Cardiovascular System: The Heart and Blood Vessels

Anatomy of the Heart

• Located in the mediastinum: the anatomical region extending from the sternum to the vertebral column, the first rib, and between the lungs.
• Apex: at the tip of the left ventricle.
• Base: the posterior surface.
• Anterior surface: deep to the sternum and ribs.
• Inferior surface: between the apex and right border.
• Right border: faces the right lung.
• Left border (pulmonary border): faces the left lung.

Pericardium

• Membrane surrounding and protecting the heart.
• Confines the heart while still allowing free movement.
• Two main parts:
  • Fibrous pericardium:
    • Tough, inelastic, dense irregular connective tissue.
    • Prevents overstretching, provides protection, and anchorage.
  • Serous pericardium:
    • Thinner, more delicate membrane.
    • Double layer: parietal layer (fused to fibrous pericardium) and visceral layer (also called epicardium).
• Pericardial fluid:
  • Reduces friction.
  • Secreted into the pericardial cavity.

Layers of the Heart Wall

• Epicardium (external layer):
  • Visceral layer of serous pericardium.
  • Smooth, slippery texture to the outermost surface.
• Myocardium:
  • 95% of the heart is cardiac muscle.
• Endocardium (inner layer):
  • Smooth lining for the chambers of the heart and valves.
  • Continuous with the lining of large blood vessels.

Chambers of the Heart

• Two atria (receiving chambers):
  • Auricles increase capacity.
• Two ventricles (pumping chambers).
• Sulci (grooves):
  • Contain coronary blood vessels.
  • Coronary sulcus.
  • Anterior interventricular sulcus.
  • Posterior interventricular sulcus.

Right Atrium

• Receives blood from:
  • Superior vena cava.
  • Inferior vena cava.
  • Coronary sinus.
• Interatrial septum has the fossa ovalis:
  • Remnant of the foramen ovale.
• Blood passes through the tricuspid valve (right atrioventricular valve) into the right ventricle.

Right Ventricle

• Forms the anterior surface of the heart.
• Trabeculae carneae:
  • Ridges formed by raised bundles of cardiac muscle fiber.
  • Part of the conduction system of the heart.
• Tricuspid valve connected to chordae tendinae connected to papillary muscles.
• Interventricular septum.
• Blood leaves through the pulmonary valve (pulmonary semilunar valve) into the pulmonary trunk and then right and left pulmonary arteries.

Left Atrium

• About the same thickness as the right atrium.
• Receives blood from the lungs through the pulmonary veins.
• Passes through the bicuspid/mitral/left atrioventricular valve into the left ventricle.

Left Ventricle

• Thickest chamber of the heart.
• Forms the apex.
• Chordae tendinae attached to papillary muscles.
• Blood passes through the aortic valve (aortic semilunar valve) into the ascending aorta.
• Some blood flows into coronary arteries; the remainder goes to the body.
• During fetal life, the ductus arteriosus shunts blood from the pulmonary trunk to the aorta (lung bypass); it closes after birth, and the remnant is called the ligamentum arteriosum.

Myocardial Thickness

• Thin-walled atria deliver blood under less pressure to the ventricles.
• Right ventricle pumps blood to the lungs:
  • Shorter distance, lower pressure, less resistance.
• Left ventricle pumps blood to the body:
  • Longer distance, higher pressure, more resistance.
• The left ventricle works harder to maintain the same rate of blood flow as the right ventricle.

Heart Valves and Circulation of Blood

• Atrioventricular valves:
  • Tricuspid and bicuspid valves.
  • Atria contracts/ventricle relaxed:
    • AV valve opens, cusps project into ventricle.
    • In the ventricle, papillary muscles are relaxed, and chordae tendinae are slack.
  • Atria relaxed/ventricle contracts:
    • Pressure drives cusps upward until edges meet and close the opening.
    • Papillary muscles contract, tightening chordae tendinae:
      • Prevents regurgitation.

Semilunar Valves

• Aortic and pulmonary valves.
• Valves open when pressure in the ventricle exceeds pressure in the arteries.
• As ventricles relax, some backflow is permitted, but blood fills valve cusps, closing them tightly.
• No valves guarding the entrance to the atria.
• As the atria contracts, it compresses and closes the opening.

Systemic and Pulmonary Circulation

• Two circuits in series:
  • Systemic circuit:
    • Left side of the heart.
    • Receives blood from the lungs.
    • Ejects blood into the aorta.
    • Systemic arteries, arterioles.
    • Gas and nutrient exchange in systemic capillaries.
    • Systemic venules and veins lead back to the right atrium.
  • Pulmonary circuit:
    • Right side of the heart.
    • Receives blood from systemic circulation.
    • Ejects blood into the pulmonary trunk, then pulmonary arteries.
    • Gas exchange in pulmonary capillaries.
    • Pulmonary veins take blood to the left atrium.

Coronary Circulation

• Myocardium has its own network of blood vessels.
• Coronary arteries branch from the ascending aorta.
• Anastomoses provide alternate routes or collateral circuits:
  • Allows the heart muscle to receive sufficient oxygen even if an artery is partially blocked.
• Coronary capillaries.
• Coronary veins:
  • Collects in the coronary sinus.
  • Empties into the right atrium.

Terms

• Anastomosis: Union or joining of two blood vessels (nerves or lymphatics).
• Collateral circulation: Alternative route of circulation to an area provided by an anastomosis.
• Ischemia: Insufficient blood supply to an organ or tissue, usually due to a blocked artery.
• Infarction: Localized necrosis resulting from an obstruction of the blood.

Autorhythmic Fibers

• Specialized cardiac muscle fibers.
• Self-excitable.
• Repeatedly generate action potentials that trigger heart contractions.
• Two important functions:
  • Act as a pacemaker.
  • Form a conduction system.

Conduction System

• Begins in the sinoatrial (SA) node in the right atrial wall.
• Propagates through the atria via gap junctions.
• Atria contract.
• Reaches the atrioventricular (AV) node in the interatrial septum.
• Enters the atrioventricular (AV) bundle (Bundle of His).
• Only site where action potentials can conduct from atria to ventricles due to the fibrous skeleton.
• Enters the right and left bundle branches, which extend through the interventricular septum toward the apex.
• Finally, large diameter Purkinje fibers conduct the action potential to the remainder of the ventricular myocardium.
• Ventricles contract.
• SA node acts as the natural pacemaker:
  • Faster than other autorhythmic fibers.
  • Initiates 100 times per second.
  • Nerve impulses from the autonomic nervous system (ANS) and hormones modify the timing and strength of each heartbeat.
  • Do not establish the fundamental rhythm.

Action Potentials and Contraction

• The action potential initiated by the SA node spreads out to excite "working" fibers called contractile fibers.
• Depolarization.
• Plateau.
• Repolarization.

Action Potential in a Ventricular Contractile Fiber

• Depolarization: Rapid depolarization due to Na^+ inflow when voltage-gated fast Na^+ channels open.
• Plateau (maintained depolarization): Due to Ca^{2+} inflow when voltage-gated slow Ca^{2+} channels open and K^+ outflow when some K^+ channels open.
• Repolarization: Due to the closure of Ca^{2+} channels and K^+ outflow when additional voltage-gated K^+ channels open.
• Refractory period.
• Contraction.

From Action Potential to Contraction

• Action potential travels down T-tubules of the myocyte.
• Calcium enters the cell.
• Calcium binds to troponin.
• The troponin-tropomyosin complex exposes myosin-binding sites on actin.
• Thin filaments slide toward the center of the sarcomere (thick filaments).
• Contraction.

Electrocardiogram (ECG or EKG)

• Composite record of action potentials produced by all the heart muscle fibers.
• Compare tracings from different leads with one another and with normal records.
• Three recognizable waves: P, QRS, and T.

Cardiac Cycle

• All events associated with one heartbeat.
• Systole and diastole of atria and ventricles.
• In each cycle, atria and ventricles alternately contract and relax.
• During atrial systole, ventricles are relaxed.
• During ventricle systole, atria are relaxed.
• Forces blood from higher pressure to lower pressure.
• During the relaxation period, both atria and ventricles are relaxed.
• The faster the heart beats, the shorter the relaxation period.
• Systole and diastole lengths shorten slightly.

Heart Sounds

• Auscultation.
• The sound of a heartbeat comes primarily from blood turbulence caused by the closing of heart valves.
• Four heart sounds in each cardiac cycle – only two loud enough to be heard:
  • Lubb – AV valves close.
  • Dupp – SL valves close.

Cardiac Output

• CO = volume of blood ejected from the left (or right) ventricle into the aorta (or pulmonary trunk) each minute.
• CO = stroke volume (SV) \times heart rate (HR)
• In a typical resting male:
  • 5.25 L/min = 70 mL/beat \times 75 beats/min
• The entire blood volume flows through pulmonary and systemic circuits each minute.
• Cardiac reserve – the difference between maximum CO and CO at rest.
• Average cardiac reserve is 4-5 times the resting value.

Regulation of Stroke Volume

• Three factors ensure left and right ventricles pump equal volumes of blood:
  • Preload.
  • Contractility.
  • Afterload.

Preload

• The degree of stretch on the heart before it contracts.
• Greater preload increases the force of contraction.
• Frank-Starling law of the heart – the more the heart fills with blood during diastole, the greater the force of contraction during systole.
• Preload is proportional to end-diastolic volume (EDV).
• Two factors determine EDV:
  • Duration of ventricular diastole.
  • Venous return – the volume of blood returning to the right ventricle.

Contractility

• Strength of contraction at any given preload.
• Positive inotropic agents increase contractility:
  • Often promote Ca^{2+} inflow during the cardiac action potential.
  • Increases stroke volume.
  • Epinephrine, norepinephrine, digitalis.
• Negative inotropic agents decrease contractility:
  • Anoxia, acidosis, some anesthetics, and increased K^+ in interstitial fluid.

Afterload

• Pressure that must be overcome before a semilunar valve can open.
• An increase in afterload causes stroke volume to decrease:
  • Blood remains in the ventricle at the end of systole.
• Hypertension and atherosclerosis increase afterload.

Regulation of Heart Beat

• Cardiac output depends on heart rate and stroke volume.
• Adjustments in heart rate are important in the short-term control of cardiac output and blood pressure.
• The autonomic nervous system and epinephrine/norepinephrine are most important.

Blood Vessels: The Vascular System

• Taking blood to the tissues and back.
• Arteries.
• Arterioles.
• Capillaries.
• Venules.
• Veins.

Differences Between Blood Vessel Types

• Walls of arteries are the thickest.
• Lumens of veins are larger.
• Skeletal muscle “milks” blood in veins toward the heart.
• Walls of capillaries are only one cell layer thick to allow for exchanges between blood and tissue.

Valves

• Valves prevent the backflow of blood.
• Venous valves are formed from folds of the endothelium.
• They are abundant in veins of the limbs and absent in veins of the ventral body cavity.

Movement of Blood Through Vessels

• Most arterial blood is pumped by the heart.
• Veins use the milking action of muscles to help move blood.

Capillary Beds

• Capillary beds consist of two types of vessels:
  • Vascular shunt – directly connects an arteriole to a venule.
  • True capillaries – exchange vessels.
    • Oxygen and nutrients cross to cells.
    • Carbon dioxide and metabolic waste products cross into blood.

Capillary Exchange

• Substances exchanged due to concentration gradients.
• Oxygen and nutrients leave the blood.
• Carbon dioxide and other wastes leave the cells.

Capillary Exchange: Mechanisms

• Direct diffusion across plasma membranes.
• Endocytosis or exocytosis.
• Some capillaries have gaps (intercellular clefts).
  • Plasma membrane not joined by tight junctions.
• Fenestrations of some capillaries.
  • Fenestrations = pores.

Venous Sinuses

• Large-diameter veins.
• They are made of endothelium.
• Examples of venous sinuses: Coronary Sinus, Cranial Sinuses

Vascular Disorders

• Arteriosclerosis
  • Causes: aging, hypertension, diabetes, smoking, hereditary factors, elevated cholesterol
  • Characteristics: abnormal thickening and abnormal hardening of the vessel walls in the arterial system
    • Smooth muscle cells and collagen fibers migrate into the tunica media
    • Lumen becomes narrowed
    • Hyperplastic arteriosclerosis
• Atherosclerosis
  • Most common cause of arteriosclerosis:
    1. Endothelial cells are injured; inflammation occurs
    2. Macrophages release free radicals and enzymes that further damage the vessel wall
    3. LDL becomes oxidized and ingested by macrophages (foam cells)
    4. Foam cells invade the wall and accumulate (fatty streak)
    5. Smooth muscle cells proliferate, secrete collagen and form a fibrous plaque
    6. Vessel becomes narrowed and obstructed; platelets adhere and lead to thrombus formation
• "Fatty Streak"
  • First lesion to appear as a result of atherogenesis, occurring possibly in early teen years. Consist primarily of foam cells in the subendothelial space.
• Varicose Veins
  • Damage to one or more valves in a vein
  • Venous distention and pooling of blood; veins become tortuous and palpable
  • Surrounding tissue may become edematous
• Aneurysm
  • Localized dilation or outpouching of a blood vessel or cardiac chamber
  • Arterial walls become weakened and may rupture; common sites are the abdominal aorta, renal arteries and cerebral arteries

Blood Pressure

• Measurements by health professionals are made on the pressure in large arteries
  • Systolic – pressure at the peak of ventricular contraction
  • Diastolic – pressure when ventricles relax
• Pressure in blood vessels decreases as the distance away from the heart increases

Blood Pressure: Effects of Factors

• Neural factors
  • Autonomic nervous system adjustments (sympathetic division)
• Renal factors
  • Regulation by altering blood volume
  • Renin – hormonal control
• Temperature
  • Heat has a vasodilation effect
  • Cold has a vasoconstricting effect
• Chemicals
  • Various substances can cause increases or decreases
• Diet

Factors Determining Blood Pressure

• Blood volume
  • Kidney conserves water and salt
• Cardiac Output = SV x HR
• Exercise
  • Activates SNS
• Centers
• Chemicals (renin, nicotine, and others)
  • Which brings about Vasoconstriction
• ↑ Peripheral resistance
• Blood viscosity
• Postural changes

Variations in Blood Pressure

• Human normal range is variable
• Normal
  • 140–110 mm Hg systolic
  • 80–75 mm Hg diastolic
• Hypotension
  • Low systolic (below 110 mm HG)
  • Often associated with illness
• Hypertension
  • High systolic (above 140 mm HG)
  • Can be dangerous if it is chronic