Module 3.1: Cardiovascular system

What is the cardiovascular system comprised of?

• heart

• blood vessels

heart

• right atrium and right ventricle: carries deoxygenated blood

• left atrium and left ventricle: oxygenated blood

• very important for respiration and waste removal

blood vessels

arteries → arterioles → capillaries → venules → veins

What is the cardiorespiratory system comprised of?

• cardiovascular system + lungs

• O2 diffuses into blood, CO2 diffuses into lungs

Functions of blood

• transport

• regulation

• protection

What is blood made of?

fluid connective tissue comprising of 3 layers

• plasma: accelular

• buffy coat: platelets, wbc

• hematocrit: rbc

what are the types of circulatory systems

• pulmonary circulation

• systemic circulation

• coronary circulation

outline pulmonary circulation

• low pressure circuit

• role is to oxygenate the blood

• short distance between heart and lungs

• right ventricle → lungs → left atrium

steps of pulmonary circulation

1. right ventricle ejects deoxygenated blood via pulmonary valve into pulmonary trunk

2. pulmonary trunk → arteries → arterioles → capillaries

3. O2 from lungs diffuses into blood, while CO2 diffuses into the lungs to be exhaled

4. oxygen rich blood carried by pulmonary veins to left atrium

outline systemic circulation

• high pressure circuit

• left ventricle → body → right atrium

• delivers oxygenated blood

steps of systemic circulation

1. left atrium ejects oxygenated blood into left ventricle via mitral valve

2. left ventricle ejects blood into aorta via aortic valve

3. aorta → arteries → arterioles → capillaries

4. O2 from blood diffuses into body, CO2 and waste from body diffuses into blood

5. superior and inferior vena cava deliver deoxygenated blood to the right atrium

6. right atrium → right ventricle to begin pulmonary circulation

myocardium

• muscle layer of the heart

• thicker in the left ventricle

• made of billions of cardiomyocytes

why is the left ventricle thicker than the right?

• right ventricle pushes blood to lower pressure circulation - less force

• left ventricle pushes blood to a higher pressure circulation

cardiomyocytes

• the muscle fibres/cells of the heart

• involuntary

structure of cardiomyocytes

• intercalated discs: join cardiomyocytes

• desmosomes: special proteins which hold cells together during contrations

• gap junctions: synchronise action potentials, allow rapid transfer of action potential between cardiomyocytes

• T-tubules: invaginations of the sarcolemma(cell membrane), allow the action potential to travel rapidly into the single cardiomyocyte

phases of cardiomyocyte action potnetial

• Phase 4: Na+ and Ca2+ channels are closed, K+ exits cardiomyocytes via opened K+ channels - starts negatively charged

• Phase 0: rapid influx of Na+ via opened Na+ channels, membrane potnetial increasing to -70mV and +30mV

• Phase 1: K+ exits cardiomyocytes via opened K+ channels, causing a drop in membrane potential

• Phase 2: Ca2+ influx via L-type Ca2+ channels balancing K+ efflux

• Phase 3: Ca2+ channels close, K+ remains open returning membrane potential to -90mV

cardiomyocyte excitation-contraction coupling

• there is a delay between initiation of a cardiomyocyte action potential and muscle contraction

1. AP travels along the sarcolemma and T-tubules

2. DHP channels open to allow Ca2+ influx

3. this triggers Ca2+ release from the sarcoplasmic reticulum via Ryanodine (RyR) receptors

4. Ca2+ binds to troponin C to expose myosin binding sites on actin. Myosin binds to binding sites on actin, hydrolyse ATP and shorten the sarcomere

5. Ca2+ return to the SR, stopping contraction

How does action potentials work in the heart?

• an action potential is required to contract the myocardium

• the heart generates its own action potentials without interacting with the nervous system

• initiates the heart’s rhythmic contraction (systole) and relaxation (diastole) to allow it to pump blood efficiently

How does the heart generate its own action potential?

1. sinoartirial (SA) node

• the heart’s primary pacemaker cell

• right atrium

• initiates ap spontaneously and sends them rapidly via specialised networks

2. atrioventricular node

• receives SA node impulse and delays it to prevent the atria and ventricles from contracting at the same time

3. Bundle of His

• from the AV node, ap travels down the Bundle of His which branches into the right and left branches at the interventricular septum which then travels into the purkinje fibres

4. cardiomyocytes repolarise and return to their resting state

Electrocardiogram (ECG/EKG)

• measures and records the heart’s electrical activity

• heart rate, regularity, size and position of chambers, presence of damage to heart, effect of drugs or devices

• since the left ventricle is bigger, it will skew the electrical impulse to itself

Different components of the ECG

• P-wave: SA node causing atrial depolarisation and contraction (systole)

• P-Q interval: AV node delays signal from SA
  QRS complex: depolarisation of ventricles to cause ventricular contraction (systole)

• T-wave: ventricular diastolic/relaxation

Blood pressure

• force that blood exerts against the walls of arteries as the heart contracts or relaxes

blood pressure parameters

• systolic pressure

• diastolic pressure

• pulse pressure

• mean arterial pressure

systolic pressure

highest pressure in the arteries when ventricles contract and blood flows into arteries

diastolic pressure

lowest pressure in the arteries when ventricles relax and blood flows into arterioles

pulse pressure

• systolic minus diastolic pressure (indicative of stroke volume)

mean arterial pressure (MAP)

• average pressure in arteries

How does the blood flow consistently?

Due to the high to low blood pressure gradient

Normal blood pressure

120mmHg/80mmHg measured from the arteries in the arm

Hypertension (what are the physiological, pathological, chronic causes)

130mmHg/80mmHg or higher

• stress, exercise

• stiff blood vessels

• left ventricular hypertrophy

hypotension

90mmHg/60mmHg or lower

• excessive bleeding

• dehydration

• drugs

baroreceptor reflex

• role is to monitor and regulate blood pressure

• either adjusts to the heart (left) or blood vessels (right)

• located in carotid sinus and aortic arch

how does the baroreceptor reflex increase/decrease blood pressure?

• increase stroke volume: to increase BP

• increase heart rate; to increase BP

• vasoconstriction: to increase BP

• vasodilation: to decrease BP

end diastolic volume, end systolic volume, stroke volume

• end diastolic volume: volume of blood in ventricle before contraction

• end systolic volume: volume of blood in ventricle after contraction

• stroke volume: volume of blood ejected by ventricle per beat - difference between end diasolic and systolic volume

formula for cardiac output

cardiac output = stroke volume x heart rate

formulas for mean arterial pressure (MAP)

MAP = cardiac output x total peripheral resistance (invasive)

MAP = diastolic pressure + 1/3 pulse pressure

relationship between stroke volume and cardiac output

increase stroke volume → increase cardiac output

preload

• stretch of the cardiac myocytes

• volume of blood in the ventricle at the end of diastole

• increase end diastolic volume → increases the preload

afterload

• how much force the heart is generating when contracting

• resistance to pushing the blood into the circulatory system

• need to overcome this resistance by producing the afterload

venous return

• how much blood volume is coming into the right atrium

• from the superior and inferior vena cava

• increase venous return → supports end diastolic volume

Frank-starling law

• increasing end diastolic volume → increase stroke volume

• increasing preload → higher stretch, contractility (afterload), increased stroke volume