Flashcards in Week 2 Deck (79):
How much the volume of a vessel changes in response to an increase in pressure; if a small increase in pressure causes a large change in volume, the vessel is said to be very compliant
What is the function of arterial compliance?
Elastic recoil of the compliant arteries continues to drive blood forward during cardiac diastole. This converts a pulsatile flow into a steadier, more continuous flow in preparation for the microcirculation.
What effect will increasing resistance have on flow and pressure difference?
Flow will decrease
Pressure difference will increase
Based on what two aspects are capillaries classified?
Position in the vascular bed
Nature of their endothelium
A simple AV shunt connects which two vessel types?
Arteriole and a muscular venule
A thoroughfare channel connects which two vessel types?
A metarteriole and a post-capillary venule
How to calculate TPR?
How to calculate MAP?
CO x TPR
Name four mechanisms for venous return
SNS innervation causing venoconstriction
Skeletal muscle pump
Intra-thoracic or cardiac pressure gradients creating larger pressure difference along the vessel length
On which side of the heart would you find the paraconal groove?
The left side
On which side of the heart would you find the subsinuosal groove?
The right side
Where on the heart surface would you find the circumflex arteries?
The coronary groove
Which vessel of the coronary circulation is subject to considerable species variation?
The right interventricular artery (subsinuosal artery)
In which species does the right interventricular (subsinuosal) artery arise from the left coronary artery?
Cows and dogs, some cats
In which species does the right interventricular (subsinuosal) artery arise from the right coronary artery?
Horses and pigs, some cats
Where is the most vascular part of the myocardium?
The subendocardial plexus
Which structure do the small coronary veins empty into?
All four chambers of the heart
At which part of the cardiac cycle are the coronary arteries most compressed?
Isovolumetric contraction phase of ventricular contraction
How does HR impact coronary blood flow?
As HR increases, time the heart spends in diastole decreases. Due to the compression of mural coronary blood vessels in the left heart during ventricular systole, 80% of coronary blood flow occurs whilst the heart is in diastole. Thus, a reduced time spent in diastole will reduce coronary blood flow, despite the increased demand (which will initially attract a vasodilation due to the accumulation of vasoactive mediators such as adenosine)
What is the ligamentum arteriosum in the adult?
The fibrous band that connects the pulmonary trunk with the aortic arch
What foetal structure becomes the ligamentum arteriosum?
The ductus arteriosus - an arterial shunt between the pulmonary trunk and aorta
Which three things caused by increased cardiac activity impact upon the coronary blood supply and how?
1. SNS stimulation to the heart results in temporary vasoconstriction of coronary vessels
2. Metabolic hyperaemia subsequently causes coronary vasodilation due to the accumulation of vasoactive mediators
3. Increased force of systolic contraction compresses vessels in the wall of the left heart, resulting in reduced pressure and flow in those vessels during that phase.
How does the pulmonary circulation maintain an extraordinarily low pressure?
Because the vessels are very compliant - will allow large change in volume for minimal increase in pressure
What is the response of the pulmonary arterioles to hypoxia? How is this different to the systemic arterioles?
They vasoconstrict (hypoxic vasoconstriction), to optimise the ventilation:perfusion ratio of the lungs. In the rest of the body, hypoxia generally causes arterioles to vasodilate to increase O2 delivery to the area.
What effect does a cold ambient temperature have on a subcutaneous AV anastamosis? What mediates this effect?
Vasoconstricts, to reduce the bloodflow to the dermis
SNS activity in the cold weather mediates the vasoconstriction
How does an AV anastamosis vasodilate?
Reduced SNS input allows vasodilation
How to calculate stroke volume?
SV = EDV - ESV
What is the basic flow equation? What is flow equivalent to?
Flow = pressure difference/resistance
Flow is equivalent to CO in L/min
Which two factors determine CO?
Resistance (TPR) and the pressure difference (MAP)
How to calculate CO?
CO = MAP/TPR
CO = HR x SV
How does MAP relate to systolic and diastolic pressure? (equation)
MAP = diastolic pressure + pulse pressure/3
Preload is the force stretching a relaxed myocardium
Which two elements can be considered to be proportionate to preload?
EDV and venous return
Afterload is the force opposing the shortening of the myocardium (opposing systole)
Which element/s of MAP affect afterload?
TPR is the only factor affecting afterload; CO does not affect it.
What does the FS law of the heart state?
Energy release during contraction depends on the initial fibre length
Define resting tension
The force required to stretch a resting muscle to different lengths
Under what circumstances does active tension develop?
When a muscle is stimulated to contract whilst length is held constant.
How to calculate total tension?
Resting tension + active tension
Why, using the sliding filament hypothesis, does stretch improve the contractility of the myocardium (within physiological limits)?
Because muscle fibres experiencing stretch can form more actin-myosin crossbridges, which increases the force generated (i.e more tension developed for more length)
What factor determines the length of cardiac muscle fibres prior to contraction in the healthy heart?
End diastolic volume.
What determines end diastolic volume ( 2 things)?
End systolic volume
How might you increase stroke volume? (2 things)
1. Increase EDV (by increasing venous return)
2. Increase contractility (by increasing intracellular Ca2+)
How to calculate ejection fraction?
EF = SV / EDV
x 100 to get as a percentage
Define 'positive inotrope'
A positive inotrope is any agent that increases peak isometric tension at a fixed length
What is the most important physiological regulator of cardiac activity?
What effect does heart rate have on cardiac muscle contractility?
Increased heart rate increases contractility because it leads to gradual accumulation of Ca2+ inside the cell (read mechanism)
Lusitrophy is the capacity of the heart to relax enough between beats to enable effective filling
What is fractional shortening and how is it measured?
Fractional shortening is the fraction of any diastolic dimension that is lost in systole
Measured with ultrasound (echocardiography)
What are the eight key components of the physical exam of CVS function?
Roses Make Excellent Plants For Courtney To Arrange
1. Respiratory rate and effort at rest
2. MM colour and CRT
3. Examine external jugular for pulsations or distension
4. Precordial palpation
5. Femoral pulse evaluation
6. Cardiac and lung auscultation
7. Thoracic percussion
8. Abdominal palpation
What is the maximum RR during sleep for a healthy animal?
What causes the S1 heart sound?
Closure of the mitral and tricuspid valves
What causes the S2 heart sound?
Closure of the semilunar valves
When is the most common part of the cardiac cycle to detect a murmur?
Systole - between S1 and S2
What causes the S3 heart sound?
The passive flow of blood into the ventricles during diastole
What causes the S4 heart sound?
Active propulsion of blood from the atria to the ventricles during atrial systole
What sort of defect is a plateau murmur most likely to represent?
AV valve incompetence
What sort of defect is a musical murmur most likely to represent?
Mitral valve disease / vibration
What sort of defect is a systolic ejection murmur most likely to represent?
Stenotic semilunar valves or vessel
What may cause a crackle sound heard upon lung auscultation?
The opening of small airways affected by oedema or fibrosis
What sound might be heard upon lung auscultation as a result of bronchial obstruction?
What are three changes seen on radiograph in cardiomegaly of either cat or dog?
Heart fills more than 70% ventral-dorsal chest on lateral rad
Trachea is elevated and parallel to thoracic spine
Caudal heart border is straighter
What event is responsible for the A wave (pressure wave)
Atrial systole causing retrograde pressure wave up the great veins as there are no valves between these structures and the atria.
What event is responsible for the C wave?
Bulging of the AV valves into the atria during isovolumetric ventricular contraction creating another pressure wave in the great veins
What effect can tachycardia have on stroke volume?
Can reduce, if HR is so high that time remaining for ventricular filling is insufficient (especially with hearts that have compromised compliance)
What event is responsible for the dichrotic notch?
Closure of the aortic valve allowing temporary retrograde blood flow into the leaflet area
What event is responsible for the V wave?
Passive emptying of venous blood into the atria during atrial systole
What are the three phases of ventricular diastole? Are the AV valves open or closed in each? Which phase involves negative pressure created by the ventricles?
Isovolumetric relaxation : AV closed
Rapid ventricular filling : AV open
* negative pressure created by ventricular recoil in this phase helps to suck blood in from the atria
Reduced refilling : AV open
Define diastolic pressure
Arterial pressure when the aortic and pulmonary valves are closed
Which compartment of the heart is being measured to visualise the A, C and V waves?
Right atrial pressure
Which compartment is being measured to visualise the dichrotic notch?
Does the FS law of the heart describe the effect of fibre length on ACTIVE tension or PASSIVE tension ?
Active tension - very short and very long fibre lengths will generate little active tension
Describe how an increase in EDV can increase contractility of the muscle in terms of tension types.
Total possible active tension generated will increase because of increased overlap of actin and myosin fibres, therefore more cross bridges made
Total resting tension will also increase - this is the passive tension generated by the muscle fibre being physically stretched.
Total tension = active + resting so overall more tension may be generated
Other than myofibre stretch, what can increase total possible tension generated?
Increased calcium inside the cell (increased contractility at a given length because more cross bridges can form)
What are the three types of Ca2+ channels on a cardiomyocyte? Which of these are modulated to increase contractility by normal physiological mediators?
Voltage gated Ca2+ channels
Ligand gated Ca2+ channels - modulated e.g noradrenaline
Ryanodine receptor channels in the SR - modulated e.g (??)
Two major mechanisms for a bounding pulse?
Aortic valve incompetence
Three potential mechanisms for a weak pulse?
Three possible causes of a jugular pulse?
R sided CHF