Cardiovascular control 2 Flashcards
How do you calculate stroke volume?
How do you calculate cardiac output?
How do you calculate blood pressure?
Describe the 2 circuits of the circulatory system
The 2 circuits- pulmonary (to the lungs) and systematic (to the body)
Pulmonary- RA-RV- lungs- heart (LHS)
Systematic- LHS heart- body
What is the venous volume distribution affected by?
Venous volume distribution affected by peripheral venous tone, gravity, skeletal muscle pump and breathing.
What does the central venous pressure (mean pressure in the RA) determine?
It determines the amount of blood flowing back to the heart.
What determines the stroke volume?
The amount of blood flowing back to the heart
How is blood flow controlled?
Constriction of veins reduces compliance and venous return.
In arterioles, constriction determines:
- Blood flow to downstream organs
- Mean arterial blood pressure
- The pattern of blood flow to organs
Flow is primarily changes by the radius
What are the 2 mechanisms to regulate blood flow?
1) Local mechanisms are intrinsic to the smooth muscle. Important for local blood flow regulation
2) Systemic mechanisms that are extrinsic to the smooth muscle.
Hormones (catecholamines) affect constriction and dilation of vessel.
Autonomic nervous system can stimulate contraction of smooth muscles.
What are the local mechanisms for regulating blood flow?
- Autoregulation
- Myogenic theory
- Metabolic theory
- Injury
What is autoregulation?
It is the intrinsic capacity to compensate for changes in perfusion pressure by chnaging vasculae resistance
What is the myogenic theory?
It states that smooth muscle fibres respond to tension in the vessel wall (as pressure rises, fibres contract; stretch sensitive channels are involved)
What is the metabolic theory?
As blood flow decreases, metabolites accumulate and the vessels dilate; subsequent increased flow will dilate diluting the effect of the xs electrolytes.
Describe the injury mechanism to regulating blood flow
If vessels are injured, there are clumping platelets to produce clot. Serotonin will constrict vessel around injured site.
Hormones can be local or circulating: what’s the difference and give some examples
Local- endothelium derived
Circulating- non-endothelium derived
What are the 2 branches of the autonomic nervous system? What are the differences between them?
Parasympathetic- rest and digest. Important for controlling the HR.
Sympathetic- fight or flight. Important for controlling circulation
PARASYMPATHETIC: Pre ganglion fibres are long and short post-ganglionic. Uses ACh
SYMPATHETIC: short pre ganglionic and long post ganglionic. Uses noradrenaline. Important for controlling ciruculation and size of vessels.
Sympathetic nervous system innervation to blood vessels
The SNS innervates the heart and all vessels except capillaries and precapillary sphincters and some metarterioles.
SNS innervation elsewhere is variable:
Heavy innervation: kidneys, gut, spleen and skin
Poorly innervated: skeletal muscle and brain
Noradrenaline preferentially binds to alpha1 adrenoreceptors to cause smooth muscle contraction and vasoconstriction
What is the vasomotor centre and where is it located?
It is located bilaterally in the reticular substance of the medulla and the lower third of the pons (midbrain-pons-medulla)
The VMC has a vasoconstrictor (pressor) area and a vasodilator (depressor) area. There is also a cardioregulatory inhibitory area.
The VMC transmits impulses distally through the spinal cord to almost all blood vessels.
What has effects over the vasomotor centre?
Many higher centres of the brain, like the hypothalamus cna have powerful excitatory or inhibitory effects on the VMC.
What do the different parts of the VMC do?
Lateral portions of the VMC control the heart activity by influencing heart rate and contractility.
Medial portion of VMC transmits signals via vagus nerve to heart that tend to decrease HR.
How is blood vessel diameter affected by nervous control?
Blood vessels receive SNS post-ganglionic innervation by noradrenalin neurotransmitter.
There is always some level of tonic activity which can be increased (vasoconstriction) and decreased (vasodilation). More tonic acitivity- constriction, less tonic activity- dilation
Generally, there is no PNS innervation to vasculture.
What will innervate the heart (i.e. what will increase the heart rate?)
- Increase in SNS activity
- Increase in plasma adrenaline
- Decrease in PNS activity
What is the controlling force of contraction?
Ca2+ channels allow Ca2+ influx in the cardiac cell.
The uptake carries into intracellular stores. The sarcoplasmic reticulum is stimulated to release internal stores of Ca2+
NB- beta 1 receptors are in the heart
alpha one receptors in skeletal muscle
How can you increase stroke volume?
- Increase SNS efferents to the heart
- Increase end diastolic volume
- Increase plasma adrenaline
End diastolic volume can be increased by higher venous return and arterial pressure. Also a greater respiratory movement (and therefore decreased intrathoracic pressure), contributes to a higher end diastolic volume.
What is the feedback mechanism for blood pressure?
Comparator (cardiovascular control centre) compares what bp is to what is should be.
Either sends signals to autonomic nervous system to increase and decrease it by the appropriate response.
If too high, decrease SNS, increase PNS, decrease angiotensin II and decrease ADH
If too low, increase SNS, decrease PNS, increase angiotensin II and increase ADH.
New BP is sensed by baroreceptors- they are the sensors that provide information- key to good blood pressure.
Barorecpeotrs exist in the aortic arch. They send afferent back to vasomotor centre from aortic arch.
Afferent- from barorecptors
Efferent- to the sinoatrial node
Therefore is a dual carriage way nerve. If there is more stretch in the carotids, they send more (i.e. when the pressure builds up). Greatest change is in the physiological range.
Describe PSN innervation
Barorecptors are responsible for bringing about change
Mechanoreceptors in carotid sinus change firing rate in response to changes in pressure.
Mechanoreceptors in the aortic arch change their firing rate in response to changes of pressure.
More information on baroreceptors
They respond to changes in arterial pressure- if there is greater pressure, there is greater stretching.
The carotid sinus baroreceptors respond to pressures between 60 and 180 mmHg byt are most sensitive between 90 and 100 mmHg
What is reciprocal innervation?
Increased afferent activity from baroreceptors stimulates the PNS nerves to the heart. SIMULTANEOUSLY, it inhibits the SNS innervation to the heart, arterioles and the veins
PNS innervation
A drop in BP leads to an increased stretch of baroreceptors.
Part of the brain responsible for this communication is the medulla oblongata- namely the cardioregulatory and vasomotor centres.
The increased parasympathetic stimulation of the heart decrease the heart rate.
The vagus nerve connects the two areas in both directions- afferent and efferent.
SNS innervation
Same situation- increase in bp, increase in baroreceptors.
The cardioregulatory and vasomotor centres in the medulla oblongata are responsible for control. However,
PSN afferent is through the vagus nerve but the sympathetic nerves are SNS efferent.
Decreased sympathetic stimulation of the heart decreases the HR and can cause vasoilation in the blood vessels
The carotid nerve activity controls reflexes
If pressures are low, less nerve activity
If pressures are high, there is greater nerve activity.
Control of venous return
The:
- greater the SNS activation of veins and
- greater the skeletal muscle pump and
- greater the blood volume and
- greater the respiratory movements
The greater the venous pressure and therefore the greater the venous return.
More blood is sent back to the RA. Therefore, for a greater venous return, increase sympathetic activation and decrease capacitance of veins and venules.
What is the feedback for blood pressure control (after someone has a haemorrhage?)
Flow diagram for maintaining arterial pressure
