole is prevented by the semilunar pulmonary valve. This deoxygenated blood reaches the lungs and enters the pulmonary circuit to get deoxygenated. At this point the blood completes its route both around the pulmonary and systemic circuits.

Series and parallel Circuits

The right and left sides of the heart are connected in a series circuit to both the pulmonary and systemic tissues respectively. By series circuit, what’s meant here is that quantitatively, the blood flow through the lungs is equal to the blood flow through the rest of the body. For simplicity in understanding, it should be considered that the lungs are connected to the rest of the body in a series circuit. During a single cardiac cycle, the right and left ventricular outputs are the same.

However, the blood supply of the systemic tissues is connected in a parallel circuit. This means that each organ system is supplied by an artery which originates as a branch of the aorta. This ensures that the blood which reaches a particular organ system is perfused at the same partial pressure of oxygen, as that of the site at which the branch originated from the aorta. If the organ systems were connected and perfused via a series circuit, by the time the blood reached the last organ system, it would have been completely depleted of oxygen and nutrients. The sum of blood flow to these individual systems adds up to the total left ventricular output (the cardiac output).

Pressures in cardiovascular system


Systole is time interval when the ventricles are contracting. Systolic BP therefore is the pressure in the systemic arteries when the left ventricle is contracting. Therefore, SBP is the highest blood pressure in the systemic arteries during a cardiac cycle.

Average SBP in healthy adults is 120 mm of Hg.


Diastole is the time interval when the ventricles are relaxing and therefore receiving blood from the atria. Diastolic BP is the pressure in the systemic arteries when the left ventricle is relaxing. DBP therefore is the lowest pressure in systemic arteries during a cardiac cycle. Average DBP in healthy adults is 80 mm of Hg.


This is the difference between the systolic blood pressure and the diastolic blood pressure in the systemic arteries at any given time. Pulse pressure can therefore be calculated as following:

Pulse pressure (PP) = Difference between the systolic & diastolic blood pressures.


PP = 120-80= 40 mm of Hg

Variations in pulse pressure can be in anemia, fever, blood loss etc. Narrow pulse pressure can be seen when there is blood loss, aortic regurgitation. Pulse pressure widens during exercise.


MAP is the average arterial pressure of the systemic arteries. However, quantitatively it’s not an arithmetic mean of the SBP & DBP. Since the ventricular muscle spends 2/3 of the time of a cardiac cycle in diastole, the MAP is closer to the DBP, than it’s to the SBP.

MAP signifies the perfusion pressure of the tissues. If the MAP of the patient decreases below 60 mm of Hg, then it should be a cause of concern for the doctor. What this signifies is that a perfusion pressure below 60 mm of Hg would not be able to meet the nutritional needs of the systemic tissues. So, the MAP, which is easier to calculate quantitatively, can be used in lieu of systemic perfusion pressure.

MAP = (CO x SVR) + CVP {CVP is negligible, so it can be ignored}

MAP = (CO x SVR)

CVP= central venous pressure

SVR (systemic vascular resistance) is the sum of resistance of all the vessels in the systemic circuit. However, the major component of systemic vascular resistance is the arteriolar resistance.

Also, MAP can be calculated by using the following formula if the SBP and DBP are known:

MAP = DBP + 1/3 (SBP – DBP)

For a normal healthy adult, SBP = 120 & DBP = 80

So, MAP = 80 + 1/3 (120 – 80)

MAP = 80 + 1/3 (40)

MAP = 80 + 13.33

MAP = 93 mm of Hg (After rounding off)

Alternatively MAP can also be calculated by the following formula:

MAP= 2/3 DBP + 1/3 SBP

MAP= (2/3 x 80) + (1/3 x 120)

MAP= 53.33 + 40 = 93 mm of Hg

The Mean Arterial Pressure is necessary pressure to bring blood supply to the systemic tissues to ensure their nutritional needs.



CVS Hemodynamics

Overview of Blood Flow

This overview begins with major steps found in blood circulation throughout the cardiovascular system. Remember this process is continuous allowing the:

1) maintenance of cell-level metabolism by the transportation of nutrients, hormones, metabolic wastes, O2 and CO2 throughout the body; 2) regulation of pH, osmotic pressure and temperature of the whole body; 3) protection from microbial and mechanical harms

Tracing Blood Flow 

There are two main functional units:

A. Right heart consisting of both the Right atrium and Right ventricle

B. Left heart consisting of both the Left atrium and Left ventricle.

Pulmonary_CircuitFrom each lung, you can trace blood being transported to the Left atrium via four pulmonary veins, which contain high O2 and low CO2 concentration. Blood flows from the Left atrium through the Left atrioventricular valve (also called Mitral valve or Bicuspid valve) reaching the Left ventricle. (Because the Left ventricle is the chamber that pumps blood from the heart, it is the most muscular and powerful of the four chambers). From the Left ventricle, blood is then pumped through the aorta to supply all systemic tissues:

1.Cerebral   2.Coronary heart  3.Splanchnic  4.Renal system 5.Musculoskeletal 6.Skin



Arteries split into arterioles, capillaries, draining into venules and then veins drains into either the Superior vena cava, Inferior vena cava, or Coronary sinus,  opening into the right atrium.

Blood from the head and neck region is drained by the Superior vena cava. Blood from the peripheral system is drained by the inferior vena cava. Blood from the heart is drained by the coronary sinus. Subsequently, all drain into the right atrium.

From the Right atrium, the deoxygenated blood (with decrease in O2 concentration, and increase in CO2 concentration) is pumped into the Right ventricle through the Right Atrium ventricular valve, also called Tricuspid valve. Finally, from the Right ventricle, blood is pumped through pulmonary arteries back to the lungs.