PHYS - Week 8 [cardiovascular system]

MAP

What does “cardiovascular” mean?

• made up of Heart (cardio)

• made up of blood vessels [arteries, capillaries and veins] (vascular)

Role of cardiovascular system. Why do we need it?

1. Transporting substances around the body

2. Chemical signalling - hormones travelling thru blood

3. Immune and inflammatory responses - WBC travel in blood to fight infection, respond to injury & mediate inflammation

4. Heat regulation - Blood helps control body temperature.

What are the 3 main blood vessel types and oxygenation rules?

1. Arteries - carriers oxy blood; except for pulmonary artery which carries deoxy blood

2. Capillaries

3. Veins - carries deoxy blood; except for pulmonary vein which carries oxy blood

Explain the systemic and pulmonary circulation sequence:

Systemic (Oxy): Left heart → aorta → large arteries → arterioles → capillaries (gas exchange occurs here) → venuoles → large veins → vena cava → right heart

Pulmonary (deoxy): Right heart → pulmonary artery → lung → Pulmonary Vein → Left Heart

Superior vs Inferior Vena Cava (regions from where they collect blood)

Superior vena cava: collect blood from head, neck and upper body

Inferior vena cava: Collects blood from lower body

What 3 factors dictates blood flow to different parts of body?

1) How much blood flows past a certain point within a given time period?

2) Decreases with increased resistance (affected by width - body can change this factor the fastest, viscosity and length)

2) Increases with increased pressure difference

What is vascular tone?

Blood vessels are never completely “off”, so they always have some baseline contraction called vascular tone. It’s like a dimmer switch that can turn up/turn down contraction, but never fully turn it off.

What is the effect of pressure gradient on flow?

Higher pressure difference/gradient = greater flow

• blood will move from area of high pressure to low pressure

Where does blood have the lowest pressure and where does it have the lowest velocity?

• Lowest blood pressure:

  • Blood pressure is lowest in vena cava because pressure steadily decreases as blood moves through systemic circulation back to the heart.

• Lowest blood velocity:

  • Blood velocity is lowest in the capillaries, because capillaries are the site of exchange. If blood moved too fast, oxygen delivery would be poor & nutrient exchange would fail.

  • also bcs cappilaries have huge cross sectional area, which slows flow velocity down.

Blood vessel structure:

From outside → inside:

1. Connective tissue

2. Elastic tissue

3. Smooth muscle

4. Endothelium

• all vessels have endothelium, but other layers vary according to function

Structural properties of arteries:

• thick walls - imp. as heart pumps in bursts. Arteries stretch when blood is pumped to them. Then, elastic recoil helps keep blooding moving even between heartbeats

• lots of elastic tissue

• high pressure

Structural properties of veins:

• thinner walls

• less elastic

• wider lumen

• low pressure

• act as blood reservoirs: large veins can store extra blood. The body can redirect this blood when needed.

• don’t stretch easily (low compliance) - but higher than arteries

Purpose of valves being present in veins:

Valves prevent backflow ad help blood move towards the heart.

What are the key differences between arteries and veins?

• Arteries don’t have valves, whilst veins do.

• arteries typicaly carry oxy blood (except for pulmonary arteries), whilst veins carry deocy blood.

• Arteries are thicker and more elastic, whilst veins are thinner and have less elasticity

Structural properties of cappilaries:

• single-cell thick - so very thin walls

• site of exchange - which is why they have the slowest velocity

Thin walls are important because diffusion works best across short distances.

What are fenestrated capillaries?

Some capillaries contain tiny windows called fenestrations.

Fenestrations are important in organs like the kidney, as they allow easier filtration.

Capillary exchange (How do we get things across body)?

Means of transport across capillaries:

• Vesicular transport: package things into vesicles and move them off

• Diffusion: Movement down a concentration gradient (ex//O2 diffusing into tissues)

• Bulk flow: Mass movement of fluid due to pressure differences.

Filtration vs Absorption

Filtration: Fluid moves out of capillaries & into tissue

Absorption: Fluid moves into capillaries from tissue

What’s Oedema?

Excess fluid in tissues, which causes swelling

The oncotic (osmotic) pressure attracting fluid absorption from interstitium into capllaries is a result of:

Proteins such as albumin

Oncotic pressure is the pulling force created by plasma proteins (mainly albumin) in the blood that draws water from the interstitial fluid back into the capillaries.

What’s hydrostatic pressure?

Pressure exerted by fluid. In capillaries, hydrostatic pressure pushes fluid OUT.

What is same and different about Plasma & Interstitial fluid

• SAME: Composition of both is same, as they have almost equal solute conc

• DIFF: Plasma have lots of proteins, whilst interstitial fluid has low amt of proteins

2 Bulk flow pressures in capillaries

Bulk flow between capillaries and interstitial fluid is controlled by 2 pressures:

(1) Oncotic/osmotic pressure (π): caused mainly by plasma proteins like albumin, which pulls water into capillaries (reabsorption)

(2) Hydrostatic pressure (P): caused by fluid pushing against vessel walls, which pushes water out of capillaries (filtration).

4 Starling forces

Starling forces are the 4 pressures that control movement of water between capillaries and tissues.

• Capillary hydrostatic pressure (Pc) pushes water out of capillaries

• plasma oncotic pressure (πp) pulls water into capillaries

• interstitial hydrostatic pressure (PIF) pushes water into capillaries

• interstitial oncotic pressure (πIF) pulls water out of capillaries into tissues.

Starling forces balance

Hydrostatic pressure pushes water out of capillaries, while oncotic (colloid osmotic) pressure pulls water into capillaries.

Net movement of water depends on which force is stronger:

• if pushing out is stronger → filtration

• if pulling in is stronger → absorption.

Hydrostatic pressure decreases along the capillary, so the balance changes along its length.

The Cardiac Cycle steps:

• electrical activity

• muscle contraction

• pressure changes

• valve opening/closing

• blood flow

Then repeats again and again.

Why is the left ventricle thicker than the right?

Because:

• pulmonary resistance is lower than systemic resistance

The:

• right ventricle → only pumps to lungs

• left ventricle → pumps to whole body

So:

• left ventricle needs MUCH more force

• therefore it has more muscle

Why does a heart attack occur? Also include another name for heart attack:

Heart attack = Myocardial infarction

This happens when:

• coronary vessels become blocked & heart muscle loses blood supply

What’s the Electrical Conduction System?

It’s a very specific sequence with which the heart contracts.

Unlike skeletal muscle, there are no normal neuromuscular junctions controlling every contraction. Instead, the heart has its own electrical conduction system.

What’s the SA node?

SA node = pacemaker (initiates electrical impulse)

• Its located in the right atrium near the superior vena cava

• It contains autorhythmic cells - means cells that make their own rhythm

  • these cells spontaneously depolarise & generate heartbeat automatically

How does the SA Node generate Action Potentials?

The SA node slowly depolarizes because of sodium leak channels.

• This causes memb potential to drift upwards until threshold is reached and then action potential fires.

• Then, repolarization occurs, but sodium leak continues, so cycle repeats automatically.

SA node heart rate = ~70 beats/min

Electrical Pathway sequence Through the Heart:

1) SA node fires

2) signal spreads across atria

3) reaches AV node

4) pauses briefly

5) travels down Bundle of His

6) down the septum

7) to apex

8) through Purkinje fibers

9) spreads upward through ventricles

Why Does Ventricular Contraction Start at the Apex?

If squeezing started randomly, blood could move inefficiently. Instead, contraction starts at apex & moves upwards. The blood gets pushed to aorta & pulmonary artery.

What’s the main role of atria?

It breifly delays conduction - This pause allows atria to finish contracting & ventricles to fill BEFORE ventricular contraction.

What’s the main role of Purkinje fibres?

Purkinje fibers rapidly conduct electrical signals & spread depolarization through ventricles.

What does ECG stand for and what does it measure?

ECG = Electrochardiogram

• It measures electrical activity of heart using surface electrodes

ECG Waves

P wave: atrial depolarisation

QRS complex: Ventricular depolarization

T wave: Ventricular repolarization

What Causes Each ECG Component?

P WAVE:

• SA node fires, electrical activity spreads thru atria, atria contract = creates P wave

QRS WAVE:

• Signal travels down septum, thru ventricles, ventricles depolarise + contract

• Large signal is creates bcs ventricles contain lots of muscle

T Wave:

• ventricular repolarisation

• Atrial repol is hidden bcs atria are small and signal gets burried in QRS complex.

Cardiac Muscle Plateau

Unlike skeletal muscle, Cardiac muscle has prolonged calcium entry creating plateau phase.

This allow longer contraction & more effective pumping

Sequence of Contractions (regarding atria & ventricles)

• atrial systole

• atria relax

• ventricular systole

• ventricular relaxation

• whole heart relaxes

Isovolumetric contraction and relaxation:

Isovolumetric Contraction: ventricles contract, ALL valves closed, blood cannot move, volume stays same (pressure rises rapidly)

Isovolumetric Relaxation: Ventricles relax - valves still closed, volume unchanged (pressure drops rapidly)

Cause for lub dub sound of heart:

“Lub”: Closure of AV valves (systole)

“Dub”: Closure of semilunar valves

When do AV valves open? (regarding atrial and ventricular pressure)

AV valves open when:

• atrial pressure > ventricular pressure

AV valves close when:

• ventricular pressure > atrial pressure

When do semilunar valves open - sames as aortic/pulmonary valve? (regarding atrial and ventricular pressure)

They open when:

• ventricular pressure > arterial pressure

They close when:

• arterial pressure > ventricular pressure

Are valves active/passive? What drives them to open?

Passive. Pressure differences alone control them.

What’s Valve Prolapse?

If valve becomes floppy:

• doesn’t close properly

• allows backflow/regurgitation

This is inefficient.

What’s Atrial Fibrillation (AFib) & Ventricular Fibrillation (VFib)?

AFib: irregular atrial electrical activity

• Danger: blood stagnates, clot forms

VFib: ventricles quiver randomly, no coordinated contraction

• Danger: life-threatening

• ECG looks chaotic

What does a Defibrillator do?

gives massive electrical shock, resets electrical activity & allows SA node to restart rhythm

If P waves are present, but QRS wave is missing from ECG, what does this mean?

SA node working, atria depolarizing, but signal NOT getting through AV node. Therefore, it must be an AV nodal block.

Describe atrial and ventricular systole:

Atrial Systole: Atria contract → atrial pressure rises → AV valves open → blood enters ventricles

This increases:

• ventricular filling

Ventricular Systole: Ventricles contract → ventricular pressure rises

• occurs when ventricular pressure > atrial pressure

What’s the ejection phase?

As ventricular pressure keeps rising, eventually, ventricular pressure > aortic pressure. This causes aortic valve to open. Blood leaves ventricle. Ventricular volume decreases.Aortic pressure increases.

When does ventricular relaxation occur?

when aortic pressure > ventricular pressure (this causes dub sound from lub dub)

When does passive Ventricular Filling occur?

When ventricular pressure < atrial pressure. AV valves reopen. Blood passively flows into ventricles even BEFORE atrial contraction. Then P wave occurs and cycle repeats.

Pressure-Volume Relationships (ventricular volume)

As:

• ventricular volume rises
  → pressure can rise

As:

• blood ejected
  → ventricular volume falls

How Blood Moves Around the Body

1. The Heart: The heart contracts and pushes blood through vessels.

2. Elastic Arteries: Arteries stretch when blood enters, then recoil. This helps keep blood moving even when the heart relaxes.

3. Gravity: Gravity is ALWAYS pulling blood downward.

4. Skeletal Muscles: Muscles squeeze veins and help push blood back to the heart (a.k.a skeletal muscle pump)

5. Breathing (Respiratory Pump): Breathing changes pressure inside your chest. This helps “suck” blood back toward the heart.

Systolic vs Diastolic Blood Pressure:

Systolic Blood Pressure: Is the highest arterial pressure

• occurs during ventricular contraction & cardiac systole

• ex// 120 in 120/80

Diastolic Blood Pressure: Is the lowest arterial pressure.

• Occurs during ventricular relaxation & cardiac diastole

• ex// 80 in 120/80

What’s pulse pressure & formula for it?

Pulse pressure = systolic pressure − diastolic pressure (ex//120-80 = 40 mmHg)

• Pulse pressure tells how strong pulse is

What’s Mean Arterial Pressure (MAP) & formula for it?

MAP = average arterial pressure. (determines whether blood can actually reach tissues properly)

Formula: MAP=DBP+1/3​(SBP−DBP)

• DBP = diastolic blood pressure

• SBP = systolic blood pressure

If MAP = average, why’s it not directly in the middle of DBP & SBP?

Because arteries spend MORE time near diastolic pressure than systolic pressure.

What happens in If MAP Is Too Low/to high?

If MAP Is Too Low: Organs don’t get enough blood.

• This can cause:

  • dizziness

  • fainting (syncope)

  • organ failure

If MAP Is Too High: Tiny vessels can rupture.

• Can cause:

  • strokes

  • aneurysms

  • vessel damage

Explain the elastic recoil of arteries:

When the heart pumps, arteries stretch & elastic energy is stored.

When the heart relaxes, arteries recoil & continue pushing blood forward. This keeps blood flowing during diastole.

What Determines Mean Arterial Pressure other than DBP & SBP?

MAP=CO×TPR

• CO = cardiac output - “How much blood the heart pumps per minute”

• TPR = total peripheral resistance - “How hard it is for blood to move through vessels”

Formula for Cardiac Output

CO=HR×SV

• HR = heart rate

• SV = stroke volume - “How much blood leaves the ventricle each beat”

Stroke Volume Formula

SV=EDV−ESV

• EDV = end diastolic volume - How much blood is in the ventricle BEFORE contraction.

• ESV = end systolic volume - How much blood is LEFT AFTER contraction.

What does Frank-Starling Mechanism say (relating to optimal conditions)?

More filling → more stretch → stronger contraction, but only up to an optimal point.

Too much stretch = weaker contraction.

What factors Affect Stroke Volume - “How much blood leaves the ventricle each beat”?

1. Venous Return:

More blood returning to heart → more filling → greater EDV →greater stroke volume.

2. Sympathetic Nervous System

Sympathetic stimulation:

• makes heart contract harder

• increases stroke volume

3. Hormones

Especially:

• adrenaline

• thyroid hormones

These increase contractility.

What’s Venous Return?

How much blood returns to the heart through veins.

Describe process of Skeletal Muscle Pump:

When muscles contract:

• they squeeze veins

• valves stop backflow

• blood is pushed upward toward heart

This helps fight gravity.

Why Long Flights Cause Swollen Ankles?

When sitting still a long time muscles aren’t pumping veins, blood pools in legs, pressure rises & fluid leaks into tissues.

This results in swollen feet and ankles.

Walking helps because muscle contractions restart venous return.

Explain Respiratory Pump:

During inspiration, thoracic pressure becomes more negative. This pulls air into lungs & also helps pull blood toward heart.

Sympathetic vs Parasympathetic effect on Heart Rate:

Sympathetic stimulation: increases sodium permeability, SA node reaches threshold faster

Parasympathetic stimulation: decreases sodium permeability, hyperpolarizes cells, slower depolarization, slower heart rate

Vasoconstriction vs Vasodilation:

Vasoconstriction (When smooth muscle contracts):

• vessel radius decreases

• resistance increases

• blood flow decreases

Vasodilation (When smooth muscle relaxes):

• vessel widens

• resistance decreases

• blood flow increases

Compensation When MAP Falls

Body responds by:

• increasing heart rate

• increasing stroke volume

• increasing cardiac output

• increasing TPR

All help restore MAP.

Standing Up Too Quickly example

When standing quickly:

• blood pools in legs

• venous return decreases

• stroke volume decreases

• cardiac output decreases

• MAP decreases

Brain gets less blood →

• dizziness

• fainting

What’s the Baroreceptor Reflex and How Baroreceptors Work?

Main SHORT-TERM blood pressure control system.

Baroreceptors are stretch receptors located in:

• carotid sinus

• aortic arch

How Baroreceptors Work:

• More stretch → more firing.

• Less stretch →less firing.

If Blood Pressure Falls:

Less stretch on baroreceptors →
LESS firing.

This causes:

• MORE sympathetic activity

• LESS parasympathetic activity

Result:

• ↑ heart rate

• ↑ contractility

• ↑ vasoconstriction

• ↑ TPR

• ↑ MAP

If Blood Pressure Rises

Opposite happens.

More stretch →
more baroreceptor firing →

• ↓ sympathetic activity

• ↑ parasympathetic activity

Result:

• lower HR

• lower TPR

• lower MAP

VERY IMPORTANT OVERALL EQUATION

MAP=HR×SV×TPR

Because:

• CO = HR × SV

• MAP = CO × TPR

So, any change in:

• HR

• SV

• TPR

will change MAP.

Long-Term Blood Pressure Control

Short-term:
mostly nervous system (baroreceptors).

Long-term:
mostly hormones + kidneys + blood volume regulation.

Standing Up (Orthostatic Challenge)

When you suddenly stand up, you go from supine (lying down) position to standing up.

• causes gravity to suddenly pull blood downwards into legs. So, less blood immediately returns to heart (less venous return)

  • That means, less blood fills the heart, less blood gets pumped out, blood pressure falls temporarily

Why does stroke volume fall when standing?

Because gravity causes blood to pool in legs. This means less blood returns to the heart (venous return)

Less venous return = less filling of ventricles = lower end-diastolic volume (EDV) = lower stroke volume

What controls heart rate quickly?

NOT HORMONES (hormones are for long term pressure stablisation).

It’s the ANS. Specifically:

• Sympathetic nervous system (SNS)

• Parasympathetic nervous system (PNS)

What’s Total Peripheral Resistance (TPR)?

how constricted all the blood vessels are overall:

If vessels constrict:

• resistance increases

If vessels dilate:

• resistance decreases

Active Standing vs Passive Tilt

Active standing: You stand up yourself.

Passive tilt: A bed tilts you upright.’

Both involve gravity, but Active standing uses muscles, whilst passive tilt doesn’t.

Anticipatory Response (“Feed-forward”)

Even THINKING about exercise can activate the cardiovascular system.Your brain prepares the body before movement begins.

Effects:

• ↑ sympathetic activity

• ↓ parasympathetic activity

• ↑ heart rate

• ↑ contractility

• slight vasoconstriction

• slight ↑ MAP

Whilst the SNS generally causes vasoconstriction for blood supply during exercise, exercising muscles need MORE blood. So how does muscle still dilate?

Local Metabolic Control: Active muscles release metabolites. These metabolites signal that more blood is needed. So local vasodilation overrides sympathetic vasoconstriction.

Blood Flow Redistribution During Exercise

At rest:

• kidneys get lots of blood

• muscles get moderate blood

During exercise:

• kidneys get less

• skeletal muscles get MUCH more

What’s Myogenic Response?

When a blood vessel stretches:

• smooth muscle stretches

• mechanically gated calcium channels open

• calcium enters

• smooth muscle contracts

This opposes overstretching. So vessels autoregulate themselves.

What happens to TPR during exercise?

This is the balancing act:

SNS → vasoconstriction

BUT

Active muscles → strong local vasodilation

Overall:

• TPR generally FALLS during dynamic exercise

Baroreceptor Reflex Resetting

Normally high blood pressure triggers reflex lowering of HR.

BUT during exercise, HR and BP both rise. This occurs because the baroreceptor reflex “set point” is reset during exercise. The brain ALLOWS higher pressure temporarily.

Static vs Dynamic Exercise

Static exercise

Example:

• holding a squat

• forearm squeeze

Muscles contract without much movement.

Dynamic exercise

Example:

• cycling

• running

Large rhythmic movements.

Does static/dynamic exercise cause larger MAP increase?

Static exercise often raises MAP MORE.

Because:

• less total muscle vasodilation occurs

• TPR stays higher

Whereas dynamic exercise:

• massive vasodilation in many muscles

• TPR drops more

So BP rise is somewhat buffered.

Thermoregulation During Exercise

Exercise creates lots of heat. To lose heat, skin blood vessels dilate & more blood goes to skin. That’s why skin gets red, you feel hot and sweat.