Cardiovascular System Anatomy and Physiology: Study Guide for Nurses

Every shift runs on this system. When you read a blood pressure, palpate a pulse, push a fluid bolus, or watch a rhythm strip, you are watching the heart and vessels do their job. Know the pump, the valves, and the plumbing cold, and the bedside picture makes sense.

Functions of the Heart

The heart does four jobs:

1. Manages blood supply. Rate and force of contraction adjust to match blood flow to tissue demand at rest, during exercise, and with changes in body position.
2. Produces blood pressure. Contractions generate the pressure that drives flow through the vessels.
3. Secures one-way flow. The valves keep blood moving in a single direction.
4. Transmits blood. The heart separates pulmonary and systemic circulation, sending oxygenated blood to the tissues.

Anatomy of the Heart

Think of the cardiovascular system as a muscular pump fitted with one-way valves and a network of large and small tubes the blood travels through.

Heart Structure

The heart is small for the work it does.

• Weight. About the size of a fist, the hollow, cone-shaped heart weighs less than a pound.
• Mediastinum. It sits in the inferior mediastinum, the medial cavity of the thorax, flanked by the lungs.
• Apex. The pointed apex points toward the left hip and rests on the diaphragm at about the level of the fifth intercostal space.
• Base. The broad posterosuperior base, where the great vessels emerge, points toward the right shoulder and lies beneath the second rib.
• Pericardium. A double-walled sac, the pericardium, encloses the heart.
• Fibrous pericardium. The loose superficial part protects the heart and anchors it to the diaphragm and sternum.
• Serous pericardium. Deep to the fibrous layer, the slippery two-layer serous pericardium lines the interior; its parietal layer covers the inside of the fibrous pericardium.

Layers of the Heart

The heart wall has three layers:

• Epicardium. The visceral, outermost layer, part of the heart wall itself.
• Myocardium. Thick bundles of cardiac muscle in ringlike arrangements. This is the layer that contracts.
• Endocardium. The innermost layer, a thin sheet of endothelium lining the chambers.

Chambers of the Heart

Four hollow chambers: two atria and two ventricles.

• Atria. The two superior receiving chambers; they play a lighter role in pumping.
• Ventricles. The two inferior, thick-walled discharging chambers, the actual pumps. When they contract, blood is propelled out of the heart into circulation.
• Septum. The wall dividing the heart longitudinally is the interventricular or interatrial septum, depending on which chambers it separates.

Associated Great Vessels

The great vessels carry the full cardiac circulation.

• Superior and inferior vena cava. Return oxygen-poor blood from the body to the heart, which pumps it out the pulmonary trunk.
• Pulmonary arteries. The pulmonary trunk splits into right and left pulmonary arteries, carrying blood to the lungs to pick up oxygen and unload carbon dioxide.
• Pulmonary veins. Oxygen-rich blood drains from the lungs back to the left side of the heart through the four pulmonary veins.
• Aorta. Blood from the left heart is pumped into the aorta, where the systemic arteries branch off to supply the body.

Heart Valves

Four valves keep blood flowing one direction through the chambers.

• Atrioventricular (AV) valves. Located between the atria and ventricles on each side; they prevent backflow into the atria when the ventricles contract.
• Bicuspid (mitral) valve. The left AV valve, two cusps of endocardium.
• Tricuspid valve. The right AV valve, three flaps.
• Semilunar valves. Guard the bases of the two large arteries leaving the ventricles: the pulmonary and aortic semilunar valves.

Cardiac Circulation Vessels

The chambers are bathed in blood constantly, but that blood does not nourish the myocardium.

• Coronary arteries. Branch from the base of the aorta and encircle the heart in the coronary sulcus (atrioventricular groove). They are compressed when the ventricles contract and fill when the heart relaxes.
• Cardiac veins. Drain the myocardium into the coronary sinus on the posterior heart.

Blood Vessels

Blood circulates inside the vessels, a closed transport system.

• Arteries. Carry blood away from the heart, starting with the large arteries.
• Arterioles. Successively smaller arteries feed into arterioles, which supply the capillary beds in the tissues.
• Veins. Venules drain the capillary beds into veins, which empty into the great veins entering the heart.

Tunics

Except for the microscopic capillaries, vessel walls have three coats (tunics).

• Tunica intima. A thin endothelial layer lining the lumen; reduces friction as blood flows.
• Tunica media. The bulky middle coat of smooth muscle and elastic fibers that constrict or dilate, raising or lowering blood pressure.
• Tunica externa. The outermost tunic of fibrous connective tissue; supports and protects the vessel.

Major Arteries of the Systemic Circulation

The major branches of the aorta, in sequence from the heart.

Ascending Aorta

The aorta rises from the left ventricle as the ascending aorta. Its only branches are the right and left coronary arteries, which serve the heart.

Aortic Arch

The aorta arches to the left.

• Brachiocephalic trunk. First branch off the arch; splits into the right common carotid artery and right subclavian artery.
• Left common carotid artery. Second branch; divides into the left internal carotid (serves the brain) and left external carotid (serves the skin and muscles of head and neck).
• Left subclavian artery. Third branch; gives off the vertebral artery, which serves part of the brain.
• Axillary artery. The subclavian becomes the axillary artery in the axilla.
• Brachial artery. Continues into the arm to supply it.
• Radial and ulnar arteries. At the elbow the brachial splits into the radial and ulnar arteries, serving the forearm.

Thoracic Aorta

The aorta runs down through the thorax along the spine. Ten pairs of intercostal arteries supply the thoracic wall muscles.

Abdominal Aorta

The aorta passes through the diaphragm into the abdominopelvic cavity as the abdominal aorta.

• Celiac trunk. First branch, with three branches: the left gastric artery (stomach), the splenic artery (spleen), and the common hepatic artery (liver).
• Superior mesenteric artery. Unpaired; supplies most of the small intestine and the first half of the colon.
• Renal arteries. Serve the kidneys.
• Gonadal arteries. Supply the gonads; ovarian arteries in females, testicular arteries in males.
• Lumbar arteries. Several pairs serving the muscles of the abdomen and trunk walls.
• Inferior mesenteric artery. Small, unpaired; supplies the second half of the large intestine.
• Common iliac arteries. The final branches of the abdominal aorta.

Major Veins of the Systemic Circulation

Major veins converge on the venae cavae, which enter the right atrium.

Draining into the Superior Vena Cava

Named distal to proximal, the direction blood flows into the superior vena cava.

• Radial and ulnar veins. Deep veins of the forearm; unite to form the deep brachial vein, which drains the arm and empties into the axillary vein.
• Cephalic vein. Superficial drainage of the lateral arm; empties into the axillary vein.
• Basilic vein. Superficial vein draining the medial arm; empties into the brachial vein proximally.
• Median cubital vein. Joins the basilic and cephalic veins at the anterior elbow. Common site for venous blood draws.
• Subclavian vein. Receives venous blood from the arm via the axillary vein and from the head via the external jugular vein.
• Vertebral vein. Drains the posterior head.
• Internal jugular vein. Drains the dural sinuses of the brain.
• Brachiocephalic veins. Right and left; receive drainage from the subclavian, vertebral, and internal jugular veins on each side.
• Azygos vein. Single vein draining the thorax; enters the superior vena cava just before it joins the heart.

Draining into the Inferior Vena Cava

The inferior vena cava, much longer than the superior, returns blood from all regions below the diaphragm.

• Tibial veins. The anterior and posterior tibial veins and the fibular vein drain the leg; the posterior tibial veins become the popliteal vein at the knee, then the femoral vein in the thigh, which becomes the external iliac vein at the pelvis.
• Great saphenous veins. The longest veins in the body; begin at the dorsal venous arch in the foot and run up the medial leg to empty into the femoral vein.
• Common iliac vein. Formed by the union of the external and internal iliac veins (the latter drains the pelvis).
• Gonadal vein. The right gonadal vein drains the right ovary (females) or right testicle (males); the left empties into the left renal vein.
• Renal veins. Drain the kidneys.
• Hepatic portal vein. Single vein draining the digestive tract; carries blood through the liver before it joins the systemic circulation.
• Hepatic veins. Drain the liver.

Physiology of the Heart

As the heart beats, blood makes continuous round trips: into and out of the heart, through the body, and back, only to be sent out again.

Intrinsic Conduction System

Cardiac muscle cells contract spontaneously in a regular, continuous rhythm.

• Cardiac muscle cells. Contract spontaneously and independently, even with all nervous connections severed.
• Rhythms. Muscle cells in different areas of the heart have different intrinsic rhythms.
• Intrinsic conduction (nodal) system. Built into the heart tissue; sets the basic rhythm.
• Composition. A special tissue found nowhere else in the body, part muscle and part nervous tissue.
• Function. Drives depolarization in one direction, atria to ventricles, and enforces a contraction rate of approximately 75 beats per minute so the heart beats as a coordinated unit.
• Sinoatrial (SA) node. Has the highest rate of depolarization in the system, so it starts the beat and sets the pace. This is the pacemaker.
• Atrial contraction. From the SA node the impulse spreads through the atria to the AV node, and the atria contract.
• Ventricular contraction. The impulse passes through the AV bundle, the bundle branches, and the Purkinje fibers, producing a wringing contraction that begins at the apex and moves toward the atria.
• Ejection. This contraction ejects blood superiorly into the large arteries leaving the heart.

Pathway of the Conduction System

The wave moves systematically: SA node initiates it, then the atrial myocardium, then the AV node (and the atria contract), then rapidly through the AV bundle, then the right and left bundle branches, and finally the Purkinje fibers in the ventricular walls, producing the contraction that ejects blood from the heart.

Cardiac Cycle and Heart Sounds

In a healthy heart, the atria contract together; as they relax, the ventricles begin to contract.

• Systole. Heart contraction.
• Diastole. Heart relaxation.
• Cardiac cycle. The events of one complete heartbeat, both atria and ventricles contracting and then relaxing.
• Length. The average heart beats about 75 times per minute, so the cardiac cycle is normally about 0.8 seconds.
• Mid-to-late diastole. The heart is fully relaxed, pressure is low, and blood flows passively through the atria into the ventricles. The semilunar valves are closed and the AV valves are open; then the atria contract and force remaining blood into the ventricles.
• Ventricular systole. Ventricular contraction begins and intraventricular pressure rises rapidly, closing the AV valves. When intraventricular pressure exceeds the pressure in the large arteries, the semilunar valves open and blood rushes out. The atria are relaxed and refilling.
• Early diastole. At the end of systole the ventricles relax, the semilunar valves snap shut, and for a moment the ventricles are closed chambers. Intraventricular pressure drops, the AV valves open, and the ventricles refill rapidly, completing the cycle.
• First heart sound ("lub"). Closing of the AV valves.
• Second heart sound ("dub"). Closing of the semilunar valves at the end of systole.

Cardiac Output

Cardiac output is the amount of blood pumped by each side of the heart in one minute. It is the product of heart rate and stroke volume.

• Stroke volume. Volume of blood pumped by a ventricle with each beat.
• Regulation of stroke volume. By Starling's law of the heart, the critical factor is how much the cardiac muscle cells are stretched just before they contract. The more they stretch, the stronger the contraction. Anything that increases the volume or speed of venous return also increases stroke volume and force of contraction.
• Factors modifying heart rate. The biggest external influence is the activity of the autonomic nervous system, plus physical factors (age, gender, exercise, body temperature).

Physiology of Circulation

You get a good read on circulatory efficiency from arterial pulse and blood pressure measurements.

Cardiovascular Vital Signs

Arterial pulse and blood pressure, along with respiratory rate and body temperature, make up the vital signs.

• Arterial pulse. Each beat of the left ventricle creates a pressure wave (the pulse) that travels the arterial system.
• Normal pulse rate. Pulse rate equals heart rate, so the pulse averages 70 to 76 beats per minute in a resting adult.
• Pressure points. Clinically important pulse points double as the points you compress to stop distal blood flow during hemorrhage.
• Blood pressure. The force blood exerts against the vessel walls; it keeps blood circulating even between beats.
• Pressure gradient. Highest in the large arteries, dropping throughout the systemic and pulmonary pathways, reaching zero or negative pressure at the venae cavae.
• Measuring blood pressure. Because the heart contracts and relaxes, two measurements are taken: systolic pressure (peak ventricular contraction) and diastolic pressure (ventricles relaxing).
• Peripheral resistance. The friction blood meets as it flows through the vessels.
• Neural factors. The parasympathetic division has little effect on blood pressure; the sympathetic division drives vasoconstriction, which raises blood pressure.
• Renal factors. The kidneys regulate arterial pressure by altering blood volume. When pressure rises, the kidneys send more water out in the urine, dropping blood volume and pressure.
• Temperature. Cold causes vasoconstriction; heat causes vasodilation.
• Chemicals. Epinephrine raises both heart rate and blood pressure; nicotine raises blood pressure through vasoconstriction; alcohol and histamine cause vasodilation and lower blood pressure.
• Diet. A diet low in salt, saturated fats, and cholesterol helps prevent hypertension.

Blood Circulation Through the Heart

The right and left sides work together for smooth circulation.

• Entrance. Oxygen-poor blood enters through the inferior and superior vena cava into the right atrium.
• Atrial contraction. Blood flows from the right atrium through the open tricuspid valve into the right ventricle.
• Tricuspid closure. When the ventricle is full, the tricuspid valve shuts to prevent backflow during contraction.
• Ventricular contraction. Blood leaves through the pulmonic valve, into the pulmonary artery, to the lungs for oxygenation.
• Oxygen-rich return. The pulmonary vein empties oxygen-rich blood from the lungs into the left atrium.
• Mitral opening. As the atrium contracts, blood flows from the left atrium through the open mitral valve into the left ventricle.
• Backflow prevention. When the ventricle is full, the mitral valve shuts, preventing backflow into the atrium during contraction.
• To systemic circulation. Blood leaves through the aortic valve, into the aorta, and out to the body.

Capillary Exchange of Gases and Nutrients

Substances move to and from cells along their concentration gradients.

• Capillary network. Capillaries weave so closely among the cells that no substance has to diffuse far to enter or leave a cell.
• Routes. Substances cross the single layer of endothelial cells by one of four routes.
• Lipid-soluble substances. Diffuse directly through the plasma membrane.
• Lipid-insoluble substances. Enter or leave within vesicles, by endocytosis or exocytosis.
• Intercellular clefts. Gaps not joined by tight junctions allow limited passage of fluid and small solutes; most capillaries have them.
• Fenestrated capillaries. Allow very free passage of small solutes and fluid; found where absorption is a priority or where filtration occurs.

Age-Related Changes

The heart's capacity for work decreases with age. Older patients respond to stress more slowly and take longer to return to baseline after physical activity. Arterial changes are common and can reduce blood supply.

Health-promotion teaching covers risk detection and reduction for cardiovascular disease, blood pressure and cholesterol monitoring, ideal weight maintenance, and a low-sodium diet.