Cardiovascular Physiology

Dr. Hussein Farouk Sakr

The primary function of cardiovascular system (CVS) is to ensure that the tissues receive an adequate flow of blood to serve their requirements: Homeostasis• Oxygen• Nutrients• Hormones • Elimination of waste products: CO2, H+,, etc

The first priority of blood pressure homeostasis is to maintain adequate perfusion to the brain & the heart

Components of Circulatory System

• Cardiovascular System (CV):• Heart:

• Pumping action creates pressure head needed to push blood through vessels.

• Blood vessels:• Permits blood flow from heart to cells and

back to the heart.• Arteries, arterioles, capillaries, venules,

veins.

• Lymphatic System:• Lymphatic vessels transport interstitial

fluid.• Lymph nodes cleanse lymph prior to return

in venous blood. The systemic & pulmonary circulation Advantages of Parallel arrangement

The functions of the heart

1. The pumping function of the heart creates blood pressure that determines the blood flow from the left ventricle to the right atrium through systemic circulation.

2. It secretes atrial natriuretic peptide

3. It contains receptors regulating the secretion of antidiuretic hormone from the posterior pituitary.

The vascular system• Arterial System1. Elastic (conducting) arteries2. Muscular arteries-

distributing arteries3. Arterioles (resistance

vessels)4. Capillaries (exchange

vessels)

Venous System1. Venules2. Veins (capacitant vessels

contains 60 % of total blood volume)

Cardiac muscle cells (fibers) Atrial muscle Ventricular muscle Specialized excitatory and conductive muscle cells (1%) – the conducting system of the heart

Like skeletal muscle, cardiac muscle has striated appearance, which results from the arrangement of numerous thick and thin filaments The thick and thin filaments in each myofibril are arranged in a repeating pattern along the length of the myofibril. One unit of this repeating pattern is known as a sarcomere The thick filaments are composed almost entirely of the protein myosin The thin filaments are principally composed of the protein actin, as well as two other proteins – troponin and tropomyosin

Cellular membranes include a T-tubule system and associated calcium-loaded sarcoplasmic reticulum. The mechanism by which these membranes interact to release calcium is different than in skeletal muscle Adjacent cells are joined end-to-end at structures called intercalated disks, within which are desmosomes that hold the cells together and to which the myofibrils are attached. Also found within the intercalated disks are gap junctions that allow rapid diffusion of ions

• Have two important functions1. Act as a pacemaker (set

the rhythm of electrical excitation)

2. Form the conductive system (network of specialized cardiac muscle fibers that provide a path for each cycle of cardiac excitation to progress through the heart)

Autorhythmic fibers

• Forms 1% of the cardiac muscle fibers

1. Sinoatrial node (SA node)Specialized region in the right atrial wall near opening of superior vena cava.

2. Atrioventricular node (AV node)Small bundle of specializedcardiac cells located at base of right atrium near septum

3. Bundle of His (atrioventricular bundle)Cells originate at AV node and enters interventricular septumDivides to form right and left bundle branches which travel down septum, curve around tip of ventricular chambers, travel back toward atria along outer walls

4. Purkinje fibersSmall, terminal fibers that extend from bundle of His and spread throughout ventricular myocardium

Cells with autorhythmicity

Cardiac properties • AutoRhythmicity: the ability of the cardiac muscle to generate

action potential spontaneously and beat regularly.• Contractility: the ability of the cardiac muscle to pump blood into

circulations.• Conductivity: the ability of the cardiac muscle to conduct impulse

from one muscle fibre to the next.• Excitability: the ability of the cardiac muscle to respond to stimuli.

Autorhythmicity

• the ability of the cardiac muscle to generate action potential spontaneously and beat regularly.

• It is myogenic in origin.• It is a property of some of the cardiac muscle fibres

like the conducting system.

Nature of automaticity

The ability of self excitation is confined to the nodal and conducting system of the heart.

The SAN has the greatest rhythm and so called the pace maker of the heart.

All the cardiac muscle fibers follow the SAN.

Rhythmicity of the different parts of the heart

• The rhythmicity of the different parts of the heart:

Sino-Atrial node: 110/min.Atrio-Ventricular node:

70/min.Bundle of His: 55/min.Purkinje fibres: 45/min.Ventricular muscle fibres: 25-

40/min.

Why SAN is the pace maker?

• The SAN is the pace-maker due to inherent permeability of the cell membrane to Na that make the membrane potential unstable.• If the SAN fails to generate the impulse, other backup node

become active and discharge through their inherent rate.

Self excitation of the SAN

•Resting membrane potential of the SAN is ranging from -55 to -60 m.v., while the remaining cardiac muscle fibers are ranging from -80 to -90.•High permeability of the membrane of the SAN for Na+

(inherent leakiness of the membrane to Na+).

Action potential of SAN• The action potential of the SAN is

formed of:1- Diastolic prepotential (phase 4): caused by the slow influx of Ca+2 through T- channels and Na+ through funny channels with decreased K+ efflux from delayed rectifier K+ channels 2- Rapid depolarization (phase 0):Caused by Ca+2 influx through L type Ca+2 channels3- Repolarization (phase 3): caused by K + efflux from delayed rectifier K + channels

Autonomic control of the SAN activity• Autonomic influences alter the rate of pacemaker firing through the following mechanisms:1) Changing the slope of phase 42) Altering the threshold for triggering phase 03) Altering the degree of hyperpolarization at the end of phase 3.

Effects of sympathetic and parasympathetic (vagal) stimulation on sinoatrial (SA) nodal pacemaker activity.Sympathetic stimulation increases the firing rate by increasing the slope of phase 4 and lowering the threshold for the action potential. Vagal stimulation has the opposite effects, and it hyperpolarizes the cell.

Sympathetic nervous system

Norepinephrine

Stimulation of B1 adrenergic receptors

Increased formation of CAMP

Increased influx of Ca and Na

Increased rate of depolarization Increased heart rate

Sympathetic stimulation: Noradrenaline stimulates B1 adrenergic receptors increasing the membrane permeability to Na+ and Ca++ rapid depolarization increasing heart rate. as occurs during exercise.

Positive Chronotropic

Parasympathetic nervous system

Acetylcholine

Stimulation of m2 cholinergic receptors

Decreased CAMP

Decreased influx of Ca

Increased hyperpolarization

Increased CGMP

Increased efflux of K

Negative Chronotropic •Parasympathetic stimulation (vagal tone): A.ch. Acts on muscrinic receptors increasing the membrane permeability to K+ increased K+

efflux hyperpolarization decreases heart rate. decreases rhythmicity from the SAN from 110 to 70/min.Increased vagal stimulation more decrease in heart rate.Sever vagal stimulation stoppage of SAN discharge and do atrial arrest, while the ventricles continue to beat by idioventricular rhythm

Sympathetic stimulation Catecholamines Thyroxine Hypokalemia Anticholinergic drugs Hyperthermia

Parasympathetic stimulation AcetylcholineB blockers Ca channels blockers Ischemia Hyperkalemia Hypothermia

Positi

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Negative chronotropic

Factors affecting autorhythmicity (chronotropism)