Blood Pressure - Reflex Control L13 Flashcards
Haemorrhage
Excessive or uncontrolled bleeding, which occurs when blood vessels are damaged, leading to blood loss.
Poiseuille’s Law
The flow is proportional to the radius to the power of four.
What is vasoconstriction?
Vasoconstriction is the narrowing of blood vessels caused by the contraction of the smooth muscle in the vessel walls. This process reduces the diameter of the blood vessels, particularly the arterioles, which in turn decreases blood flow to certain areas and increases blood pressure.
Why Vasoconstriction Happens During Hemorrhage?
Vasoconstriction (systemic and local) to try and maintain BP (Blood Pressure):
When the body loses a significant amount of blood, as in haemorrhage, blood pressure drops. The body compensates by constricting blood vessels (vasoconstriction). This is a reflex controlled by the nervous system to try to maintain adequate blood pressure and ensure that vital organs like the brain and heart continue to receive sufficient blood.
Systemic vasoconstriction affects the entire body, while local vasoconstriction targets specific areas, prioritizing blood flow to essential organs.
HR (Heart Rate) increase - Sympathetic Nervous System activated:
The sympathetic nervous system is part of the autonomic nervous system that responds to stress. In the case of haemorrhage, it triggers an increase in heart rate (tachycardia). This is a compensatory mechanism aimed at maintaining cardiac output (the volume of blood the heart pumps) to make up for the reduced blood volume.
The increase in heart rate helps circulate the remaining blood faster to meet the body’s needs.
Maintain stroke volume - increase contractility / venous constriction to increase venous return/preload:
Stroke volume is the amount of blood pumped by the heart with each beat. In haemorrhage, the body works to maintain this despite the loss of blood. One way it does this is by increasing the contractility of the heart muscle (the heart squeezes harder to pump more effectively).
Venous constriction involves the tightening of veins to push more blood back to the heart. This increases venous return (the amount of blood returning to the heart) and preload (the volume of blood filling the heart before it contracts). By doing so, the heart can maintain stroke volume, even with a reduced overall blood volume.
What are arterial baroreceptors?
- Specialized sensory nerve endings located primarily in the walls of large arteries.
- Baroreceptors detect the stretch in the arterial walls caused by changes in blood pressure. When blood pressure rises, the arterial walls stretch more, and when it falls, they stretch less.
- Signals connect up to the brain via cranial nerves
- Tonically active: can respond to increases and decreases in BP
The reflex path of baroreceptors - neural control of BP
From baroreceptors: monitor blood pressure and then travels to the medulla, then through nerves travel to the heart and blood vessels
What is every arterial and artery in the body innervated by?
The sympathetic nervous system
What happens if you increase your firing rate?
Increase in norepinephrine release onto a receptor, which causes the blood vessel to constrict
What happens if you decrease your firing rate?
Decrease in norepinephrine release onto a receptor, which causes the blood vessel to dilate
Graph showing arterial pressure over time, focusing on how vascular tone is affected by two key interventions: total spinal anesthesia and norepinephrine injection.
Baseline Arterial Pressure (Before Intervention):
At the start, the arterial pressure is stable around 100 mm Hg. This represents the normal functioning of the cardiovascular system with typical vascular tone.
2. Total Spinal Anesthesia:
Spinal anesthesia blocks the sympathetic nervous system, which normally maintains vascular tone by causing vasoconstriction (narrowing of blood vessels). When total spinal anesthesia is applied, there is an immediate drop in arterial pressure.
This happens because the blockade of the sympathetic nervous system causes vasodilation (widening of blood vessels), leading to a significant drop in systemic vascular resistance and blood pressure, as seen in the graph where arterial pressure drops below 50 mm Hg.
3. Steady Low Arterial Pressure:
After the application of spinal anesthesia, arterial pressure remains low for a few minutes. This is because the body’s sympathetic nervous system is not able to constrict the blood vessels, keeping blood pressure low.
4. Injection of Norepinephrine:
Around the 10-minute mark, norepinephrine is injected. Norepinephrine is a potent vasoconstrictor that works by stimulating the sympathetic nervous system receptors (α1-adrenergic receptors) on blood vessels.
This causes an immediate and sharp increase in arterial pressure, as norepinephrine constricts the blood vessels, thereby increasing vascular resistance and raising blood pressure.
However, this spike in blood pressure is temporary, and after a short period, the arterial pressure decreases back toward a more stable level (though still higher than the post-anesthesia level).
Where do we have the arterial receptors ?
In the aortic arch and carotid artery
Flowchart of baroreceptor reflex, a key mechanism in the regulation of blood pressure.
Stimulus: A decrease in blood pressure disrupts homeostasis.
Controlled Condition: The body is trying to maintain blood pressure at an appropriate level.
Receptors: Baroreceptors in the carotid sinus and the aortic arch detect the stretch or pressure in these vessels. When blood pressure decreases, the baroreceptors stretch less, which reduces the rate of nerve impulses.
Control Centers:
The cardiovascular (CV) center in the medulla oblongata responds by increasing sympathetic stimulation and decreasing parasympathetic stimulation.
The adrenal medulla also secretes more epinephrine and norepinephrine, which are hormones that increase heart rate and constrict blood vessels.
Effectors:
Heart: Increased heart rate and stroke volume boost cardiac output (CO).
Blood Vessels: Constriction of blood vessels increases systemic vascular resistance (SVR), helping raise blood pressure.
Response: These changes lead to increased blood pressure, helping to return it to normal.
Return to Homeostasis: As cardiac output and vascular resistance increase, blood pressure rises back to normal levels, restoring homeostasis.
BP = CO x TPR
CO = HR x SV
MEMORISE THIS FLOW CHART
Hormones that increase cardiac output - heart rate and contractility
Norepinephrine, epinephrine
Hormones that increase blood volume
Aldosterone, antidiuretic hormone
