Small Animal Medicine: Craig Flashcards
Describe the heart as a pump
The heart is regulated as 2 separate pumps. The left heart is a high pressure system, the common problem in left heart failure is the inability to supply blood.
The right heart is a low pressure system and the common problem is backwards failure. It cannot get rid of the blood from the caudal vena cava and the veins. Therefore we see systemic problems: ascites, oedema.
Describe forward and backward heart failure
Forward failure for left heart: Cannot supply blood peripherally
Backward failure of left lung: Unable to drain enough blood from the lungs.
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Describe the arrangement of the CVS (cardiovascular system)
The heart can preferentially shunt blood to different organs based on priority.
E.g. in a flight response: Blood is shunted to muscles and lungs, as it is needed for exercise
This way blood does not need to flow through one system to get to the other system; this would increase pressure in the circuit if this was the case.
EXCEPTION: Blood supplied to the liver is through the GI tract through the portal system.
Describe the cardiac cycle (Diastole and Systole)
DIASTOLE: Ventricles relaxing, atria contracting. The left and right ventricle will expand, actively relaxing to receive blood. Through the atria/AV valves it will fill the left and right ventricles. At the end of diastole, both will be full and have end diastolic volume.
During diastole the aorta and pulmonary artery still maintain pressure, as they are stretched. During diastole, the Aorta/PA will contract so pressure will not go from 0 to 120, rather like 120 to 180.
Systole: Ventricles rapidly contract and atria relaxes. Blood flows back into the atria and the AV valves will close.
Lub-dub sound in heart beat: Low pressure closure of the atrial valves.
In the ventricles, it squeezes until blood pressure is higher than that of the PA/Aorta so that it will push through the semi lunar valves and go to the artiers.
Low pressure closure of the AV valves, high pressure closure of the semi lunar valves.
Note: the heart does not empty completely as if it does, the surface tension of the walls against each other keeps it closed and it cannot open.
E.g. at the end of diastole, there is 100mL of blood, and the end of systole it is 50mL. 50mL left.
What is preload and afterload
Preload: Amount of blood in the ventricle at the end of diastole (maximum volume of the heart).
–> Expanding with blood
Afterload: Maximum pressure against which the heart contracts
The pressure against which the heart has to pump
Preload has a positive effect on stroke volume:
Increase preload, increase stroke volume. End diastolic volume: maximum filling.
The amount of blood ejected is higher if the volume sis higher. There is a limit to the amount of blood that can be ejected, if too high it can cause heart failure.
Diseases that increase preload will increase volume retention; more blood in the same space.
Causes congestion.
Leaky valves (e.g. mitral valve disease) can also increase pre load: As it cannot eject all of the the blood. At the end of every heart squeeze more blood is leftover due to the damaged valve.
Afterload: has a negative effect on stroke volume.
Increase after load, decrease stroke volume. End systolic volume: maximum emptying.
Opens under higher pressure and the heart has to squeeze much harder to get blood out, apposing stroke volume
Diseases that increase after load can be like pulmonic stenosis: has to squeeze against a higher gradient to get blood out of the left ventricle.
Explain MAP (mean arterial pressure), SVR (systemic vascular resistance) and CVP (Central venous pressure)
MAP is mean arterial pressure
Central venous pressure is the the amount of pressure at the atria and the number should be close to 0 as possible.
The heart pumps all the blood it receives as much as possible so there is no CVP.
If the CVP increases: there is increased resistance in the tissue beds.
Systemic Vascular resistance: Cardiac output delivers an arterial pressure then travels through vascular beds.
There is a systemic vascular resistance here.
Any left over resistance in the veins is the Central venous pressure.
Explain how cardiac output is regulated
There is intrinsic and extrinsic regulation of cardiac output.
Intrinsic: The ventricle will stretch and maximally fill, leads to reflex contraction that will increase cardiac output.
Frank-Starling Law of the heart: Heart pumps all the blood it recieves
Extrinsic: Cardiac output being managed from the outside. Neurohormonal activation, SNS, RAAS system.
When cardiac output starts to fall, these systems activate and try attempt fix falling cardiac output.
SNS: alpha 1 receptors cause vasoconstriction
Beta 1 receptors on the heart and AV node. To increase heart rate, contractility and speed conduction.
When MAP starts to fall there are pressure receptors in the aorta and carotids, supplied by the vagus nerve to the brain.
Result in falling cardiac output. There will be release in catecholamines, noreadrenaline and adrenaline.
Explain RAAS Activation
Renin released in associated with the kidney perceiving there is less renal perfusion due to less cardiac output.
Renin activates a cascade –> causes release of angiotensin II causing vasoconstriction. This also results in release of aldosterone/ADH. Strictly conserve sodium and fluid and vasoconstriction to increase preload and afterload.
This is not good when sustained for a long time, and if the heart is failing puts more work load on the heart.
Also causes tissue fibrosis and scarring.
