Cardiac Physiology and Circulation

Cardiovascular Physiology

• This subject covers the structure and function of the heart and blood vessels.
• Objectives include describing cardiac tissues, blood flow, and the conduction system.
• Additional objectives include identifying physiologic factors affecting cardiac output and blood pressure, blood components and clotting, and the process of blood clotting.

Medical Terminology

• Angi/o - Vessel
• Vas/o - Vessel
• Ven/o - Vein
• Phleb/o - Vein
• Thromb/o - Clot (thrombus)
• Aort/o - Aorta
• Arteri/o - Artery
• Ather/o - Thick, fatty
• Atri/o - Atria
• Valv/o - Valve
• Valvul/o - Valve
• Vascul/o - Blood vessel
• Cardi/o - Heart
• Ventricul/o - Ventricle
• Hem/o - Blood
• Scler/o - Hardening
• Hemat/o - Blood

Medical Terminology (Prefixes)

• Brady- - Slow
• Ecto- - Out, outside
• En- - In, within, inner
• Endo- - In, within, inner
• Macro- - Large
• Micro- - Small
• Oligo- - Deficiency, few
• Pre- - Before
• Pro- - Before, forward
• Re- - Behind, back
• Retro- - Behind, back
• Tachy- - Rapid

Medical Terminology (Suffixes)

• -ary - Pertaining to
• -cyte - Cell
• -dynia - Pain
• -edema - Edema
• -emesis - Vomiting, producing
• -genesis -Creating, producing
• -gram - Record
• -lysis - Destruction
• lysis- Destruction
• -megaly - Enlargement
• -oid - Resembling
• -sclerosis - Abnormal hardening
• -stasis - Stopping, cessation

Cardiac Muscle

• Composed of cardiac muscle cells (myocytes).
• Generate their own action potentials, don't need nerve impulses.
• Intercalated discs connect cells, forming junctions.
• Folds in the membrane create a large surface area, allowing rapid spread of electrical activity.
• Functions as an endocrine tissue releasing ANP (atrial natriuretic protein) in response to increased atrial pressure, exercise, and/or cold temperatures.
• ANP decreases sodium reabsorption by the kidneys, increasing fluid excretion and relaxing blood vessels' smooth muscles.

Structure of the Heart

• Located in the mediastinum between the lungs.
• Base is uppermost, located behind the sternum.
• Apex (tip) is just above diaphragm to the left of midline.
• Encased in pericardial membranes.
• Fibrous pericardium: outermost layer, a loose-fitting sac.
• Serous pericardium: folded membrane with parietal & visceral layers.
• Parietal pericardium lines the fibrous pericardium; visceral pericardium (epicardium) lines the heart.
• Serous fluid between parietal and visceral layers reduces friction.
• Myocardium: thickest portion, composed of cardiac muscle.
• Endocardium: simple squamous epithelium lining chambers and valves, continuing into vessels' lining (endothelium).

Structure of the Heart (Chambers)

• Atria (right & left): relatively thin walls, receive blood from body or lungs.
• Ventricles (right & left): thicker walls, pump blood to lungs or body.
• Atria are separated by interatrial septum; ventricles by interventricular septum.
• Left atrium (LA) receives oxygenated blood from lungs. Pumps blood to body.
• Right atrium (RA) receives deoxygenated blood from body. Pumps blood to lungs.
• Left ventricle (LV) pumps blood to the body via the aorta.
• Right ventricle (RV) pumps blood to the lungs via pulmonary artery.

Heart Valves

• Atrioventricular valves (tricuspid and mitral/bicuspid) prevent backflow of blood between atria and ventricles.
• Tricuspid: prevents backflow from RV to RA
• Mitral/Bicuspid: prevents backflow from LV to LA.
• Semilunar valves (pulmonary and aortic) prevent backflow from ventricles to arteries.
• Pulmonary: prevents backflow from pulmonary artery to RV.
• Aortic: prevents backflow from the aorta to the LV.

Structures & Function of the Heart

• Papillary muscles: myocardial projections into ventricles.
• Chordae tendineae: fibrous strands connecting papillary muscles to flaps of AV valves.
• Prevent inversion of AV valves during ventricular contraction.
• Fibrous skeleton of the heart: fibrous connective tissue anchoring valves and separating atrial and ventricular myocardium.

Coronary Circulation

• Coronary vessels circulate oxygenated blood to the myocardium.
• Right and left coronary arteries branch off the aorta.
• Branch into smaller arteries & arterioles, then to capillaries.
• Coronary capillaries form coronary veins.
• Coronary veins return blood to the right atrium via a large coronary sinus.

Cardiac Cycle & Heart Sounds

• Cardiac cycle: sequence of events in one heartbeat.
• Simultaneous contraction of atria and then ventricles.
• Systole: myocardial contraction, increases pressure to eject blood.
• Diastole: myocardial relaxation, allows chamber filling.
• Cycle begins with atria relaxed, filling with blood, then atria contract, then ventricles contract to eject blood, then both relax to allow for filling again.
• Heart sounds (lub-dub): S1 from AV valve closure, S2 from aortic and pulmonary valve closure.

Pathways of Circulation

• Pulmonary: blood from right ventricle to lungs via pulmonary artery, divides into right and left pulmonary arteries within the lungs, branches into smaller arteries/capillaries to surround alveoli of lungs (site of gas exchange). Capillaries, venules, and veins carry blood back to the left atrium via pulmonary veins.
• Systemic: blood from the left ventricle to the body via aorta. Aorta branches take blood into arterioles & capillary networks. Capillaries merge to form venules and veins (also carrying blood from the lower body to the inferior vena cava, and blood from the upper body to the superior vena cava). These merge into caval veins that return blood to the right atrium.

Cardiac Conduction System

• Regulates cardiac cycle by electrical activity of the myocardium.
• Cardiac muscle cells contract spontaneously.
• Intercalated discs facilitate rapid electrical impulse spread between cells.
• Allows the atria to contract as a unit, followed by simultaneous ventricle contraction.
• Electrical impulses follow a specific path through the myocardium.

Conduction Pathways

• Sinoatrial (SA) node (natural pacemaker): initiates heartbeat, located in the wall of the right atrium, depolarizes more rapidly than other cells because it's more permeable to sodium. Depolarization occurs 60–80 times/minute.
• Atrioventricular (AV) node: located in the lower interatrial septum, receives impulses from the SA node, transmission to the AV node is responsible to the ventricles for atrial systole.
• AV bundle (bundle of His): only pathway for impulses from atria to the ventricles. Located within the upper interventricular septum. Receives impulses from the AV node and transmits to right and left bundle branches.
• Bundle branches: impulses travel from bundle branches to Purkinje fibers.
• Purkinje fibers: terminal fibers, transmit impulses to the rest of the ventricular myocardium and stimulate ventricular systole.

Resting Heart Rate (HR)

• Healthy adult resting HR: 60 - 80 beats per minute (bpm).
• Parasympathetic impulses (vagus nerves) decrease rate.
• Sympathetic impulses increase rate (<60 bpm: bradycardia; >100 bpm: tachycardia).
• Well-conditioned individuals may have lower resting HR (e.g., 35 bpm).

Heart Rate

• If the SA node malfunctions, AV node takes over (50-60 bpm). AV bundle can also trigger contractions (15-40 bpm).
• Irregular heartbeats (arrhythmias or dysrhythmias) range from harmless to life-threatening.

Cardiac Physiology

• Cardiac output (CO): volume of blood ejected by a ventricle in 1 minute (CO = SV × HR). Average resting CO: 5 - 6 L/min.
• Stroke volume (SV): volume of blood pumped out of a ventricle in 1 contraction. Average resting SV: 60-80 mL/beat.
• Ejection fraction: percentage of blood ejected per beat from within a ventricle. Average ejection fraction in healthy adults: 60–70%.
• Preload: force stretching myocardial muscle prior to contraction; is related to the amount of blood in venous return.
• Afterload: force required to eject blood from ventricles to arteries; determined by peripheral resistance in arteries.

Starling's Law of the Heart

• The more cardiac muscle fibers are stretched, the more forcefully they contract.
• During exercise, increased venous return stretches the myocardium of ventricles, leading to increased force of contraction, and hence increasing stroke volume.
• Cardiac reserve: difference between resting CO and maximum CO during exercise. Average cardiac reserve is ~15 L or more.

Regulation of Heart Rate

• Cardiac control center in the medulla oblongata controls rate & force of contraction via the autonomic nervous system (ANS).
• Sympathetic stimulation increases HR and contractility.
• Parasympathetic stimulation decreases HR.
• Sensory inputs (baroreceptors in aorta and internal carotid arteries, chemoreceptors in carotid bodies and aortic body) to the cardiac control center influence the heart rate via the glossopharyngeal (9th cranial) and vagus (10th cranial) nerves.

Vascular Physiology

• Arteries and arterioles: carry blood from heart to capillaries, three-layered walls (intima, media, externa); smooth endothelium prevents clotting, media contributes to BP maintenance.
• Veins and venules: carry blood to heart from capillaries; three-layered walls (intima, media, externa); thin media, valves prevent backflow, greater capacity than arteries.

Anastomoses

• Anastomoses are connections joining vessels. They provide alternate pathways for blood flow if/when one vessel is obstructed (e.g., artery to artery or vein to vein.)

Capillaries

• Capillaries carry blood from arterioles to venules; walls are 1 cell thick.
• Precapillary sphincters (smooth muscle) regulate blood flow into capillary networks, and they respond to tissue needs (as opposed to nervous control).
• Extensive networks reflect metabolic activity; some tissues don’t have them (e.g., epidermis, cartilage).
• Sinusoids are highly permeable specialized capillaries found in red bone marrow, spleen, lymph nodes, liver, and some endocrine glands.
• Capillary exchange: movement of material between blood and interstitial fluid via diffusion (from high to low concentrations). Filtration occurs at the arterial end, pushing fluid from the blood into the interstitial tissue fluid (blood pressure > interstitial pressure). Osmosis brings fluid and waste back to capillaries at the venous end (colloid osmotic pressure > interstitial pressure).

Blood Components

• Viscosity (thickness): blood is thicker than water, contributing to blood pressure.
• Volume: 4–6 liters.
• Composition: 38-48% formed elements (cells); 52-62% plasma.
• Arterial blood: bright red; venous blood: darker, dull red. Normal pH range: 7.35–7.45 (slightly lower in venous blood due to CO2).
• Plasma: mostly water, transports substances (e.g., glucose, amino acids, vitamins, minerals, hormones, antibodies, waste products).
• Plasma proteins (albumin: osmotic pressure; globulins: carriers, antibodies; prothrombin & fibrinogen: blood clotting).
• Blood cells: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

Erythrocytes

• Biconcave discs, lack nuclei.
• Most abundant type of blood cell.
• Hemoglobin (protein) carries oxygen from lungs to tissues and some CO2 from tissues to lungs.
• Normal RBC count: 4.5–6.0 million/µL; Hematocrit (percentage of blood volume occupied by RBCs): 38–48%.
• Production in red bone marrow from stem cells, regulated by oxygen levels (via erythropoietin). Removal by spleen & liver.
• Needed nutrients: protein, iron, copper, folic acid & vitamin B12.

Leukocytes

• Larger than erythrocytes, have nuclei.
• Normal WBC count: 5,000–10,000/µL.
• Types include granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes, lymphocytes).
• Function as part of the immune system.

Thrombocytes

• Platelets: cell fragments involved in hemostasis (preventing/reducing blood loss).
• Produced from megakaryocytes in red bone marrow.
• Hemostasis mechanisms include vascular spasm, platelet plugs, and chemical clotting.
• Regulated by thrombopoietin, a liver hormone.
• Normal platelet count: 150,000–300,000/µL.
• Chemical clotting: series of reactions lead to blood clot formation (prothrombin > thrombin > fibrinogen > fibrin), with vitamin K needed.
• Clot retraction & fibrinolysis occur after a clot has formed (involving platelets, ATP, & factor 13). Clot breakdown (fibrinolysis) occurs with the help of plasmin, an enzyme, produced by tissue plasminogen activator (tPA), secreted by endothelial cells, which is triggered by several stimuli.
• Heparin, antithrombin, & fibrin limit clot formation to what is needed to appropriately repair a vessel.

Blood Pressure

• Force of blood against vessel walls.
• Systolic pressure: pressure during ventricular contraction.
• Diastolic pressure: pressure during ventricular relaxation.
• Blood pressure altered by cardiac output, blood volume, and peripheral resistance.
• Peripheral resistance influenced by blood vessel diameter, with vasoconstriction or dilation increasing/decreasing peripheral resistance.

Factors Influencing Blood Pressure

• Factors: venous return; cardiac contractility; peripheral resistance (influence of vessel diameter); elasticity of large arteries; viscosity (influence of RBCs & plasma proteins).
• Blood loss will temporarily decrease BP leading to compensatory response in HR and blood vessel constriction. Severe loss exceeding the body's capacity to compensate will result in severe health complications.
• Hormones influencing blood pressure: Norepinephrine & epinephrine (stimulate vasoconstriction, increase heart rate & contractility); antidiuretic hormone (ADH increases BP); aldosterone (increases blood volume and therefore BP); Renin-Angiotensin-Aldosterone system causes vasoconstriction increasing BP.

Test your knowledge of cardiac physiology with these multiple-choice questions. Topics covered include pressure differences in ventricles, heart sounds (S1), gas exchange in pulmonary circulation, systemic circulation pathways, blood return to the heart, and effects of valvular malfunctions and pulmonary hypertension.