Control Of The Circulation Flashcards
Why do we need to control the circulation
To maintain blood flow
To maintain arterial pressure
To distribute blood flow
Auto-regulation/homeostasis
To function normally
To prevent catastrophe
To maladapt in disease
Characteristics of arteries
Low resistance conduits
Elastic
Cushion systole
Maintain blood flow to organs during diastole
Where is the principle site of resistance of vascular flow
Arterioles
What is TPR
Total arteriolar resistance
What is TPR determined by
Local, neural and hormonal factors
What are 2 major roles of arterioles
Major role in determining arterial pressure
Major role in distributing flow to tissue/organs
What determines the radius of the arterioles
Vascular smooth muscle
What happens if the vascular smooth muscle contracts
The radius decrease so the resistance increases and the flow decreases
What happens when vascular smooth muscle relaxes
When the vascular muscle relaxes the radius increases which causes the resistance to decrease and the flow to increase
Why does vascular smooth muscle never completely relaxed
Myogenic tone
What does the slow flow of capillaries allow
It allows time for nutrients and waste to be exchanged
How is flow determined inside the capillaries
Arteriolar resistance
Number of open pre-capillary sphincters
Characteristics of veins
Compliant
Low resistance conduits
Valves present to aid venous return against gravity
Skeletal muscle/respiratory pump aids venous return
Vasoconstriction maintains venous return/pressure
How is cardiac output calculated
Heart rate X stroke volume
How is blood pressure calculated
Cardiac output X Total peripheral resistance
How is pulse pressure calculated
Systolic - diastolic pressure
How is mean arterial pressure calculated
Diastolic pressure + 1/3 pulse pressure
What governs flow
Ohms law and poiseuilles law
What is ohms law
Flow = pressure gradient/resistance
What is poiseuille’s equation
Flow = radius to the power of 4
What is the frank starling mechanism
Stroke volume increases as end diastolic volume increases, due to the length-tension relationship of muscle the end diastolic volume would increase so the stretch would increase along with the increased force of contraction. At rest the cardiac muscle is not at its optimum length. Increased vascular resistance increases end diastolic volume which increases stroke volume and increased cardiac output even if heart rate is constant
What is the goal of control of circulation
Maintain blood flow
CO = SV X HR
This needs pressure to bush blood through the peripheral resistance
What is blood pressure
Blood pressure is the pressure of blood within and against the walls of the arteries
When is systolic blood pressure the highest
It is highest when the ventricles contract
When is diastolic blood pressure the lowest
It is lowest when the ventricles relax
How is blood pressure measured
Using the brachial artery
Why is the brachial artery used to measure blood pressure
Convenient to compress, and it is at the level of the heart
In what ways can blood pressure be controlled
Auto regulation
Local mediators
Humoral factors
Baroreceptors
Central (neural) control
What is myogenic auto regulation
Myogenic autoregulation of the blood vessels is when the smooth muscle of the blood vessel stretches under the pressure of the blood within the lumen, so when this decreases the vessel will contract until the diameter normalises or is slightly reduced in normal size
Why does autoregulation vary
It is an intrinsic ability of an organ and so will have constant flow despite perfusion pressure changes
Where has excellent autoregulation
Renal, cerebral and coronary
Where has moderate levels of autoregulation
Skeletal muscle/splanchnic
Where has poor autoregulation
Cutaneous vessels
How is blood flow maintained to vital organs such as the brain and heart
Intrinsic control
What is examples of a vasoconstrictor
Endothelin-1
Internal blood pressure/myogenic contraction
What are examples of vasodilators
Hypoxia
Bradykinin
Tissue breakdown products
What is essential for control of the circulation
Endothelium
What are examples of circulating hormonal vasoconstrictors
Epinephrine - skin
Angiotensin II
Vasopressin
What are examples of circulating hormonal vasodilators
Epinephrine- muscle
Atrial natriuretic peptide
Where are primary baroreceptors found
Carotid sinus and aortic arch
Where are secondary baroreceptors found
Veins
Myocardium
Pulmonary vessels
How do baroreceptors respond to increased blood pressure
Blood pressure increase is detected which results in increased firing which results in a decreases in cardiac output so reduces blood pressure
What are the role of arterial baroreceptors in the short term
Key role in short term regulation of blood pressure is minute to minute control in response to exercise or haemorrhage
What happens to baroreceptors if the arterial pressure deviates from norm for more than few days
The baroreceptors would adapt to a new baseline pressure which would be seen in hypertension
What is a major factor in long-term blood pressure control
Blood volume
What are the main neural influences on the medulla
Baroreceptors
Chemoreceptors
Hypothalamus
Cerebral cortex
Skin
Changes in blood oxygen and carbon dioxide
What does stimulation of the anterior hypothalamus causes on cardiovascular reflexes
Decreased blood pressure and heart rate
What is important in the regulation of skin blood flow in the response to temperature
Hypothalamus
How can the cerebral cortex affect blood flow and pressure
Stimulation of the cerebral cortex can usually cause increased vasoconstriction
Emotion can increase vasodilation and depressor responses
Where are central chemoreceptors found
In the medulla
How is short term blood pressure changes controlled
Baroreceptors
How is long term blood pressure change controlled
Volume of blood
Na+
H2O
Renin-angiotensin-aldosterone
ADH
What are the key central effectors
These are peripheral
Blood vessel vasodilation and vasoconstriction
Heart rate and contractility
Kidneys fluid balance in longer term control
