Week 5b – Cardiovascular Physiology 1:
Role of the cardiovascular system:
• Supply of nutrients & hormones
• Removal of metabolic waste products
• Exchange of nutrients & waste products (at capillaries) Ø 5% of blood is found in capillaries
• Helps to produce heat/warm the body up
Design of the cardiovascular system:
Ø Pulmonary circulation:
o Right ventricle to lung to left atria
o Role is to oxygenate blood and remove CO2
Ø Systemic circulation:
o Left ventricle to tissues to right atria o Supplies oxygen to the bodies’ tissues
• Arteries carry blood away from the heart. (Note – not all arterial blood is oxygenated).
• Veins carry blood back towards the heart. (Note – not all venous blood is de- oxygenated).
Ø Arterioles are resistance vessels – the smooth muscle surrounding them can constrict or relax depending on what blood pressure is required.
Ø Veins are known as capacitance vessels (they can hold up to 60% of blood volume)
Pressure, flow & resistance:
• Blood flow (F) is always from a region of high hydrostatic pressure to a region of low hydrostatic pressure
• Hydrostatic pressure is the pressure exerted by a liquid in response to an applied force (e.g. heart beat)
• Blood pressure is the hydrostatic pressure pushing blood through the circulatory system
• Blood flow = pressure difference (ΔP)/resistance (friction force impeding flow) Determinants of resistance:
o Blood viscosity (the thickness of blood) – viscosity increases with haematocrit, however is normally kept constant.
o Length of blood vessels – kept constant throughout adulthood.
o Blood vessel radius (the most important determinant).
Heart valves:
• Function is to ensure unidirectional blood flow
• The opening & closing of valves is determined by the pressure difference across the valves
• The papillary muscle & chordae tendinae prevent valve eversion
Structure of cardiac muscle cells:
• Striated due to the arrangement of thick & thin filaments into sarcomeres
• Shorter than skeletal muscle, and branched
• Mononucleated (have 1 nucleus), rich in mitochondria but poor in sarcoplasmic reticulum
• Cells are connected via intercalated disks and gap junctions
Heart beat coordination:
• The atria contract first, then the ventricles.
• Depolarization begins at the sinoatrial (SA) node located in the posterior wall of the right atrium.
Cardiomyocte action potentials:
• Cardiomyocte AP’s are different from neurons & skeletal muscle cells.
• Contain a plateau phase, which is ~200 times longer than the excitation phase in other cells.
• The plateau phase is due to the influx of Ca2+ via slow voltage-activated Ca2+
channels, matched by the efflux of K+ via K+ channels. Na+ influx becomes reduced markedly because Na+ channels close.
• Repolarization occurs when K+ and Ca2+ permeability return to the resting state (i.e.
when enough K+ leaves the cell).
Pacemaker potential:
• Where resting potential is not steady but gradually depolarizes
• Pacemaker potential results from the influx of Na+ via special Na+ channels activated by the repolarizing phase of the previous action potential
• Other cells of the conducting system also display depolarization during the resting phase, but their rate of depolarization is normally slower than SA node cells
Week 6a – Cardiovascular Physiology 2:
The electrocardiogram (ECG):
• P wave – atrial depolarization.
• QRS complex – ventricular depolarization (and atrial repolarization).
• T wave – ventricular repolarization.
• ST segment – ventricular systole (ventricular contraction & emptying).
• TP interval – ventricular diastole (ventricular relaxation & filling).
Þ With a partial block of the AV node (the node between the atria and the ventricles), there is less ventricular contraction occurring (but still the same amount of atrial contraction).
Þ With a complete block of the AV node, there is no association between the P wave &
the QRS complex.
Excitation-contraction coupling:
• The process by which an action potential in the muscle cell membrane induces muscle contraction.
• In cardiac muscle, excitation-contraction coupling is dependent upon extracellular Ca2+ entering cells
• Ca2+ channel antagonists can block calcium channels from opening. This reduces Ca2+ release from the sarcoplasmic reticulum, which reduces the strength of contraction. This is useful for conditions such as high BP.
Cardiac muscle refractory period:
• Unlike skeletal muscle, cardiac muscle cannot summate contractions (this allows for filling of the heart). This prevents uncoordinated contractions. This occurs due to a long absolute refractory period (~250msecs).
• During the absolute refractory period, contraction cannot occur no matter how strong the stimulus is.
• Cells can only be excited again when the cardiac muscle twitch is almost complete.
This ensures that max HR is around 200bpm.
