Physiology of the heart 3

Question 1

Where do the coronary arteries come from and how do they supply the heart?

Answer 1

First branch off the aorta

coronary arteries run over the surface of the heart- blood perfuses into the myocardium

Question 2

Whats does coronary blood flow =

Answer 2

coronary blood flow= perfusion pressure/ resistance

Question 3

What is perfusion pressure?

Answer 3

Pressure at either end of the coronary circulation

pressure gradient that drives coronary blood pressure

Question 4

What does resistance depend on?

Answer 4

flow going down the tube
length of the tube
viscosity of the contents
diameter of the artery

Question 5

When does coronary blood flow occur?

Answer 5

In diastole- when the heart is not contracting

Question 6

How does the perfusion pressure differ from the top of the coronary arteries to the bottom?

Answer 6

Top= equivalent to the arterial diastolic pressure 
bottom= hard to measure as its small capillaries at the bottom- equates to the pressure in the ventricles

Question 7

What is the oxygen consumption of an arrested heart, resting and active?

Answer 7

arrested (alive but not contracting)= 2ml/min/100g
resting= 8
heavy exercise= 70

Question 8

Whats the perfusion of the heart like?

Answer 8

One of the worst perfused organs

uses all the oxygen it can get

Question 9

What does oxygen delivery =?

Answer 9

arterial oxygen concentration x coronoary blood flow

Question 10

What are the characteristics of arterial oxygen concentration?

Answer 10

mainly determined by oxygen bound to haemoglobin

generally maximised- theres little change

Question 11

What are the characteristics of coronary blood flow?

Answer 11

controlled by :
neural factors
hormones
local metabolites
mechanical factors

Question 12

What does the left ventricle to aorta pressure trace look like?

Answer 12

use a catheter to get the measurements
systolic BP is the same
arterial diastolic pressure is higher than the LVEDP due to the aortic valve closing
difference between ADP and LVEDP= perfusion pressure

Question 13

Why is the window between systoles important?

Answer 13

This is the time for perfusion to occur

Question 14

Which events affect perfusion and why?

Answer 14

1. Tachycardia- reduces diastole and thus time for perfusion
2. Raised LVEDP (due to heart failure, hypertension etc.)- decreases perfusion pressure
3. Reduced arterial diastolic pressure- decreased perfusion pressure

Question 15

How does the heart exhibit autoregulation of coronary blood flow?

Answer 15

If arterial pressure dropped and thus blood flow through the capillaries dropped-
heart goes into autoregulation mode
resistance of vessels decreases- maintains the coronary flow
maintained flow despite the pressure remaining reduced
*only within a certain range of pressures- if pressure is way too high or low- autoregulation is overridden

Question 16

What is autoregulation of blood flow?

Answer 16

Ability of an organ to maintain constant blood flow despite changes in perfusion pressure for example

Question 17

How do they know that metabolites are important in vascular control?

Answer 17

Hypoxia causes marked coronary vasolidation in situ
but not in an isolated coronary artery
there is something in the heart responding to hypoxia and causing vasodilation

Question 18

Which receptors cause vasodilation and vasoconstriction?

Answer 18

Alpha receptors on the larger coronary arteries cause vasoconstriction
Smaller beta 2 receptors on smaller arteries- cause vasodilation
*resistance is all in the small vessels
big vessels matter pathologically- tight stenosis in the coronaries needs fixing

Question 19

Why is the heart technically an endocrine organ and happens as a result?

Answer 19

Secretes natriuretic peptides
atrial natriuretic peptide
brain natriuretic peptide
-released due to stretching of atrial and ventricular muscle cells/ raised atrial or venticular pressure = overload
Main effects:
increases renal extraction- natriuresis and diuresis
relax vascular smooth muscle (apart from efferent arterioles of renal glomeruli- maintain filtration pressure to get rid of na and water)
increased vascular permeability
inhibit the release/ actions of:
aldosterone (retain na and h20), angiotensin 2 (vasocontrictor), endothelin and ADH

Question 20

What is CNP and what inhibits its degradation?

Answer 20

Cardiac natriuretic peptides
broken down by neutral endopeptidases (neprilysin)
inhibits its breakdown- potentiates the effects of the CNPs- have longer lasting effects
drugs that do this:
sacubitril- neprilysin inhibitor
valsartan - angiotensin 2 blocker
both used as a novel therapy of heart failure- block angiotensin 2 and potentiate the effects of good CNPs