The Regulation of Blood Pressure Flashcards
How do the kidneys play a role in the regulation of blood pressure?
They are largely responsible for sodium and water transport and we use fluid dynamics to increase or decrease blood volumes
What is blood pressure?
the force placed on the walls of blood vessels as blood flows through it
What are changes in blood flow controlled by?
changes in the diameter of arteries and cardiac output
What is the minimum pressure needed for capillary exchange?
capillary hydrostatic blood pressure
what is capillary exchange?
When fluids and substances are exchanged across a capillary network
How do you work out blood pressure?
cardiac output x total peripheral resistance
How do you work out cardiac output?
heart rate x stroke volume
What is the total peripheral resistance dependent upon
changes in the arteriolar radius
What are the three processes that control blood vessel tone (changes in arteriolar radius)?
- automatic/ intrinsic
- short-term
- long-term
Why may homeostasis of blood flow be disturbed?
- Physical stress (trauma)
- Chemical Changes
- Increased tissue activity
What will homeostasis being disturbed lead to?
inadequate local blood pressure and blood flow
What does autoregulation do?
- Allows blood vessels to alter their diameters as a consequence of their environment around them
- Leads to a local decrease in resistance and increase in blood flow (in response to a low blood pressure)
What is the intrinsic control mechanism driven by?
vasodilatory metabolites
Why are vasodilatory metabolites produced?
- as a consequence of active tissue metabolism
- A inactive constricted vessel becomes active as a consequence of metabolism and produces vasodilatory metabolites
What are some examples of vasodilatory metabolites?
- H+
- CO2
- K+
- adenosine
- lactate
What do vasodilatory mechanisms do?
- Can act by themselves on the blood vessels in order to bring about a change in shape
- they will work on different systems in order to bring about vasodilation
- if vasodilation takes place more blood will be supplied to the tissue and that is exactly what you need when there is a low blood flow
Summarise the autoregulatory process
It is an intrinsic one in which healthy blood pressure can be restored to a tissue as a consequence of disturbance of homeostasis. The chemical mediators responsible for the intrinsic control are classified as Vasodilatory Metabolites
However what happens in pulmonary circulation?
A decrease in O2 and increase in CO2 causes vasoconstriction
What happens in the short-term regulation of blood flow?
- Stimulation of receptors (chemoreceptors and baroreceptors) sensitive to changes in blood pressure and blood chemistry. The receptors send signals send signals
- These signals activate cardiovascular centres in the CNS
- These centres bring about short-term elevation of BP by SNS stimulation of the heart rate and peripheral vasoconstriction
- Leads to homeostasis: healthy blood pressure and blood volume
What are baroreceptors?
pressure detectors that can respond to changes in blood pressure by generating electrical responses that can be relayed to the central nervous system – sensory cells that detect pressure changes
What are chemoreceptors?
chemical sensors
Where are chemoreceptors found?
- in the carotid body
- inside the blood vessels of the CNS
What do chemoreceptors do?
send signals to the cardio regulatory centres in the medulla oblongata
How do baroreceptors work in response to low blood pressure?
- If there is an inadequate blood supply or fall in blood pressure baroreceptors become inhibited (they are no longer stretched)
- This is then detected by the CNS through afferent fibres
- CNS responds to fall in blood pressure/ inadequate blood supply by increasing cardiac output, heart rate and bringing about vasoconstriction
What are the centres in the CNS that receive signals from the baroreceptors and are they activated or inhibited in response to the inhibition of baroreceptors (low blood pressure)?
- Cardioacceloratory centre (activated due to inhibition of baroreceptors)
- Vasomotor centres (activated due to inhibition of baroreceptors)
- Cardioinhibitory centre (inhibited due to inhibition of baroreceptors)
If there is a rise in blood pressure what do the baroreceptors do?
- Stimulation of the baroreceptors
- Baroreceptors send afferent signals
- Inhibition of carioacceloratory centre
- Inhibition of vasomotor centres
- Activation of cardioinhibitory centre
- Leads to decrease in cardiac output and heart rate and vasodilation
What does it mean that baroreceptors are spontaneously active?
- they are always stretched and detecting blood pressure
- They can then become supressed or further activated in response to changes in blood pressure
What do the carotid chemoreceptors do?
- In carotid body detect rise in CO2 or decrease in pH (due to respiration)
- This stimulates cardioacceleratory centre
- Inhibits cardioinhibitory centres
- Stimulates vasomotor centres
- Brings about rise in cardiac output and rise in heart rate
- Also responds to a decrease in O2 by increasing respiration rate
Summarise the role of chemoreceptors
Chemoreceptors are peripheral sensors of changes of blood chemistry and work in association with baroreceptors to bring about changes in heart rate and vasomotor tone as a consequence of changes in metabolic status of tissues
What do CNS chemoreceptors do?
- Detect a rise in CO2 or decrease in pH
- This results in vasodilation of cerebral blood vessels
- Increases blood flow to the brain
What does long term regulation of blood flow involve?
Stimulation of the endocrine system
What does the stimulation of endocrine responses bring about?
- Long-term increases in blood pressure (vasoconstriction by the hormone angiotensin II) and blood volume (by water being reabsorbed and put into blood circulation – involves aldosterone and ADH). Leads to:
- Homeostasis: healthy blood pressure and blood volume
What happens when low blood pressure is detected by the kidneys?
- Kidney releases renin which acts upon angiotensinogen (released from the liver). This produces:
- Angiotensin I
- Angiotensin I converted in lungs by ACE (angiotensin converting enzyme) into Angiotensin II
- Angiotensin II is a vasoconstrictor bringing about a decrease in blood diameter and bringing about an increase in blood pressure
- Angiotensin II also a signal for the initiation of the release of the steroid aldosterone. Aldosterone is released from adrenal glands:
Increases Na+ retention
Water will flow by osmosis
Fluid volume increase
Blood volume increased
what are the two main role of Angiotensin II?
- stimulating vasoconstriction
- stimulating fluid retention
What does antidiuretic hormone (ADH) do to increase blood volume?
increases thirst increasing fluid volume of blood
What does erythropoietin (EPO) do to increase blood volume?
increases formation of RBC’s which increases the viscosity of blood and therefore acts to increase blood volume
Why in pulmonary circulation does decreased O2 and increased CO2 bring about constriction of the blood vessels?
Because under normal O2 rich conditions, the pulmonary vessels (alveoli) dilate and blood flow increases to absorb O2. However when O2 is depleted, pulmonary vessels constrict to shunt O2 to other more oxygen rich alveoli cells.
What are the other ways in which coronary vessels regulate blood flow to the heart?
- Metabolic
- Mechanical
- Neural
- Hormonal
- Paracrine
When does the vessel lumen diameter decrease?
as the vessels branch out more and go closer to the capillaries
what are blood pressure and speed of blood flow proportional to?
the cross section of the vessels involved
In terms of cross-sectional area what is important?
the combined cross area of all of a particular vessel type
Describe blood flow velocity as it goes through the body
as the total cross-sectional area of the vessel lumens increase from the aorta toward the capillaries the velocity of blood flow decreases. Blood flow velocity then increases as the total cross-sectional area decreases from the capillaries toward the venae cavae
What do we use to report a single blood pressure value?
The mean arterial pressure
What does a high blood pressure do?
puts more stress of the blood walls and accelerates the development of arteriosclerosis. It also increases the risk of aneurysms, heart attacks and strokes
Why di the MAP and pulse pressure decline as the distance from the heart increases?
- MAP declines as arterial branches becomes smaller. Blood pressure decreases as it overcomes friction
- Pulse pressure lessens due to the cumulative effects of elastic rebound along the arterial system (the elastic rebound is not as strong)
Why does pressure decline very slowly as blood moves through the venous system?
veins offer little resistance
What are the most important processes that move materials across capillary walls?
diffusion, filtration and reabsorption
What is tissue perfusion?
The amount of blood flow through the tissues
What is vasomotion?
rhythmic changes in vessel diameter cause by the precapillary sphincter contracting and relaxing. Means that blood flow across capillaries occurs in pulses
What are vasodilators and what do they do?
- factors that promote the dilation of precapillary sphincters
- When the dissolved O2 concentration decreases within a tissue local vasodilators cause the capillary sphincters to relax so blood flow to the area increases
What do local vasodilators include?
- Decreased O2 level or increased CO2 level in tissue
- Lactate generated by tissue cells
- Nitric oxide (NO) released from endothelial cells
- Rising concentrations of potassium ions or hydrogen ions in the interstitial fluid
- Chemicals released during local inflammation including histamine
- Elevated local temperature
What are examples of vasoconstrictors?
- Prostaglandins and thromboxanes released by activated platelets and white blood cells
- Endothelins released by damaged endothelial cells
through what mechanism do the different centre in the CNS affect cardiac output and vasomotor responses?
- Carioacceleratory centre: increases cardiac output through sympathetic innervation
- Cardioinhibitory centre decreases cardiac output through parasympathetic innervation
- Vasomotor centre directs vasomotor responses in blood vessels
What two populations of neurones does the vasomotor centre contain?
- A very large group responsible for widespread vasoconstriction
- A smaller group responsible for the vasodilation of arterioles in skeletal muscles and the brain
What are the vasodilator neurons innervating peripheral blood vessels in most tissues?
adrenergic (release norepinephrine) NE stimulates smooth muscles in the walls of arterioles producing vasoconstriction
How do the vasodilator neurones innervating the blood vessels in the skeletal muscle and the brain bring about vasodilation?
The stimulation of these neurones relaxes muscle cells in the walls of arterioles producing vasodilation. This relaxation is triggered by NO
Where are baroreceptors found?
In the aortic sinuses
Why are carotid sinus receptors extremely sensitive?
because blood flow to the brain must remain constant
What does the Bainbridge reflex respond to?
a stretching of the right wall of the atrium
The Valsalva manoeuvre checks normal cardiovascular responses to changes in arterial pressure and venous return. How does it work?
Involves trying to exhale forcefully so that no air can leave the lungs and pressure in the thoracic cavity rises sharply. The action causes reflexive changes in blood pressure and cardiac output
What are the four important functions of Angiotensin II?
- It stimulates the adrenal production of aldosterone causing Na+ reabsorption and K+ loss by the kidneys
- It stimulates the secretion of ADH
- It stimulates thirst
- It stimulates cardiac output and triggers the constriction of the arterioles increasing blood pressure
What do cardiac muscle cells in the right atrium produce in response to excessive stretching during diastole?
atrial natriuretic peptide (ANP)
How do peptide hormones reduce blood volume and blood pressure?
- increasing sodium ion excretion by kidneys
- Increasing water loss by increasing volume of urine
- Reducing thirst
- Block the release of ADH, aldosterone, epinephrine and norepinephrine
- Stimulate peripheral vasodilation
What does the cardiovascular system treat as the top priority?
Blood flow to the brain
