Unit 6: Physiology of the Cardiovascular System

Unit 6: Physiology of the Cardiovascular System-CVS

• The cardiovascular system (CVS) is composed of the heart, blood vessels, and blood.
• The CVS is responsible for transporting materials to and from all parts of the body.
• Substances transported include nutrients, water, gases, oxygen, nutrients from gastrointestinal tract, wastes, immune cells, antibodies, clotting proteins, hormones, metabolic wastes, heat, and carbon dioxide.
• The heart acts as the pumping center.
• Atria receive blood returning from blood vessels.
• Ventricles pump blood into blood vessels.
• Blood vessels are categorized as arteries, capillaries, and veins.
• Arteries distribute blood to various organs.
• Capillaries are the major sites of nutrient, metabolic end product, and fluid exchange between blood and tissues.
• Veins collect blood and return it to the heart.
• Blood is a fluid transporting materials to and from cells.
• The CVS involves two circulations: pulmonary and systemic.
• Pulmonary circulation circulates blood from the right ventricle to the lungs and back to the left atrium.
• Systemic circulation circulates blood from the left ventricle to the tissues and back to the right atrium.
• The blood flow pathway through the heart and lungs involves vena cavae, pulmonary arteries, left atrium, left ventricle, right atrium, right ventricle, pulmonary veins, aorta, and branches.
• Pulmonary circulation has low resistance and low pressure.
• Systemic circulation has high resistance and high pressure (120/80 mmHg).
• Blood flows through multiple subcircuits in different organs.
• Blood flow is unidirectional.

Functions of the CVS

• The main function of the CVS is to transport materials throughout the body.
• This includes delivering oxygen, nutrients, and other essential substances to cells and removing waste products.
• The CVS also plays roles in regulating blood pressure and temperature, and in transporting hormones and immune cells.

Components of the CVS

• The heart acts as the pumping center.
• Blood vessels (arteries, capillaries, and veins) conduct blood to various organs, exchange materials between blood and tissues, and return blood to the heart, respectively.
• Blood itself is a fluid circulating around the body, carrying materials to and from cells.

Heart

• The heart is a hollow muscular organ that plays a central role in pumping.
• Located in the thoracic cavity.
• It's enclosed by a membranous sac called the pericardium, which contains pericardial fluid that lubricates the heart.
• The heart wall consists of three layers: epicardium (outer), myocardium (middle), and endocardium (inner).
• The heart is vertically divided into left and right sides by a septum.
• The heart has four chambers: two atria and two ventricles.
• Atrioventricular (AV) valves (tricuspid and mitral) prevent backflow of blood between atria and ventricles.
• Semilunar valves (pulmonary and aortic) prevent backflow of blood between ventricles and arteries.
• The heart receives arterial blood from coronary arteries.
• Resting coronary blood flow = 250 ml/min, 5% CO.

Cardiac Muscle (Myocardium)

• Two types of cardiac muscle: contractile and autorhythmic.
• Contractile muscles make up 99% of the heart and responsible for pumping.
• Autorhythmic muscles (1%) make pacemakers and conduct signals.
• Pacemakers include the SA node (sinoatrial node).
• The SA node is the primary pacemaker of the heart.
• Conduction fibers transmit action potentials through the heart.
• Cardiac muscle cells are smaller than skeletal muscles, striated, mono-nucleated, and connected through intercalated disks with desmosomes and gap junctions.

Excitation-Contraction Coupling in Cardiac Muscle

• Action potentials originate spontaneously in pacemaker cells and spread to contractile cells via gap junctions.
• Myocardial cell excitation results in Na+ influx, depolarization of sarcolemma and T-tubules, Ca2+ influx, Ca2+ release, Ca2+ binding to troponin C, and myocardial contraction.
• Relaxation occurs when Ca2+ unbinds from and is pumped back into the sarcoplasmic reticulum.

Myocardial Action Potentials

• Myocardial action potentials differ from skeletal muscle action potentials primarily due to the plateau phase.
• The plateau phase occurs during depolarization.
• It prevents tetanic contraction and allows ventricular filling, which is crucial for efficient cardiac function.

Electrical Excitation of the Heart

• Electrical communication in the heart begins with action potential generation in the autorhythmic cells.
• SA node discharges impulses, causing the wave of depolarization to spread through the atria and to the ventricles.
• The AV node delays the impulse before the action potential enters the ventricles, ensuring that atrial contraction completes before ventricular contraction.

Sequence of Excitation

• Sinoatrial (SA) node (pacemaker) in the right atrium initiates the electrical impulse for the heartbeat.
• Internodal pathways conduct the impulse from the SA node to the AV node.
• Atrioventricular (AV) node in the base of the right atrium delays the impulse to allow for complete atrial contraction.
• AV bundle (bundle of His) transmits the impulse to the ventricles.
• Bundle branches conduct impulses to the left and right ventricles.
• Purkinje fibers rapidly spread the impulse throughout the ventricles, initiating ventricular contraction.

Control of Heart Rate

• Heart rate is regulated by the autonomic nervous system (sympathetic and parasympathetic divisions) and hormones.
• Sympathetic stimulation speeds up the heart rate by increasing the slope of the pacemaker potential and increasing the permeability to Na+ and Ca2+ ions.
• Parasympathetic stimulation slows down the heart rate by increasing K+ permeability, hyperpolarizing the cell, and decreasing Ca2+ permeability.
• Hormones such as epinephrine and thyroid hormones can increase heart rate, and insulin and glucagon can have less of an immediate response.

Control of Stroke Volume

• Stroke volume is affected by factors such as preload, contractility, and afterload.
• Preload, or end-diastolic volume (EDV), is the volume of blood in the ventricles before contraction. Increased EDV stretches the ventricular walls, increasing the force of contraction (Frank-Starling mechanism).
• Contractility is the force of contraction of the ventricular myocardium, which is also regulated by nervous and hormonal systems.
• Afterload refers to the resistance the ventricles must overcome to eject blood into the aorta.

Heart Sounds

• Heart sounds are generated by the vibrations created by the closure of heart valves.
• First heart sound (S1) is caused by the closure of the atrioventricular valves.
• Second heart sound (S2) is caused by the closure of the semilunar valves.

Abnormal heart sounds (Murmurs)

• Abnormal heart sounds (murmurs) often indicate valvular diseases such as stenosis (stiff/narrowed valves) or insufficiency (leaky valves).

Cardiovascular Shock

• Circulatory shock is a life-threatening condition where the body's organs and tissues lack adequate blood flow.
• Shock can be classified into different types, such as hypovolemic, cardiogenic, neurogenic, anaphylactic, and septic shock.

Control of Blood Pressure

• Blood pressure is regulated by multiple mechanisms, including local, neural, and hormonal factors.
• Local control: tissues regulate their own blood flow based on metabolic demand.
• Neural control: the central nervous system, particularly the medulla oblongata, coordinates reflex mechanisms to control blood pressure, including the baroreceptor reflex.
• Hormonal control: Various hormones like epinephrine, ADH, and angiotensin II can alter blood pressure. Kidneys play a crucial role in long-term blood pressure control by regulating blood volume through the renin-angiotensin-aldosterone system.

Hypertension

• Hypertension is characterized by persistently elevated blood pressure.
• Categorized as primary (essential) or secondary hypertension, based on an underlying cause (or the lack of one, for essential).
• Common causes of secondary hypertension include renal disorders, endocrine issues, and some vascular or nervous system problems.
• Hypertension causes significant damage to blood vessels over time, increasing the risk of many serious health consequences.

Hypotension

• Hypotension is characterized by significantly low blood pressure.
• It can lead to insufficient blood flow and can be life threatening.

Electrocardiography (ECG)

• An ECG is a tool for evaluating the electrical events of the heart.
• It's a recording of the electrical activity generated by the heart, spread by body fluids (like blood or interstitial fluid), and recorded on the surface of the skin.
• An ECG records the sum of multiple action potentials from many heart muscle cells.
• Different waves and components on ECG correspond to specific phases of cardiac excitation and contraction.
• The ECG displays P-wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization).
• Different intervals and segments on ECG measure specific events of heart activity, such as P-R intervals (delay through AV node) or Q-T Intervals (ventricular depolarization and repolarization).