Physiology of Heart• Heart muscle cells contract,

without nerve impulses, in a regular, continuous way

• Heart is autorhythmic

• Initiate, conduct and impulse

• Heart contains special tissue that produces & sends electrical impulses to the heart muscle to contract

• Autorhythmic cardiac cells are found in the following areas:

• Sinoatrial (SA) node• Atrioventricular

(AV)node• Bundle of His• Bundle branches• Purkinje fibers

Physiology of Heart

– Sinoatrial (SA) Node• Electrical impulse that

causes rhythmic contraction of heart muscles arises in the SA node

• Located in R. atrium• Pacemaker of the heart • Generates impulses 70 to 80

times a minute• The electrical impulse from

the SA node spreads over the right and left atria

• Causes atrial contraction

Physiology of Heart

Physiology of Heart• AV Node:

• Then impulses are conducted to the atrioventicular (AV) node

• Impulse is delayed at AV node for 0.1 sec

• Allows completion of atrial contraction before venticular contraction begins

• Bundle of His (AV bundle)

• Then electrical impulse is relayed down to Bundle of His

• Bundle of His passes impulse to right and left bundle branches

• Bundle Branches• Right and left Bundle branches• Branch into purkinje fibers

Physiology of Heart

• Purkinje Fibers

• Enter myocardium of ventricle walls, and apex of the heart

• Purkinje fibers transmit the impulse first to apex of the heart

• Contraction begins at apex and pushes the blood to aorta and pulmonary trunk

CARDIAC CONDUCTION SYSTEM SUMMARY

Sinoatrial Node

AV Node

AV Bundle

Bundle Branches

Purkinje Fibers

• Conducted cell to cell

• Takes 200-500 ms to complete

• Resting membrane potential is electronegative

• Unstable resting membrane potential

• Continuously depolarize • AP takes place in SA node

• When spontaneously changing potentials, called prepotential reaches threshold

• Voltage gated Na+ channels open, Na + influx, K+ channels close

• Depolarization takes place

• Depolarized to +20 mV

• Repolarization takes place

• Na+ channels close, K+ channels open

• Conduction of action potential produces electric current that can be measured at the surface of the body

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Alterations in an ElectrocardiogramAlterations in an

Electrocardiogram

Normal

SA Node Dysfunction no P waves

Ventricular Fibrillation

Cardiac Cycle• Heart is two pumps that work together, right and left

half

• Each pump consists of

• Primer pump – Atrium

• Power pump – Ventricle

Cardiac Cycle• Cardiac cycle: Is the sequence of

events in one heartbeat

• It is the repetitive pumping process that begins with onset of cardiac muscle contraction and ends with beginning of next contraction

• Cardiac muscle contraction is responsible for pressure and blood movement. How?

• Blood moves from high pressure to low pressure

  

Cardiac Cycle• The length of cardiac cycle is about 0.8 sec• Interval from end of one contraction to the following contraction• Consists of Two Phases:

– Systole phase– Diastole phase

CARDIAC CYCLECARDIAC CYCLE• Systole Phase

– Contraction phase

– Blood ejected

– Atrial Systole (0.1 sec.)• Following passive filling

with blood

• Atrial pressure rises above ventricular pressure

• And AV valves open, semilunar valves closed

• Ventricles fill with blood

RA

LA

RVLV

semilunar valves (closed)

tricuspid (open)

bicuspid (open)

CARDIAC CYCLECARDIAC CYCLE• Systole Phase (cont.)

– Ventricular Systole (0.3 sec.)• AV and semilunar

valves closed until pressure opens semilunar valves

• Blood pushed into pulmonary trunk and Aorta

• 120 mm Hg pressure• Atria in diastole

RA

LA

RVLV

tricuspid (closed)

bicuspid (closed)

semilunar valves (open)

CARDIAC CYCLECARDIAC CYCLE

• Diastole Phase– Relaxation phase

– Ventricular Diastole• Follows ventricular

systole• AV valves reopen and

filling begins• 80 mm Hg pressure

RA

LA

RVLV

semilunar valves (closed)

tricuspid (open)

bicuspid (open)

Heart Sounds

• First heart sound or “lubb”– Atrioventricular valves and surrounding fluid

vibrations as valves close at beginning of ventricular systole

• Second heart sound or “dupp”– Results from closure of aortic and pulmonary

semilunar valves at beginning of ventricular diastole, lasts longer

Mean Arterial Blood Pressure

• BP is important for blood movement• Blood flows from higher to lower pressure• During one cardiac cycle, blood flows from high pressure in aorta

from contraction of ventricles• Then towards the lower pressure in relaxed R. atrium

• Mean Arterial Pressure (MAP) = CO x PR– CO (Cardiac output) is amount of blood pumped by heart per

minute

– PR (Peripheral resistance) is total resistance against which blood must be pumped

• CO = HR x SV– HR: Heart rate (number of times heart beats per

minute)– SV: Stroke volume (blood pumped during each heart

beat)

CO = 72 bpm X 70 ml/beat = 5040ml/min (app. 5L/min)

• Starling’s law of the heart— the more the cardiac muscle is stretched, the stronger the contraction

• Important factor for stretching the heart muscle is venous return

• Greater the volume of blood returned to the heart by the veins, Greater the volume of blood the heart will pump

Cardiac Output

Regulation of the Heart

• To maintain homeostasis, amount of blood pumped by heart must vary:

• Eg. Cardiac output increases more during exercise than resting

• Intrinsic regulation: Results from normal functional characteristics of heart, not depend on neural or hormonal regulation

Regulation of the Heart• Extrinsic regulation: Involves neural and

hormonal control • Neural Control

– Parasympathetic stimulation• Supplied by vagus nerve,

acetylcholine is secreted,

decreases heart rate, maintain

heart beat average of 70 beats/min.

– Sympathetic stimulation– Supplied by cardiac nerves– Increases heart rate and force of

contraction. – Epinephrine and norepinephrine released.– Increased heart beat causes increased

cardiac output

Regulation of the Heart

– Hormonal Control– Epinephrine and

norepinephrine from the adrenal medulla

– Increases rate and force of heart contraction

– Occurs in response to increased physical activity, emotional excitement, stress

Heart and Homeostasis

• Effect of blood pressure– Baroreceptors monitor blood

pressure; in walls of internal carotids and aorta. This sensory information goes to centers in the medulla oblongata

Baroreceptor Reflex

Heart and Homeostasis• Effect of pH, carbon dioxide,

oxygen– Receptors that measure pH

and carbon dioxide levels found in hypothalamus

– Chemoreceptors monitoring oxygen levels found in aorta and internal carotids. Prolonged lowered oxygen levels causes increased heart rate, which increases blood pressure and can thus deliver more oxygen to the tissues.

Chemoreceptor Reflex-pH

Heart and Homeostasis• Effect of extracellular ion

concentration– Increase or decrease in

extracellular K+ decrease the heart rate

• Effect of body temperature– Heart rate increases when body

temperature increases, heart rate decreases when body temperature decreases

Effects of Aging on the Heart• Gradual changes in heart function, minor under resting

condition, more significant during exercise

• Hypertrophy of left ventricle

• Maximum heart rate decreases

• Increased tendency for valves to function abnormally

• Increased oxygen consumption required to pump same amount of blood

DISORDERSDISORDERS• Tachycardia

– Abnormally high heart rate (over 100)

• Bradycardia– Abnormally low heart rate (under 60)

• Fibrillation– Rapid and out of phase contractions

• Atherosclerosis– Formation of fatty plaque on artery walls– Decrease in vessel elasticity and possible blockage

Interrelationships between–Pressure–Flow–Resistance–And the control mechanisms that

regulate blood pressure and blood flowPlay important role in circulatory system

Laminar flow

– Blood flow in Streamlined fashion

– Interior of blood vessel is smooth and of equal diameter along its length

– Outermost layer moving slowest (move against resistance of stationary wall)

– And center layer moving fastest

Turbulent flow

– Interrupted

– Fluid passes a constriction, sharp turn, rough surface

– Partially responsible for heart sounds

– Sounds due to turbulence not normal in arteries and is probably due to some abnormal constriction

– And increases the probability of thrombosis

• Blood pressure: Measure of force exerted by blood against the blood vessel wall

• Blood moves through vessels because of blood pressure

• BP is measured in mm Hg

• Measured by Sphygmomanometer

• Measured by listening for Korotkoff sounds produced by turbulent flow in arteries as pressure released from blood pressure cuff (systolic pressure)

• No sound, continuous laminar flow, Diastolic pressure

• Pulse Pressure: Difference between systolic and diastolic pressures

• Healthy person 120 mm Hg systolic, 80 mm Hg diastolic

• Pulse pressure is 40 mm Hg

• Pulse pressure increases when stroke volume increases

• eg. During exercise, stroke volume increases, pulse pressure also increases

• Pulse pressure can be used to take a pulse to determine heart rate

• Most frequent site used to measure pulse rate is in the carpus with the radial artery- the radial pulse

Rate of flow through a tube is expressed as the volume that passes a specific point per unit of time. E.g.; cardiac output at rest is 5L/min, thus blood flow through the aorta is 5L/min

Blood flow = (P1 – P2/R)

P1 and P2 are pressures in the vessel at points one and two; R is the resistance to flow

Blood flow is directly proportional to pressure differences, inversely proportional to resistance

Resistance = 8vl/r4

v is viscosity, l = length of the vessel, r is the radius of the vessel, 8 and are constant

radius and viscosity determines resistance

Blood flow decreases when resistance increases

Since resistance is proportional to blood vessel diameter, constriction of a blood vessel increases resistance and thus decreases flow

Blood Flow =(P1 –P2)/R Poiseuille`s law = (P1 –P2)r4/8vl

According to Poiseuille`s law, small change in radius dramatically changes resistance to flow, r is raised to power 4

During exercise, blood vessels in skeletal muscle vasodilate, decreases resistance to blood flow

And blood flow through blood vessels increases dramatically

Viscosity: Is measure of resistance of liquid to flow

As viscosity increases, pressure required to flow increases

Viscosity of blood is influenced largely by hematocrit (percentage of the total blood volume composed of red blood cells)

Dehydration and/or uncontrolled production of RBCs

can increase hematocrit, thus increase viscosity

Higher viscosity increases the workload on the heart, heart failure can result

Blood Pressure varies directly with the following:

Blood Volume

Mainly regulated by kidneys

in blood volume = in B.P.

in blood vol. = decrease in B.P.

• By nervous system, kidneys and chemical controls

Nervous System Regulation:

Sympathetic nerve fibers:Vasoconstriction of blood vessels

diameter, resistance B.P.

Vasomotor center in medulla:Controls cardiac outputControls degree of vessel constriction

• Epinephrine and Norepinephrine - Vasoconstriction - cardiac output, B.P.

• ANF (Atrial Natriuretic Factor) - ANF act on kidney - Release of more sodium and water in urine - Loss of water and sodium in urine - blood volume B.P.

• ADH (Antidiuretic Hormone) - Stimulates kidneys to reabsorb water - blood volume B.P.

• Renin (Enzyme) - Released from kidneys in response to low B.P. - Stimulates angiotensin/aldosterone system - Kidneys reabsorb sodium and water blood volume and B.P.

RENIN / ANGIOTENSIN / ALDOSTERONE SYSTEMRENIN / ANGIOTENSIN /

ALDOSTERONE SYSTEM

• Kidneys may alter B.P. directly - Increased B.P. more blood filtered by kidneys - More urine produced and released - blood volume B.P.

• Kidneys may alter B.P. indirectly - Renin/angiotensin system activated with B.P. - Vasoconstriction, water reabsorption due to aldosterone release - blood volume B.P

RENAL REGULATION OF B.P.