Week 1 - Heart rate and Stroke Volume, and clotting... Flashcards
What is the equation for cardiac output?
cardiac output = heart rate x stroke volume
What is preload?
the amount of blood in the ventricle is what stretches the cardiac muscle. the more volume of blood inside, the more the muscle is stretched so the stronger the contraction will be and thus more stroke volume
What is afterload?
this is the resistance against which the muscles in the ventricles are trying to contract. if the arterioles have great resistance, ventricles will have to work harder to build the pressure to exceed the aorta, so less energy pushes blood out and stroke volume decreases.
How does contractility affect stroke volume? about ANS
sympathetic NS - b1 receptors on myosin can be activated with noradrenaline and cause more Ca+ to be released, forming more cross bridges and having a larger contraction and thus larger stroke volume.
Parasympathetic isnt involves as vagus nerve doesnt innervate ventricular muscle
What is contractility?
it’s the ability for the heart to contract in regards to cross bridges being formed.
What parts of the heart does the parasympathetic system innervate?
vagus nerve innervates SA and AV nodes, but not ventricular muscle
What is the root of the vagus nerve?
10th pair of cranial nerves
What 3 factors affect stroke volume?
preload, afterload and contractility effect by Sympathetic NS
How does increasing heart rate affect stroke volume?
increased heart rate decreases stroke volume as it reduced the cardiac interval, so less time for blood to fill. this is ofsetted by many factors.
Why does heart rate increase when we exercise? (5 factors)
- decreased vagal tone
- increased sympathetic tone
- contractility (more cross bridges)
- venous return increases - venoconstriction. maintains preload
- total peripheral resistance falls - arterioles dilate in muscle, skin and heart, reducing afterload
How much do the 5 factors (causing heart rate to increase) increase cardiac output by?
4-6x greater cardiac output
How does our body prevent clotting?
epithelium has many mechanisms
How does blood clot when we have low pressure?
blood may pool in legs and clot as static blood is more likely to coagulate. this causes DVT and may cause pulmonary embolism, myocardial infarction or ischaemic storke.
What does the epithelium secrete if a clot is formed?
t-PA. tissue plasminogen activator. converts plasminogen to plasmin, which dissolves clot. this is thrombolysis.
What does the endothelium produce that inactivates thrombin? (3)
- TFPI (tssue factor pathway inhibitor) stops thrombin production, so fibrin isnt formed
- thrombomodulin binds to thrombin and inactivates it
- expresses heparin - inactivates thrombin
