The cardiovascular system

• Functions

  • Transport of materials around the body

    (to and from all parts of the body)

    -   Circulation of oxygen, nutrients (glucose, fatty acids) & water

    -   Removal of waste products (CO2, metabolic waste, heat)

  • Cell-cell communication (hormones)• Immune defence (White blood cells, antibodies)
• Pulmonary circulation

  •The right side receives blood from tissues and sends it to lungs for oxygenation

  Systemic circulation

  •The left side receives newly oxygenated blood from lungs and pumps it to tissues
• •Blood flows because liquids move from high to low pressure regions•• Contraction of the heart creates pressure without changing the volume of blood. Pressure in ventricles is called driving pressure

  •Hydrostatic Pressure: exerted by a fluid which is not moving and force is exerted equally in all directions
• •Flow usually means flow rate, the volume of blood that passes a given point in the system per unit of time.


  •Velocity of flow (or simply velocity), the distance a fixed volume of blood travels in a given period of time.

  •The tendency of the cardiovascular system to oppose blood flow is called Resistance to flow

  •Velocity depends on the flow rate and the cross-sectional area
• (1) The resistance to fluid flow offered by a tube increases as the length of the tube increases

  (2) Resistance increases as the viscosity (thickness) of the fluid increases

  (3) Resistance decreases as the tubes radius increases

  A decrease in blood vessel diameter is known as vasoconstriction. An increase in blood vessel diameter is called vasodilation
• •Divided by a central wall or septum

  •Each half functions as an independent pump that consists of an atrium and a ventricle

  •However, the two sides contract in a coordinated fashion;

  •First, the atria contract together, then the ventricles contract together

  •Contracts continually, resting only in the milliseconds long pause between beats
• •Atria receive blood returning to the heart, right-from tissues, left from lungs

  •Ventricles pump blood away from heart, right ventricle to lungs, left ventricle to the tissues.

  •Encased in a tough, membranous, fluid-filled sac – pericardium lubricates the heart as it beats

  •Major blood vessels all connected to the base of the heart.−Superior vena cava – right atrium−Pulmonary artery – right ventricle−Pulmonary vein – left atrium−Aorta – left ventricle
• Atrioventricular valves

  •Control blood flow between atria and ventricles•Connected to ventricular wall via collagenous tendons- Chordae tendineae•Chordae tendinea- tethered to extensions of the ventricular muscle - Papillary muscles•The muscles do not control the opening of the valve – the valves open and close passively when blood  pushes on them

  Semilunar valves

  •Control blood flow between the ventricles and the arteries•Aortic valve•Pulmonary valve•3 cuplike leaflets, snap closed when blood attempting to flow backwards fills them•Don’t need connective tendons
• During ventricular contraction the semilunar valves are open but the Atrioventricular valves remain closed to prevent blood flow back into the atria
• During ventricular relaxation the semilunar valves prevent blood that has entered the arteries from flowing back into the ventricles
• Contractile

  •Straited fibres organised into sarcomeres


  •Smaller than skeletal muscle cells, have a single nucleus per fibre and mitochondria occupy 1/3 cell volumeintercalated discs which contain desmosomes•Gap junctions – allow waves of depolarisation to spread rapidly from cell to cell – so all heart muscle cells contract almost simultaneously

  •Attached to each other by

  Autorhythmic (pacemaker)

  •Approx. 1% of heart muscle cells•Signal for contraction, have no organised sarcomeres•Signal is myogenic (originates in the heart)•Smaller and fewer contractile fibres
• Action potential:

  Similar to those of neurons and skeletal muscle,  but much longer, 200 msec or more. Stable resting potential of -90mV
• •Muscle  twitch: period when a muscle is contracting, then relaxing.Refractory period: period during which another action potential cannot be triggered•Muscle fibre will have relaxed fully before another action potential can take place – so ventricles can fill with blood.•If refractory period was shorter muscle twitches would run into one another = Summation, leading to sustained contraction – Tetanus•Long action potential means refractory period and muscle twitch end together – prevents tetanus
• Spontaneously generate - Action Potentials

  AC myocytes have an unstable membrane potential – Pacemaker potential, starts at -60mV & slowly drifts up.

  AC myocytes possess If channels (I=current f= funny). Permeable to both Na+ and K+.

  1. At -60mV: If channels open. Na+ enters, and exceeds K+ efflux.

  2. If channels close. One set of  Ca++ channels open; Ca++ enters.

  3. Depolarisation continues, more Ca++ channels open; more Ca++ enters-rapid depolarisation

  4. Ca++ channels close; slow K+ channels open. Efflux of K+ out of cell causes repolarisation

  5. K+ channels close.
• Ca++ important for action potential of both contractile and autorythmic cells
• Sinoatrial (SA) Node

  Group of Autorhythmic cells in the right atrium heart as main heart pacemaker
• When depolarisation at the SA occurs

  1. Depolarisation wave spreads rapidly via Internodal pathways – specialised noncontractile autorhythmic fibres to Atrioventricular (AV) node

  2. From AV node depolarisation moves into the ventricles where Purkinje fibres specialised conducting cells of the ventricles conduct electrical signals very rapidly down the Atrioventricular bundle/Bundle of His in the ventricular septum

  3. AV bundle branches divides into left & right branches-continue down to Apex of heart & divide into smaller Purkinje fibres -spread out among contractile cells
• Atrioventricular (AV) node

  Group of autorhythmic cells near the floor of the right atrium
• ECG


  Records electrical activity of the heart
• ECG


  • Electrodes on both arms & left leg
  • Hypothetical triangle around heart – "Einthoven's triangle"
  • Recorded from one lead at a time – one electrode acts as the positive electrode, 2nd is negative electrode
• Lead 1

  • Left arm electrode designated +ve, right arm electrode –ve
• When an electrical wave moving through the heart is directed to the +ve electrode


  The ECG wave goes up
• If net charge movement through the heart is towards the negative electrode

  The wave points downward
• ECG is not the same as one action potential. It's the sum of multiple action potentials
• ECG: 2 main components:

  Waves: go above or below baseline

  Segments: Sections of baseline between waves

  Intervals: Combinations of waves & segments

  3 major waves on normal ECG

  P wave

  • Depolarization of the atria

  QRS complex (trio of waves)

  • Depolarization of the ventricles

  T wave

   Re-polarization of the ventricles
• ECG Interpretative Questions:

  (1)What is the heart rate?

  -Beginning of one P/R wave to beginning of next

  -normal 60-100 beats
• (2) Is the rhythm irregular?

  -an irregular rhythm, or arrhythmia, can result from a benign extra beat or a more serious condition
• Is there one QRS for each P wave? If yes, is the P-R segment constant in length?

  -if not, a problem with conduction of signals through one of the nodes may exist
• Two main phases of the Cardiac Cycle:

  • Diastole:  Time during when cardiac muscle relaxes and the chamber fills with blood {diastole; dilation}

  • Systole:  Time during when cardiac muscle contracts and blood is pushed out of the chamber {systole; contraction}

  •The atria and ventricle chambers do not contract and relax at the same time
• •Cardiac cycle begins with the atria & ventricles at rest.

  •ECG begins with atrial depolarisation - P wave.

  •Atrial contraction begins during latter part of P wave & continues during P-R segment

  •During the P-R segment, electrical current is slowing down as it passes through AV node.

  •Ventricular contraction begins just after Q wave and & continues through to T wave.

  •The ventricles are repolarising during the T wave, which is followed by ventricular relaxationDuring the T-P segment the heart is electrically quiet.
• Because depolarisation initiates muscle contraction, the cardiac cycle lags slightly behind electrical signal.
• Blood pressure: measured in mm Hg

  A millimeter of mercury is a manometric unit of pressure

  No longer standard but these manometric units are still encountered

  Lung pressures in centimeters of water are still common, as in settings for Continuous Positive Airway Pressure (CPAP) machines

  Formerly defined as the extra pressure generated by a column of mercury one millimetre high at 0◦C on Earth

  Now defined as precisely 133.322387415 pascals. It is denoted by the symbol "mmHg"
• Another way to describe the cardiac cycle is using pressure and volume
• Remember:

  •Blood flow through heart governed by pressure (P)•Fluid flows from high to low pressure

  A: mitral valve opens

  B: mitral valve closes

  C: aortic valve opens

  D: aortic valve closes

  A-B: Passive filling & atrial contraction

  B-C: Isovolumetric contraction

  C-D: Ejection of blood into aorta

  D-A: Isovolumetric relaxation
• Represents:

  •Changes in left ventricular pressure/volume during one cardiac cycle


  •Time passing as the heart fills with blood, then contracts
• •At C the atrial pressure increases because pressure on the mitral valve pushes the valve back into the atrium, decreasing atrial volume. Ventricular pressure shoots up when the ventricles contract on a fixed volume of blood••Atrial pressure decreases during the initial part of ventricular systole as the atrium relaxes then increases as it fills with blood••Atrial pressure begins to decrease at point D, when the mitral valve opens & blood flows down into the ventricles
• Isovolumic - unchanging volume

  Ventricular ejection - as ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected

  End Diastolic Volume (EDV) – the maximum volume of blood that the ventricles hold during a cardiac cycle

  End-Systolic Volume (ESV) – the amount of blood left in a ventricle at the end of contraction

  “lub-dup” – heart beat sounds caused by vibrations of valves closing

  Stroke volume – the amount of blood pumped by one ventricle during one contraction
• Heart: 2 separate pumps

  •Systemic circulation:

      LHS heart        tissues       RHS heart. 

  • Pulmonary circulation:

      right ventricle      lungs      left ventricle

  Systemic circulation

  Blood leaving LHS

  Arterial system: Aorta       arteries       arterioles        branch - capillaries

  From capillaries

  Venus system: small vessels      venules      veins        Vena cava        RHS
• Cardiac Output = Total Blood Flow
• •Capillaries leaky epithelium allows exchange of materials •Arterioles the site of variable resistance. Direct distribution of blood flow to individual tissues by selectively constricting and dilating •At the distal end of the capillaries, blood flows into the venous side of the circulation

  •The veins act as a volume reservoir. Blood can be sent to the arterial side if blood pressure falls too low