CARDIOVASCULAR

PHYSIOLOGY

LECTURE 1

Ana-Maria Zagrean MD, PhD

Circulation – an evolutionary need

• Isolated/single cells vs. multicellular organisms

• Connection between external - internal milieu*: from simple diffusion to complex, highly regulated, circulatory systems

- Cellular membranes exchange surfaces with the external milieu (lungs, gut, kidney)

- The need to develop two pumps, a dual circulatory system in mammals and birds

Role of the circulatory system in promoting diffusion.

Nutrients and wastes exchange across two barriers: a surface for equilibration between the external milieu and blood, and another surface between blood and the central cell. Blood is the conduit that connects the external milieu (e.g., lumina of lung, gut, and kidney) to the internal milieu (i.e., extracellular fluid bathing central cells). The system is far more efficient, using one circuit for exchange of gases with the external milieu and another circuit for exchange of nutrients and nongaseous wastes.

Components of the Cardiovascular System:

1. a dual pump - the heart 2. the blood 3. a tubular system 4. a regulation system One pump (the heart) that circulates a liquid (the blood) through a set of containers (the vessels) in a regulated fashion.

Components of the Cardiovascular System:

1. a dual pump - admission chamber - atriums Right heart (RH) - ejection chamber - ventricles Left heart (LH)

2. the blood 3. a tubular system (two serial circuits): - systemic (high-pressure)/pulmonary (low-pressure) circulation

- vessels: arteries, veins, capillaries, lymphatics - self-repairing/self-expanding capacity (angiogenesis)

4. an integrated regulation system to adapt to variable demands: - intrinsic: automatism

- systemic: nervous system endocrine system - local/metabolic regulation ! level of activity – metabolic rate coupling e.g., increase in muscular metabolic rate during physical activity

muscular blood perfusion – resting cond. - 4ml/100g/min muscular blood perfusion - physical activity - 80ml/100g/min

2

2

1

1 3

3

CV System

A- atrium; V-ventricle; R-right; L-left; 1. Systemic circulation 2. Pulmonary Circulation; 3. Lymphatic Circulation

RA LA

RV LV

CV System

- Systemic & pulmonary

circulations arranged in series

- Circulations to individual

organs arranged in parallel…

Circuitry of the Cardiovascular System

• Cardiac output

Cardiac output of the LH (systemic blood flow) equals cardiac output of the RH (pulmonary blood flow)

• Direction of blood flow…

• Blood flow rate (V) depends on driving pressure (DP) and vascular resistance (R) (Poiseuille’s Low) – see an

analogy to Ohm’s low for electrical circuits

LV RA

CV system - continuous circuit, closed system !? - pressure changes in the system…

Functions of the cardiovascular system

CV System: an interplay of fast & complex mechanisms

to maintain body homeostasis and adaptive capacity.

1. TRANSPORT SYSTEM FUNCTION: BLOOD…

2. SECRETORY FUNCTION: ANP, NO, EDHF, PG2, ET…

1. CV system as a TRANSPORT SYSTEM : FUNCTIONAL SIGNIFICANCE OF BLOOD

- Nutrition, growth and repair transport between all parts of the body: nutrients, metabolic products, respiratory gases (O2, CO2) relation CO2 pH homeostasis

- Fast signaling: hormones, neurotransmitters, signalling molecules

- ‘Channel’ for the immune response/defense mechanisms:

immune cells, antibodies, clotting proteins - Maintenance of circulatory parameters: cardiac output – 5L/min at rest, for an adult blood pressure, blood volume,

blood fluidity/viscosity - Thermoregulation: heat dissipation from the body core to the surface

84 % — systemic circulation 64 % - veins 13 % - arteris 7 % - arteriols & capillaris 9 % — pulmonary system 7 % — heart

Blood distribution through the CV system

N.B. The same volume of blood must flow through each segment of the circulation each minute - the velocity of blood flow is inversely proportional to vascular cross-sectional area: under resting conditions, the velocity averages ~33 cm/sec in the aorta (2.5 cm2), ~0.3 mm/sec. in the capillaries (2500 cm2 and 0.3-1 mm length).

Distribution of blood flow (% Cardiac Output) at rest & during heavy exercise

2. SECRETORY FUNCTION OF THE CV SYSTEM:

- Atrial Natriuretic Peptide (ANP):

powerful vasodilator, reduce water & sodium on the circulatory

system, thereby reducing blood pressure

- NO (Endothelial Derived Relaxing Factor):

vasodilator, inhibits platelets adherence & aggregation

- Endothelial Derived Hyperpolarizing Factor,

- Endothelin: vasoconstrictor on ETA, ETB2 receptors

- Prostaglandins PGE2, PGI2 (prostacyclin):

increase Na+ excretion by the kidneys, vasodilators

Heart – morphological data:

- G = 250 - 340 g - Longitudinal diam. =100 - 120 mm - Transversal diam. = 80 - 100 mm - Measurement methods: - clinical: aria of cardiac dullness - echocardiography - radiology

aria of cardiac dullness

Morphological components of the heart

-Pericardium: thin-walled membranous cavity surrounding the heart; small volume of pericardial fluid in the space between its two surfaces.

- Myocardium, cardiac conduction system, peacemaker cells

- Fibrous skeleton

- 4 intracardiac valves:

AV valves: Right (tricuspid, 3-leaflet)

Left (mitral, 2-leaflet)

Semilunar valves: Aortic & Pulmonary

- chordae tendineae

- Endocardium

- Coronary circulation

The cardiac muscle: myocardium

• Peacemaker cells

• Conductive system

• Working myocardium

Muscles - major categories Criteria: • location • histological structure • control of their action

Skeletal Muscle: interactions with the external environment, usually associated with bones of the skeleton Smooth Muscle: visceral muscle, involuntary contraction, regulation of the internal environment Cardiac Muscle: located exclusively in the heart, involuntary contraction; -striated in appearance (sarcomeres and myofilaments are much like those of skeletal muscle); small cells, as present in smooth muscle, and electrical and mechanical cell-to-cell communication (syncitium); - a mechanical performance considerably more complex and subtle.

Myocardial cell (MC) structure

Sarcolema - T tubules & terminal cistern:

continuous with the cell mb, invaginate the cell at the Z lines

(carry the AP; more developed in the ventricles )

- sarcoplasmic reticulum (SR):

small-diameter tubules in close proximity to the contractile elem.

site of storage and release of Ca (Excitation-Contraction coupling)

Sarcomere: contractile unit of the MC

between 2 Z lines

contains: thin filaments - actin, troponin, tropomyosin

thick filaments – myosin

Intercalated disks: occur at the ends of the cells

maintain cell-to-cell cohesion: hold the cells (desmosomes)

and connect them electrically (gap junctions),

heart behaves as an electrical syncytium (sync, synch: informal for synchronization)

Mitochondria (> than in skeletal muscle)

Intercalated discs contain two types of specialized junctions:

1. desmosomes (which act like rivets & hold the cells tightly together)

2. gap junctions (which permit action potentials to easily spread from one cardiac muscle cell to adjacent cells).

Myocardial properties

1. Excitability (bathmotropia)

2. Rhythmic activity/automaticity (chronotropia)

3. Conductibility (dromotropia)

4. Contractility (inotropia)

5. Relaxation (lusitropia)

Excitability: Overview of the transport of molecules across membranes

• Membranes as permeability barriers

• Transport as: 1. Diffusion 2. Osmosis 3. Protein-mediated transport

-random thermal motion (brownian motion) -for lipid soluble substances and for very small uncharged water-soluble molecules

-flow of water across a semipermeable-membrane, when there is a solute concentration difference

Facilitated transport: • mediates the transport down the conc. grad. Active transport: • require energy to pump against the conc. grad. • primary or secondary active transport

Protein mediated-transport

Classification according to the mechanism that mediates the transport:

• Pore proteins… (water channels, connexons of gap junctions…)

• Transporters (carriers - exchangers, pumps…)

• Channels…

Properties of the protein-mediated transport:

• Functional state: opened or closed (…opening probability)

• Rate of transport … (109/sec - aquaporins, 106-108/sec. -

gated channels, 102-105/sec. - transporters)

• Transport mechanism: diffusion or binding of substrates…

• Saturation…

• Chemical specificity…

• Regulation…(e.g. by phosphorilation)

Membrane Ionic Transport System

Channel proteins: IONIC CHANNELS

- water-filled passageways to link intra- & extra-cellular compartments

- faster but less selective, for ions and very small molecules

Carrier proteins: IONIC PUMPS

IONIC EXCHANGERS

- never form a direct connection between intra- & extra-cellular compartments

- slower but more selective, can also move larger molecules

Membrane Ionic Transport System

Ionic channels

Ionic pumps

Ionic exchangers

Ionic Channels

- Def: trans-membrane protein segments that surround a

water-filled pore (selectivity determined by the diameter

& electrical charge of the amino-acids that line the channel)

- Classification ion channels variety :

– open/leak channels (non-gated)

– gated channels: chemically, voltage or mechanically gated

– time dependency

– ionic selectivity & conductivity

– regulation via extracellular agonists/intracellular messengers

- Named according to the ion(s) they allow to pass − K+ channels − fast Na+ channels, slow Na+ and Ca2+ channels − Ca2+ channels, Cl- channels − nonspecific cation channels (Na+, K+, Li+), etc...

Ionic pumps

-Na+/K+ pump: function action mechanism; blockers – ouabain -Ca2+ pumps: function distribution action mechanism

SR Ca2+ Pump (especially in the network part of SR)

- Na+/H+ antiporter

Ionic exchangers

- Cl-/HCO3- antiporter

- 1Na+/1K+/2Cl - exchanger

- K+/Cl- , Na+/Cl- symporters

- 3Na +/1Ca++ antiporter

-Na+/aa, Na+/G, H+/oligopeptide symporters

- Na+/HCO3- symporter

Na+/Ca2+ exchanger (NCX1)

-location: sarcolema

-transport mechanism: 3Na+/1Ca2+ (net inward current)

-increased activity during the late part of the AP plateau phase

-functional inter-relations with Na/K pump

-[discuss the effect of cardiac glycosides (digitalis):

1. on Na/K pump & indirectly on Na/Ca exchanger

2. on Ca permeability of Na channels ]

-reverse function during injury…

sarcoplasmic reticulum:

calsequestrin

Ca2+ ATPase

Ca2+ channels

Na+/Ca2+ exchanger

Na+/K+ ATPase Ca2+ ATPase

Ca2+ in myocardial fiber: transport through channels, pumps and exchangers

Ionic equilibrium & resting membrane potential

• ELECTROCHEMICAL POTENTIAL DIFFERENCE of ions across the cell membrane - RESTING MEMBRANE POTENTIAL – potential energy to be used to perform work EXCITABILITY

• ELECTROCHEMICAL EQUILIBRIUM across a membrane: no net force on the ion, no net movement of the ion (Nernst eq.)

(electrical force equal and opposite to the concentration force)

K+ equilibrium potential:

the membrane potential difference at which the movement down the concentration gradient is exactly equal to and opposite of the movement down the electrical gradient (Nernst equation).

Eion=RT/zF (log[ion]out/[ion]in )

R=ideal gas constant, T=absolute temperature,

z = electrical charge of the ion, F=Faraday’s number,

RT/zF = 61 at 37°C degree, for a univalent ion

E.g. K+, 10x conc. difference…

Rule of a thumb… A 60mV potential difference is required to balance a 10 fold concentration difference of a univalent ion

Resting membrane potential (RMP): -generated by the diffusion of K+ down the electrochemical gradients the relation between [K+]o (hypo/hyperkalemia) and level of RMP excitability

-contribution of the Na/K-ATPase

Action potential (AP): -generated by a depolarization (rapid change in the membrane potential) equal or over the threshold value -Propagation of AP without size decrement or change in shape -All or none response

Changes in the membrane potential: -Depolarization (local or propagated response)… -Hyperpolarization…

Membrane Potential (MP) Resting MP

Depolarization

Action Potential (AP) – propagation…

Repolarization

+/- Hyperpolarization

Heart Excitability

• Heart is an excitable tissue capable of generating and responding to electrical signals

Heart contains cells that generate spontaneously

APs = Pacemaker Cells (exhibits automaticity)

slow response AP induce a pace…

Working myocardium – respond to electrical stimuli

fast response AP contraction…

Action Potentials along the heart

atrium

Purkinje fb.

SAN AVN

ventricles Hiss b.

Me

mb

rane

po

tentia

l (m

V)

0

-50

-100

0

-50

-100

300ms

Slow response AP

Fast response AP

Transmembrane potential recorded from a fast-response (left) and a slow-response (right) cardiac fiber

in isolated cardiac tissue immersed in an electrolyte solution.

ERP, effective refractory period; RRP, relative refractory period.

Effect of changes in external [K+] on the transmembrane action potentials recorded from a Purkinje fiber. Stimulus artifact (St) - to the left of the upstroke of the action potential.

Fast responses may change to slow responses under certain pathological conditions (e.g., ischemia [K+] rises in the interstitial fluid because

inadequate perfusion).

atrium

Purkinje fb. ventricles

Me

mb

rane

po

tentia

l (m

V)

0

-50

-100

0

-50

-100

300 ms

• Normal atrium and ventricular myocardium

• Purkinje fibers in the specialized conductive system

Fast Action Potentials in the myocardium

Resting potential

Mem

bra

ne p

ote

ntial (m

V)

-

+

0

depola

riza

tion

threshold

overshoot

repolarization

repola

riza

tion

posthyperpolarization

Fast Action Potentials

stimulus

AP Phases

0 50 100 150 200 250 300 ms

Mem

bra

ne P

ote

ntial (m

V)

0

-50

-100

0

2 1

3

4 4

AP Phases: 0- depolarization/upstroke of the AP; 1-initial repolarization; 2-plateau; 3-repolarization; 4- resting membrane potential

0 0.15 0.30

Time (sec.)

Mem

bra

ne

Pote

nti

al

(mV

) 0

-50

-100

10

1.0

0.1

PNa+

PK+

PCa2+

AP

Membrane currents in the heart

-time-dependent

-voltage-dependant

Classification

1.Na Currents (INa)

2.Ca Currents (ICa)

3.K Currents (IK)

4.Pacemaker Currents (if)

Principal ionic currents & channels that generate AP in a cardiac cell

-Na inflow through fast voltage-dependent channels: 200/µm²; a, b1 subunits: a subunit has a phosphorylation site

for cAMP-dependent PK

1.Na Currents (INa)

-opening/activation (m activation gate) in response to local depolarization, at ~ -55 mV (0,1- 0,2 ms)…

-closing/inactivation (h inactivation gate) to a positive potential (~1 ms); 99% of Na channels are inactivated at the peak of the AP

-channel reset to open again after the membrane becomes repolarized below -50 mV !

- Responsible for rapid depolarization (Ph 0) of the AP in atrial and ventricular muscle and in Purkinje fibers.

-INa blocking: TTX, local anesthetics (lidocaine – used as antiarrhythmic drug)

Note that during AP [Na+]i increases by only 0.02%

1. Na Currents (INa)

- activate Ca & K channels

- participate to Ph 1 & Ph 2 of AP in atrial and ventricular muscle

The [Na+] in the external medium is the main determinant of the peak value of the upstroke of the action potential, with little influence on the resting membrane potential

- Ca inflow through slow L-type voltage-dependent

channels/dihydropyridine receptors (L from long lived)

- Responsible for depolarization (Ph 0) of AP in SAN, AVN and their neighboring cells

2. Ca Current (ICa)

- activation ~ 1 ms

- inactivation ~ 10-20 ms

- participate in Ph 2 (plateau phase) of AP in atrial and ventricular muscle long refractory period of the AP

2. Ca Current (ICa)

-Activates calcium-dependent Ca release from SR;

-initiates excitation-contraction coupling in myocardium

-L-type Ca channels blockers: nifedipin, verapamil, diltiazem

- Other Ca channels: T-type (transient) -activated at a lower voltage threshold (< - 30 mV), inactivated at a lower voltage… - initiation of the AP, automatism … N-type, P/Q-type

T (transverse) tubule

Ca2+ dependent Ca2+

releasing channel

(ryanodine receptor)

Sarcoplasmic

Reticulum

L-type Ca2+ channel

(Cav1.2. dihydropiridine

receptors)

Sarcoplasmic

Reticulum (SR)

Tetrad: 4 L-type Ca channels

faces a single Ca release

channel on SR

Triad: T tub + 2 SR cisternae

3. K Currents

IK… -repolarization currents resulting from the slow opening (20-200 ms) of KV channels -responsible for Ph 3 of the AP -deactivating at the diastolic membrane voltage

Ito - early transient outward K current (A-type) - slow and fast transient outward K channels - activated by depolarization; rapidly inactivates - contributes to Ph 1 of the AP

IK 1 - Inward rectifying K channel - responsible for the resting membrane potential (Phase 4)

G-protein activated inwardly-rectifying K current -vagal nerv stim. of SAN & AVN Ach muscarinic rec G-protein (bg subunit) GIRK K channels: outward K current hyperpolarization slows pacemaker rate & slows AP

conduction through AVN

ATP-sensitive K channels: Low probability to open at normal [ATP]~5 mM K currents dependent on ATP/ADP ratio; hypoxia/ischemia

ATP & ADP K channels activation & K outflow

(early repolarisation, short plateau, weak beat; possible role in electrical regulation of contractile behavior by coupling cellular metabolism & membrane excitability; cardioprotection…)

Note that the [K+]i changes just by 0.001% during AP.

3. K Currents

4. Pacemaker Current If

- found in SAN & AVN cells and in Purkinje fibers

- slow activation (100 ms) by hyperpolarization at the end of Ph 3 (“f” from funny)

-Produces an inward, depolarizing current

-If through a nonspecific cation channel (permeable for Na & K) called HCN (hyperpolarization-activated cyclic nucleotide (AMPc, GMPc)-gated channels)

-other pacemaker currents: Ica, IK

Name

Voltage

(V)-Gated

or Ligand

(L)-Gated Functional Role

Voltage-gated Na+

channel (fast, INa)

V Phase 0 of action potential (permits influx of Na+)

Voltage-gated Ca2+

channel (slow, ICa)

V Contributes to phase 2 of action potential (permits influx of Ca++

when membrane is depolarized); β-adrenergic agents increase

the probability of channel opening and raise Ca2+ influx;

Acetylcholine (ACh) lowers the probability of channel opening

Inward rectifying K+

channel (IK1)

V Maintains resting membrane potential (phase 4) by permitting

outflux of K+ at highly negative membrane potentials

Outward (transient)

rectifying K+

channel (Ito)

V Contributes briefly to phase 1 by transiently permitting outflow of

K+ at positive membrane potentials

Outward (delayed)

rectifying K+

channels (iKr, iKs)

V Cause phase 3 of action potential by permitting outflow of K+

after a delay when membrane depolarizes; IKr channel is also

called HERG channel (‘r’ for rapid, ‘s’ for slow).

G protein–activated

K+ channel (iK.G,

iK.ACh, iK.ado)

L G protein–operated channel, opened by ACh and adenosine

(ado); this channel hyperpolarizes membrane during phase 4

and shortens phase 2

Major Ion Channels Involved in Purkinje and Ventricular

Myocyte Membrane Potentials

Events associated with the

ventricular action potential

0 50 100 150 200 250 300 ms

Mem

bra

ne

Pote

nti

al (

mV

) 0

-50

-100

ERP RRP

Refractory periods

ERP/ARP-effective/absolute refractory period;

RRP-relative refractory period

Changes in action potential amplitude and slope of the upstroke as premature (P) action potentials are initiated at different stages of the relative refractory period of the preceding excitation in a fast-response fiber (bar = 100 msec).

Premature contraction

P

early

delayed

AP – electrical activity

Contraction – mechanical activity

P- premature contraction

Compensatory pause