What is Afterload?
The resistance that the heart has to work against to eject blood (systole)
What does an increase in systemic vascualr resistance cause?
Increased afterload (more resistance the heart has to work agaisnt for the ejection of blood)
Increased afterload result in increased cardiac workload (may reuslt in heart failure)
What are the results of an Increased or Decreased Afterload?
- Increased: Reduces stroke volume, increases end diastolic pressure (heart has to do more work, to maintain stroke volume the heart has to contract more forcefully - via increasing sympathetic stimulation to maintain cardiac output)
- Decreased; Increases stroke volume, reduces end diastolic pressure
What factors affect the arterial blood pressure?
- Physical Factors - arterial blood volume and arterial compliance
- Physiological Factors - Cardiac Output and Total Peripheral Resistance
- Many Others - Age, Gender, Position where measured, Posture, Sleep, Pregnancy, Exercise, Emotion, Haemorrhage, Disease
How is Mean Arterial Pressure regulated?
Sensors (baroreceptors)
Intergrating system/brain assessing the correctness of pressure
Effectors or mechanisms to return the pressure to the required levels
What is the role of chemoreceptors in afterload/MAP?
Respond to chemical changes (CO2, H+, O2)
What is the role of the carotid sinus in context of blood pressure?
Recieves baroreceptor nerve fibres via the carotid sinus nerve
When arterial pressure is elevated tunica media becomes more elastic and allows expansion
Tunica adventitia is thickened to accomodate rich network of afferent nerve endings
Describe the baroreceptor activation
- The baroreceptor detects an increase in blood pressure
- Diameter of arteries increases
- Elastic conversion of tunica media amplifies increase in blood vessel diameter (detect length not pressure)
- Baroreceptor nerve endings within the adventitia stretch (mechanical distortion of nerve endings)
- Discharge of action potentials occurs
- Impulse in carotid sinus nerve
How is total peripheral resistance controlled in the vessels?
Arterioles are the main site of resistance meaning the greatest fall in pressure occurs across the arterioles
The diameter of the arterioles determines resistance to blood and therefore controls blood flow
Arteriolar radius controls TPR (increased blood pressure/hypertension is a consequence of arteriolar narrowing)
What factors effect arteriolar radius?
- Metabolic
- Neural
- Hormonal
- Mechanical
- Endothelial
All vascular smooth muscle is in a state of basal tone - the level of basal arterial tone dictates the variation of vascualr resistance that can be chieved in vessel
Explain the metabolic factors affecting the arteriolar radius
Rate of blood flow closely relates to the metabolic activity
Oxygen is the main controlling factor - (decreased oxygen results in reduced basal tone = vasodilation)
So an increased amount of tissue work requires increased blood flow to the tissues for metabolism (oxygen demand increases)
Explain the neural factors affecting the arteriolar radius
Arteries, arterioles, veins, muscular venules, arteriovenous anastomoses all innervated by sympathetic vasomotor fibres.
Vasoconstircitve effects predominate the sympathetic vasomotor fibres through alpha adreno receptors on the vascualr smooth muscle cells
Explain the hormonal factors affecting the arteriolar radius
Hormones - Noradrenaline and adrenaline (from adrenal medulla when its sympathetic nerves are stimulated) cause vasoconstricition
RAAS - Angiotensin II stong vasoconstrictor
ADH - vasoconstrictor in response to decreased blood volume
How do vasoconstrictors work?
Usually raise concentration of cellular calcium ions to cause vasoconstricition of smooth muscle
