Blood Vessels Flashcards
How does cross-sectional area change as blood flows into capillary beds?
Why?
Cross-sectional area increases as blood flows into capillary beds
This gives a larger surface area for exchange of gases and nutrients
In which type of vessel is the speed of blood flow greatest?
Speed of flow is greater when blood is travelling through a wider blood vessel e.g. aorta
What is flow like in the capillaries and why?
There is continuous flow and it is not pulsatile
This allows for the most efficient exchange
Why are arteries more muscular than veins?
To withstand the high-pressure blood coming from the heart
What are the roles of accessory pumps in the veins?
They help to move blood from the capillaries back to the heart, against gravity
Why are arteries always found next to veins?
The artery acts as an accessory pump as it pulsates to push blood up in the vein
What are the 2 types of artery?
Muscular and elastic
How are muscular and elastic arteries connected?
Muscular arteries draw blood from an elastic artery and branch into “resistance vessels”
e.g. small arteries and arterioles
How does the structure of a muscular artery differ to an elastic artery?
The elastic artery has a much larger lumen and thinner layer of tunica media
The muscular artery has a much thicker tunic media and narrower lumen
What is the role of the muscular artery?
They are involved in distributing blood between different circulations
What is the key property of elastic arteries?
Where are they found?
They have the ability to stretch in response to each pulse
They receive blood directly from the heart - pulmonary artery and aorta
What is the endothelium and what type of blood vessels is it present on?
It is a single layer of cells that is present on every blood vessel
What are the three layers of a blood vessel?
- tunica intima (innermost)
- tunica media
- tunica externa (outermost)
What is the structure of the tunica intima?
A single layer of endothelial cells supported by an internal elastic lamina
The endothelial cells are in direct contact with the blood flow
What is the structure of the tunica media?
What is its main component?
It consists of smooth muscle, elastic tissue and collagen
Its main component is smooth muscle
What is the tunica externa comprised of?
It is mainly comprised of collagen fibres
It also consists of the vasa vasorum
What is the vasa vasorum?
A network of small blood vessels that supply the walls of large blood vessels
Where are the internal and external elastic laminae found?
internal elastic lamina is between the tunica intima and tunica media
external elastic lamina is between the tunica media and tunica externa
What is the venous return?
The amount of blood returned to the heart each minute via the veins
What is the pressure in the veins?
What is significant about veins being ‘compliant’?
pressure is virtually nothing
veins are compliant so can expand to fill with larger volumes of blood
What is significant about veins having a large diameter and low resistance?
They favour flow as there is little loss of pressure due to friction
What 3 factors help venous return?
- arterial pulse acting as an external pressure
- skeletal muscle contraction acting as an external pressure
- negative pressure in the thorax during inspiration
Why does negative pressure in the thorax during inspiration aid venous return?
During inspiration, negative pressure sucks blood back into the chest towards the heart
What does the area surrounding an artery and a vein contain?
Fatty cells
What is the most common pathology of the veins?
Deep vein thrombosis (DVT)
This is the blockage of a vein by a thrombus
Which veins may be affected by a DVT if it extends?
If it extends above the knee - femoral vein
If it extends into the pelvis - iliac veins
If it extends into the abdomen - inferior vena cava
What is thrombophilia?
The tendency to develop thrombosis
It presents itself with recurrent thromboses
What are common types of hereditary thrombophilia?
- deficiencies in anticoagulation factors
these are protein C, protein S and antithrombin
- mutations in the factor V and prothrombin genes
How does the pressure of pulmonary circulation compare to systemic circulation?
Why?
Pulmonary circulation is at a much lower pressure than systemic circulation in ALL types of blood vessel
Not much pressure is needed to move blood to the lungs, compared with moving it around the whole body
What would happen if the pressure in pulmonary circulation was too high?
It could cause leakage from the capillaries in the lungs
What is normal arterial pressure in systemic and pulmonary circulation?
Systemic - 120/80 mmHg
Pulmonary - 25/10 mmHg
What is normal capillary pressure in systemic and pulmonary circulation?
Systemic - 30 mmHg
Pulmonary - 10 mmHg
What is normal venous pressure in systemic and pulmonary circulation?
In both circulations it is 2 - 5 mmHg
What is local “metabolic” control of pulmonary vessels?
Low alveolar pO2 causes constriction of the surrounding blood vessels
This allows blood to be sent somewhere else to be oxygenated
What is the less important means of control of the pulmonary vessels?
Autonomic control
This goes to the outer layer of the blood vessel to stimulate contraction or relaxation
What is a pulmonary embolism?
What is it caused by?
Blockage of part of the pulmonary arterial tree
Due to part of a blood clot breaking off and moving
May also be a fat embolus or amniotic fluid embolus
How does pulmonary embolism lead to low oxygen levels in the blood?
It results in impaired perfusion of the alveoli in the lungs
What are the 5 common symptoms of pulmonary embolism?
- shortness of breath/breathing rapidly
- sharp, stabbing chest pain when taking a deep breath
- coughing or coughing up blood
- rapid heart rate
- dizziness and fainting
What is the capacity of cerebral circulation?
750 millilitres of blood per minute
15% of total cardiac output
What is the blood pressure in the brain?
it is kept between 50 and 150 mmHg
What is autoregulation of blood vessels in the brain?
The pressure is regulated by dilation or constriction of the cerebral blood vessels
Maintaining this pressure is autoregulation
Under what conditions can autoregulation not occur?
If there is either a very high or a very low blood pressure
What happens if blood pressure falls below 50 mmHg in the brain?
- impaired dilation
- artery collapse
- ischaemia
this can lead to cerebral infarction (stroke)
What happens if blood pressure rises above 150 mmHg in the brain?
- force-mediated dilation
- increased flow
- vasogenic oedema
this can lead to cerebral infarction (stroke)
What is flow of blood determined by?
- viscosity of the blood
- dilation of blood vessels
- net cerebral perfusion pressure, which is determined by blood pressure
What is does cerebral infarction begin with?
Diseases of the wall of the arteries going to or within the brain
e.g. atherosclerosis
How does atrial fibrillation impact cerebral infarction?
Dilated atria of the heart allow blood clots to form
What does impaired blood flow to the brain lead to?
Ischaemia of brain tissue as there is a reduced oxygen supply
This may lead to death of cells in brain tissue (necrosis)
What is the equation that involves pressure, flow and resistance?
V = IR
pressure = flow x resistance
What three factors influence the severity of a stroke?
- increased blood volume (flow)
- increased blood pressure
- increased resistance to blood flow
How does hypertension lead to increased resistance to blood flow?
It begins with salt and water retention, which increases blood volume
This increases blood pressure
Resistance to flow is increased as a response to protect vital organs
What is hypertrophy in relation to blood vessels?
Thickening of arteries in response to high blood pressure
Without this, the vessels would burst
What is the difference between arterosclerosis and atherosclerosis?
Arterosclerosis involves thickening of the tunica media to increase the stiffness of the blood vessel
Atherosclerosis involves plaques in the tunica intima which reduce the blood flow
Why does the heart have the highest arterial/venous oxygen difference of any major organ?
Flow of blood through the heart is tightly linked with oxygen demand
The heart has a very high basal oxygen consumption
Between what values must the heart maintain perfusion pressure?
60 and 200 mmHg
What happens to the blood flow in the heart when aortic pressure changes?
The heart has very good autoregulation so maintains normal blood flow
What is the role of adenosine on the heart?
It is an important mediator of active hyperaemia and autoregulation
What is hyperaemia?
An excess of blood in the vessels supplying the heart
What is the role of nitric oxide on the heart?
It is an important regulator in coronary blood flow
Why must the heart receive all of its blood supply from coronary vessels during diastole?
During systole, all of the blood vessels are compressed due to ventricular contraction
What is the role of sympathetic and parasympathetic stimulation of the heart?
Sympathetic activation leads to vasodilation due to increased metabolic activity
Parasympathetic stimulation leads to moderate coronary vasoconstriction through the vagus nerve
Which blood vessels in the heart are most vulnerable to ischaemia?
Why?
The blood vessels that are furthest on the inside of the heart
If there is a pressure inside the heart, this region is compressed the most
Blood cannot reach the innermost part of the heart
What is myocardial infarction caused by?
Narrowing of the arteries as a result of coronary atheroma (cholesterol deposits in the wall of the artery)
Or due to coronary thrombosis
What does myocardial infarction lead to?
Ischaemia due to reduced blood supply
This then leads to death of heart cells in the myocardium - necrosis of the heart muscle
How does myocardial infarction affect contraction of the myocardium and electrical activity?
It leads to impaired contraction of the myocardium and abnormal electrical activity of the heart cells
What factors cause chronic injury to the endothelium?
What does this chronic injury lead to?
- smoking
- diabetes
- high blood pressure
- age
- high cholesterol
This leads to the accumulation of oxidised LDL cholesterol
What do happens to macrophages when they encounter oxidised LDL cholesterol?
They will ingest the cholesterol as they recognise it as foreign
They then die and become foam cells
How does an atherosclerotic plaque form after the foam cells have accumulated?
Smooth muscle cells change and proliferate and the extracellular matrix accumulates
The plaque will become inflamed and eventually rupture
What is the fibrous cap of the atherosclerotic plaque made of?
What increases the possibility of rupture?
Smooth muscle and collagen
Calcification of the fibrous cap increases possibility of rupture
what leads to thrombus formation when the atherosclerotic plaque ruptures?
Exposure of the collagen
Which vessels are the most vulnerable to atherosclerosis?
Arteries
This occurs when cholesterol deposits cause the arteries to narrow
Which vessels are the most vulnerable to thrombosis?
What are the 2 types
Veins
This may be a white thrombus or a red thrombus
When does a white thrombus occur?
When there is a fast flow of blood
There is no stagnation of blood flow, but the vessel is damaged
Platelets aggregate around the site of injury
What drugs are used to treat a white thrombus and why?
antiplatelet drugs
The white thrombus is rich in platelets
when does a red thrombus occur?
When there is a slow flow of blood
It contains a lot of red blood cells which turn to jelly
What drugs are used to treat a red thrombus?
Anticoagulants are used to treat this and prevent clotting
What chemical is produced by the endothelium of blood vessels?
Nitric oxide
Where does nitric oxide act on and what does it cause?
It acts on the smooth muscle in the tunica media to cause relaxation of blood vessels
What is meant by nitric oxide being volatile?
It remains in the circulation for a matter of seconds
What is the result of interactions between vascular endothelial cells and vascular smooth muscle cells?
The release of nitric oxide and endothelin
What is released by the sympathetic nervous system that acts on blood vessels?
adrenaline and noradrenalin
What circulating hormones will act on the blood vessels?
angiotensin II and natriuretic peptides
Where is heparin found and what is its role?
It is naturally attached to the endothelium and prevents blood cells from sticking to it
What is the role of bradykinin?
It is a naturally occurring substance that will release nitric oxide, causing relaxation of smooth muscle
What is glyceryl trinitrate (GTN) spray used as a treatment for?
Angina patients as it mimics nitric oxide
Why must GTN not be given orally?
It undergoes rapid inactivation in the liver by nitrate reductase
What is the action of GTN and what are the side effects?
It rapidly dilates the blood vessels
Side effects include headaches and hypotension
What is meant by glyceryl trinitrate having a high first pass metabolism?
It enters the circulation very quickly, but also leaves the system quickly
What is isosorbide mononitrate given as a treatment for?
Angina management
It mainly affects the venous circulation but has some effect on arterial circulation and coronary arteries
How may the renin-angiotensin system become pathological?
It helps to keep salt and water in the body
This is not helpful in hypertension or heart failure
What are the 4 stages of platelet activity?
- adhesion
- activation
- aggregation
- platelet plug
At which stage of platelet activity do aspirin and clopidogrel act?
What is their mechanism of action?
Activation
Aspirin inhibits COX-1
Clopidogrel inhibits P2Y12 - a receptor for ADP
At which stage of platelet activity does Abciximab act?
What is its mechanism of action?
Aggregation
It is an a(IIb)b(3) inhibitor
This prevents fibrinogen or vWF from binding to the platelet
What are the 3 main antiplatelet therapies?
- COX-1 inhibition
- P2Y12 ADP receptor inhibitors
- a(IIb)b(3)-integrin inhibitors
What are heparin and low-molecular weight heparins used for?
Anti-coagulants
Why does the activity of Warfarin (anticoagulant) have to be monitored?
Lots of medications and foods interact with warfarin
Monitoring ensures an adequate, yet safe, dose has been taken
What mechanism is used to monitor warfarin?
Blood testing for INR
international normalised ratio
What is the INR target therapeutic range?
2.0 - 4.0
what happens if an INR reading is less than 2.0?
The blood will clot more quickly
There is a higher risk of developing a blood clot
what happens if an INR reading is greater than 4.0?
The blood will clot more slowly
There is a higher risk of bleeding
