733 Cardiovascular System

conduction of the heart

consists of specialized cardiac muscle cells that generate and transmit electrical impulses to ensure a coordinated heartbeat. This system is autorhythmic meaning it can initiate its own electrical signals without input from the nervous system, which only serves to modify the heart rate.

electrical impulse traveling through the heart first step

sinoatrial (SA) node

electrical impulse traveling through the heart second step

Atrioventricular (AV) node

electrical impulse traveling through the heart third step

Atrioventricular (AV) Bundle of His

electrical impulse traveling through the heart fourth step

right and left bundle branches

electrical impulse traveling through the heart fifth step

Purkinjie fibers

SA node

the pacemaker initiating the electrical impulse across the atria

AV node

receives the signal from the SA node and delays it for about 0.1seconds to allow the atria to finish contracting and pushing blood into the ventricles before the ventricles contract

AV Bundle of His

is an electrical impulse between the atria and the ventricles and carries impulse from the AV node to the bundle branches

right and left bundle branches

conduct impulses through the interventricular septum to the Purkinjie fibers

Purkinjie fibers

rapidly distribute the electrical impulse, stimulating the ventricular muscle to contract in a coordinated, upward motion from the apex

SA node intrinsic rate

60-100 beats per min

AV node intrinsic rate

40-60 beats per min

Purkinjie fibers intrinsic rate

20-40 beats per min

cardiac plexus

branches of the autonomic nervous system located both superficial and deep to the aortic arch that supply the conduction system (SA and AV nodes), coronary vessels, and cardiac muscle cells

sympathetic innervation of the cardiac plexus

• Cervical and upper thoracic ganglia travel as cardiac nerves in the cardiac plexus

• Sympathetic activation increases the rate and force of contraction

• T1-T5 sympathetic nerves that travel back to the spinal cord are responsible for angina

parsympathetic innervation of the cardiac plexus

• Parasympathetic fibers travel in the vagus nerve, leave cardiac branches, and synapse in the cardiac plexus and heart

• Parasympathetic activation decreases the rate and force of contraction

• Sensory fibers traveling with the vagus nerve back to the brain stem mediate cardiac reflexes  

day 19 of cardiovascular embryonic development

heart development begins with the formation of a pair of lateral endocardial tubes which will soon fuse to form the primitive heart tube

day 22 of cardiovascular embryonic development

the primitive heart tube begins to beat

day 23 of cardiovascular embryonic development

the tube starts to fold and loop, displacing the future chambers into their general adult positions

day 24 of cardiovascular embryonic development

the heart begins to circulate blood

sinus venosus

develops into the smooth-walled part of the right atrium, the coronary sinus, and the SA node

primitive atrium

gives rise to the muscular, ridged parts of the right and left atria, known as the pectinate muscles

primitive ventricle

becomes trabeculated (muscular, ridged)portions of the right and left ventricles. It is the strongest pumping chamber of the developing heart

bulbus cordis

the lower part differentiates into the smooth outflow tracts of both ventricles

truncus arteriosus

this cranial extension of the bulbus cordis ultimately forms the ascending aorta and the pulmonary trunk

atrial separation embryonic development of the heart

two septa, the septum primum and the septum secundum, grow from the roof of the primitive atrium to divide it into right and left chambers

foramen ovale embryonic development of the heart

an opening that allows for the continued shunting of blood from the right atrium to the left atrium and at birth, the rise in left atrium pressure forces this shut

ventricular and outflow tract separation embryonic development of the heart

divides into the ascending aorta and the pulmonary trunk by a pair of spiraling truncoconal septa which grow downwards to complete the interventricular septum

valve formation embryonic development of the heart

the AV valves and their supporting chordae tendinae and papillary muscles are sculpted from surrounding myocardium between weeks 5-8. By the end of week 8, a heart with all its definitive structures is functioning

the pathway of fetal blood flow

1. Oxygen from the placenta

2. first shunt (ductus venosus)

3. Oxygenated blood mixes with deoxygenated blood in the IVC before entering RA

4. Second shunt (foramen ovale)

5. Third shunt (ductus arteriosus)

6. Return to placenta for re-oxygenation via two umbilical arteries

fetal blood flow transition at birth

• lungs inflate causing a drop in pulmonary resistance and allowing blood to flow into the pulmonry circulation

• pressure in LA rises above RA functionally closing the foramen ovale

• increased blood oxygen triggers the constriction of the ductus arteriosus and closes within days

• the ductus venosus also closes forcing blood to pass through the liver

cardiac cycle

the sequence of mechanical and electrical events that occur during a single heartbeat. It is compromised of 2 principle phases: systole and diastole. (0.8 seconds)

diastole

the period of ventricular relaxation and filling

systole

the period of ventricular contraction and ejection

phases of the cardiac cycle

1. isovolumetric relaxation

2. ventricular filling

3. atrial systole

4. isovolumetric contraction

5. ventricular ejection

Isovolumetric relaxation

the initial phase of diastole immediately following ventricular ejection. The semilunar valves close producing the second heart sound (S2). All cardiac valves are closed. The ventricular myocardium relaxes, leading to rapid decline in intraventricular pressure.

ventricular filling

when intraventricular pressure falls below the pressure within the atria, the atrioventricular valves (mitral and tricuspid) open. This allows blood that has accumulated in the atria to flow passively into the ventricles.

atrial systole (atrial kick)

to complete ventricular filling, the atria contract, actively ejecting the final 20-30% of blood into the ventricles. The stretch on these ventricular muscle fibers is defined as preload.

isovolumetric contraction

ventricles begin to contract, increases intraventricular pressure, AV valves close and produces the first heart sound (S1). For a brief interval, all valves remain closed and ventricular pressure rises sharply but ventricular volume remains constant.

ventricular ejection

once intraventricular pressure surpasses the pressure in the aorta and pulmonary artery, the semilunar valves are forced open and ventricular ejection begins

when is S1 sound heard

isovolumetric contraction

when is S2 sound heard

isovolumetric relaxation

afterload

the resistance the ventricles must overcome to eject blood

stroke volume

the volume of blood ejected from a ventricle during a single contraction

ejection fraction

the proportion of blood ejected from the ventricle relative to its end-diastolic volume (55-70%)

preload

the stretching of the cardiac muscle fibers at the end of diastole just before contraction- it is determined by the volume of blood in the ventricles at the end of this phase.

contractility

the intrinsic ability of the heart muscle to pump blood with a given force

cardiac output

the total blood volume pumped by the heart per minute (CO= SV * HR)

Frank Starling Mechanism

the rule that the heart pumps out more blood if more blood fills it up (more in, more out)

blood pressure

=cardiac output * total peripheral resistance

total peripheral resistance (TPR)

is determined primarily by the diameter of the arterioles

vasoconstriction

raises TPR and blood pressure

vasodilation

lowers TPR and blood pressure

baroreceptor reflex

when baroreceptors found in the carotid sinus and the aortic arch continuously sense arterial wall stretch to signal an increase or decrease in sympathetic/parasympathetic activity to control heart rate and blood vessels to increase/decrease blood pressure

baroreceptor reflex pathway example

decrease VP → decrease baroreceptor firing→ increase sympathetic activity → increase HR, contractility, vasoconstriction

baroreceptor reflex importance

this reflex is the body’s primary rapid mechanisms for stabilizing blod pressure, such as with postural changes or exertion

basic pathophysiology principle of the baroreceptor reflex

Because BP and perfusion both depend on CO and TPR, a change in any single input (heart rate, contractility, preload, afterload, or vessel diameter) can shift blood pressure or reduce downstream tissue perfusion even when the other inputs are normal

heart sounds

Normal is “lub-dub.” The presence of abnormal sounds, such as murmurs or gallops can indicate valvular heart disease, heart failure, or other structural defects

ECG/EKG

a test recording the electrical activity of the heart

ECG/EKG importance

It detects arrhythmias, myocardial infarction, and other cardiac abnormalities by analyzing the heart’s electrical signals

cardiac biomarkers

are proteins and enzymes released into the blood when the heart muscle is damaged, stressed, or lacking oxygen

troponin levels

elevated levels are a key marker for diagnosing MI and assessing the extent of heart damage

lipid profile

high levels of LDL and triglycerides increase the risk of atherosclerosis and heart disease

beta blockers

decrease heart rate and contractility by blocking sympathetic B1 effects on the SA node and the myocardium. This leads to decreased cardiac output, decreased myocardial O2 demand, decreased heart rate, increase EDV, increase preload, and decreasing afterload.

calcium channel blockers

relax smooth muscle which leads to a decrease in TPR/afterload, some agents decrease HR and contractility and have little to no effect on preload.

ACE inhibitors/ARBs

block angiotensin II formation or action which leads to vasodilation (decreasing afterload) and aldosterone-driven fluid retention (decreasing preload) making it easier for the heart to pump blood.

diuretics

decrease the circulating blood volume which results in a decrease in preload and a decrease in venous return.