Coronary Circulation & Tissue Perfusion Flashcards
When does the heart perfuse itself and why?
During diastole
There is more time for it to perfuse itself during diastole
During systole it is perfusing the rest of the body
What is the heart’s main source of energy?
Fat as it carries more ATP per gram
What is used if a coronary artery is blocked?
A coronary artery bypass
This involves taking a vessel from the leg to work as a graft to bypass the blockage
What are the two types of graft?
Which lasts longer and why?
Arterial and venous
Arterial grafts last longer as arteries have thicker walls
How long does an arterial graft work?
12-15 years
What is the main function both upstream and downstream of the capillary?
Conduction
What is the blood pressure like upstream of the capillary?
Why?
Higher blood pressure is being delivered by the arterial system
The arteries have more muscle in their tunica media
What is the purpose of elastic tissue in the arteries?
It stores energy during systole and then recoils to propel the blood along
How is the elastic recoil of the arteries damaged in an aneurysm?
There is damage to the elastic tissue
The arterial lumen is dilated
How does the composition of the arteries change as you get closer to the capillaries?
The proportion of muscle in the artery increases
They change from being elastic arteries to muscular arteries
What is the role of resistance vessels in blood distribution?
They decide how much blood different parts of the body require
They supply the correct amount of blood for maximum efficiency
What is blood flow like in a capillary?
There is a single-file line of RBCs
It is a continuous flow of blood that is not pulsatile
What is the role of counter-pulsation?
It helps to smooth the flow of blood so that the blood is moving at all times
This gives the maximum amount of time to exchange substances across the capillary
Where are capillaries positioned in relation to cardiac myocytes and why?
The capillaries are in close proximity to the cells
Cardiac myocytes need a lot of perfusion as the heart is working all the time
What happens to capillaries in the myocardium during systole?
Contraction of the heart compresses the capillaries
It is difficult for blood to flow through them
If there are plaques present in the capillaries in the myocardium, how does this affect relative pressures?
The pressure must be higher in the arterioles than in the capillaries
How does the heart compensate for plaques in the myocardium capillaries?
It dilates the blood vessels or increases the force of contraction
This increases the pressure of the blood
From the aorta to the capillary venules, what is the branching pattern of blood vessels?
- aorta
- pre-arterioles (200-500 micrometres)
- arterioles (40-100 micrometres)
- smaller arterioles (<40 micrometres)
- capillary venules
In relation to the epicardium and endocardium, where are the capillary venules found?
Closer to the endocardium
The pre-arterioles are closer to the epicardium
When does perfusion of the heart occur and why?
During diastole
During systole, blood cannot flow through the smaller vessels due to the increased pressure in the heart
How does the amount of blood needed vary in hypertrophy?
More blood is needed to perfuse all of the muscle
There is more muscle in the ventricle walls
When is the main flow through the aorta and right coronary artery?
During systole
When is the main flow through the left coronary artery?
During diastole
There is little flow in the left coronary artery during systole
How does a myocardial infarction affect the amount of perfusion to the ventricle?
Myocytes in a particular region have died and become scar tissue
Scar tissue does not need perfusion as it is not contracting
How can compromised blood flow (e.g. blocked coronary artery) be improved?
Through a stent
As blood flow increases, what happens to the rate of metabolism in the heart?
The rate of metabolism also increases
What happens to metabolism of the heart during exercise?
The heart metabolises more and uses more oxygen and nutrients
It also produces more metabolites (waste products)
How do metabolites (waste products) alter blood flow to the heart?
Blood vessels can sense waste products such as CO2 and lactic acid
They send a message to increase blood flow to the heart
What is the process by which producing more metabolites leads to increased blood flow to the heart?
Autoregulation
What metabolites will stimulate relaxation?
How does this affect blood flow to the heart?
Adenosine, CO2, H+ and K+
These cause blood vessels to dilate and increase blood flow to the heart
How is coronary blood flow affected by different blood pressures?
Coronary blood flow decreases when blood pressure is lower
It increases when blood pressure is higher
What is involved in autoregulation?
Adjusting resistance and dilation of the arteries to smooth out the perfusion to the heart
What does autoregulation allow for?
Providing maintained coronary blood flow to the heart
How does normal cardiac output vary from during rest and during exercise?
Normal cardiac output is 5L per minute
This can increase to 20L during exercise
What 2 factors work to increase the cardiac output during exercise?
- increasing the heart rate
2. increasing the volume of blood pumped out of the heart with each beat (stroke volume)
What is required in order for the heart to maintain a high cardiac output?
A lot of adaptation
Where is nitric oxide produced?
Endothelium
How does blood flow affect nitric oxide production by the endothelium?
Increased blood flow means that more nitric oxide is produced
What is the effect of nitric oxide on blood vessels?
It causes the blood vessels to dilate more
How is the endothelium stimulated to produce nitric oxide?
The endothelium senses the blood flow and experiences sheer stress due to the blood flowing past it
Blood flow stimulates it to produce nitric oxide
What is produced by the parasympathetic nervous system that affects blood vessels?
What is its effect?
Acetylcholine
It causes the blood vessels to relax (dilate)
What is produced by the sympathetic nervous system that affects blood vessels?
What is its effect?
Noradrenaline
It causes blood vessels to contract through action on alpha receptors
How does adrenaline affect blood vessels?
It causes blood vessels to dilate
What are ‘endo-en paracrine’ influences on blood vessels?
They are locally produced hormones
- angiotensin II
- bradykinin
- histamine
How does histamine affect blood vessels?
It is a vasodilator which is released in response to inflammation
How does bradykinin affect blood vessels?
It is produced by the endothelium during exercise
It causes dilation of blood vessels
What are the metabolic influences on blood vessels?
- pCO2, H+, K+
- adenosine
- pO2
What are extravascular influences on blood vessels?
compression
What is the effect of adenosine on blood vessels?
It causes blood vessels to dilate (coronary and all over the body)
This increases blood supply to the heart
What is the diameter and length of a capillary?
diameter: 5 - 10 micrometres
length: 0.5 - 1 mm
What are capillaries made from?
A single layer of endothelial cells
This is surrounded by a basal lamina
what is the blood cell viscosity of capillaries?
They have a slow blood cell velocity of approximately 0.3 - 1.0 mm/sec
What is the diffusion distance like between each capillary and the cell it serves?
There is a short distance between each capillary and the cell it serves
What are intercellular junctions like between cells?
Junctions between capillary cells are “tight”
What are the 2 exceptions to capillary cell junctions being “tight”?
In the liver and bone marrow there are large clefts between cells
Why are there large clefts between capillary cells in the liver and bone marrow?
Cells are leaving and entering the circulation at this point
The holes allow larger blood cells to enter or exit the circulation
What would happen if there were large clefts between capillary cells all over the body?
There would be swellings as too much fluid would leak out of the capillary
What are the capillaries like in the central nervous system?
The “blood-brain barrier” in the CNS has capillaries which are very tightly bound together
Why are the capillaries tightly bound together in the brain?
To prevent swelling in the brain
This causes headache due to a pressure on the nervous system
What are the types of blood vessels which lead to the capillary beds?
- collateral arteries
- arterioles
- metarterioles
- capillary
What is a metarteriole?
a short microvessel in the microcirculation that links arterioles and capillaries
What is a precapillary sphincter?
a band of smooth muscle that adjusts blood flow into the capillaries
Where is a precapillary sphincter found?
at a point where the capillary originates from the metarteriole
What is an arteriovenous anastomosis?
a connection between an artery and a vein
blood flows from an artery to a vein without passing through a capillary network
What are the 2 different types of capillary?
- continuous capillary
2. fenestrated capillary
What is a continuous capillary and what is its composition like?
the endothelial cells provide an uninterrupted lining
only small molecules, such as water and ions, can pass through intercellular clefts
where are continuous capillaries found?
blood-brain barrier
What is a fenestrated capillary and what is its composition like?
they contain fenestrations or pores which allow larger cells to pass through
How is blood flow to the capillaries controlled?
By resistance vessels
These are arterioles that determine how much blood enters the capillaries and perfuses an organ/tissue
