Neural Reflexes and Control of Arterial Blood Pressure (B2: W3) Flashcards Preview

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Flashcards in Neural Reflexes and Control of Arterial Blood Pressure (B2: W3) Deck (42):
1

What two major physical factors determine the arterial (pulse) pressure?

  1. Arterial blood volume
  2. Areterial compliance

2

What physiological factors influence arterial blood volume and compliance?

  • Cardiac output (stroke volume x heart rate)
    • SV 
    • HR
  • Peripheral resistance (diameter, for the most part)

If we are able to change/regulate these factors, we can change/regulate the arterial blood pressure

3

What is responsible for feedback control of blood pressure? 

Baroreceptors

When they sense an increase in blood pressure, they send a message to the brain stem that regulates HR, SV and vessel diameter (resistance)

4

What are the components of the baroreceptor reflex?

  1. Signal - change in arterial pressure
  2. Dectectors/sensors - arterial baroreceptos
  3. Afferent pathways that translate the signal to a coordinating center
  4. A neural network (CNS, coordinating center), which compares a signal from the sesors with a comand signal which orginates in CNS
  5. A neural output which connects the nervous system to the target cells in the effector organs (efferent pathways)
  6. The target organs themselves (heart and peripheral blood vessels)

5

Where are the baroreceptors located?

  • In the wall of the carotid sinus, at the biforcation of the common carotid artery
  • In the aortic arch

They are branched and coiled bare ends of myelinated sensory nerve fibers

The cell bodies are in the ganglia near the brainstem

6

What exactly do the arterial baroreceptors sense?

Stretch

They are stretch receptors

Vascular wall tension indicates change in blood pressure

7

What is the effect of increased or decreased blood pressure on the baroreceptors?

Sudden increase in BP causes increased activity of baroreceptors

Sudden decrease in BP does the opposite

 

8

What is the purpose of the arterial baroreceptors?

To buffer acute changes in blood pressure

Second to second, minute to minute changes

9

At what pressure will the baroreceptors be fired up?

Pressures range over which the baroreceptors can monitor systemic arterial blood pressure

  • Aortic: 100-200 mmHg (very high pressure)
  • Carotid: 50-200 mmHg (range is wider)

10

How do changes in arterial blood pressure affect carotid sinus nerve activity?

Lower pressure, lower activity

Higher pressure, higher activity

11

Where are the cardiopulmonary baroreceptors (low pressure baroreceptors)?

At strategic low-pressure sites

  • Pulmonary artery
  • The junction of the atria with their corresponding veins
  • Atria themselves
  • Ventricles

12

What is the function and mechanism of the cardiopulmonary baroreceptors?

The purpose is the monitor venous volume

Help control blood volume through reflex release of antidiuretic hormone

13

What exactly do the cardiopulmonary baroreceptors sense?

Stretch!!

Stretches in venous return to the heart indicate changes in blood volume

14

Where is the coordinating center for the cardiovascular system?

Medullary cardiovascular center - in the medulla

Points on the medulla: vasoconstrictor, cardioinhibitor, vasodilator

15

What is the medullary cardiovascular center and what does it do?

Collection of neurons in the medulla that receives sendory information from a variety of sources

  • Compares this information with the set point for systemic arterial blood pressure
  • Initiates the rsponses to maintain an appropriate blood pressure
  • Also receives input from higher areas
    • Can override the homeostatic activity of the cardiovascular system

16

What are the efferent organs of cardiovascular control?

Target organs

  • All of the efferent pathways come down to the heart and blood vessels
  • Efferent pathways are parasympathetic AND sympathetic fibers

17

What happens when the mean arterial pressure decreases (e.g. from standing up or from acute hemorrhage)?

  • Decrease in stretch on the baroreceptors
    • Decrease nerve activity (Hering's nerve)
      • Decreased parasympathetic outflow to heart and blood vessels
    • Increased sympathetic outflow to heart and blood vessels
      • ß: increase HR and contractility
      • a: increase resistance
  • All of this increases mean arterial pressure

18

What happens when there is a sudden increase in arterial pressure?

  • Increase in aortic pressure
  • Baroreceptors sense stretch → increased afferent nerve activity
  • Reciprocal changes in efferent sympathetic and parasympathetic nerve activity to the heart and blood vessels
  • Decrease in aortic pressure back to normal

19

Massage of the neck over the carotid sinus area in a person experiencing a bout of paroxysmal atrial tachicardia is often effective in terminating the episode. Why?

Decreases the HR

  • Fools the baroreceptors to think that ther is high pressure
  • They will respond by controlling the heart rate and decreasing it

20

What would happen to blood pressure if the arterial baroreceptors were surgically cut?

The mean arterial pressue would not change, but the fluctuations around the mean would

  • BP is determined by input from several systems, largely coordinated by the kidneys - RAAS, ADH, ANP
  • The others just don't respond instantaneously
  • Range increases
  • Fluctuations will be around 100 mmHg
  • Blood pressure is less stable

21

What would happen to blood pressure if BOTH the arterial and cardiopulmonary baroreceptors were surgically cut?

Mean arterial pressure and fluctuation around the mean would increase

  • Unstable BP
  • There will be no acute response to sudden change in BP

22

What happens to baroreceptors during chronic hypertenstion?

Baroreceptors reset at a higher level of blood pressure

  • They are not important in regulating control of hypertension because they measure accute changes only
  • After about two days they adjust their set point

23

Aside from baroreceptors, which type of receptors play a role in acute blood pressure control?

Peripheral (arterial) chemoreceptors

Located in the exact same places as the baroreceptors

24

What is the purpose of the arterial chemoreceptors?

They buffer acute changes in pO2, pCO2, and pH

25

What do the arterial chemoreceptors sense?

Hypoxemia (low oxygen)

Hypercapnia (increased carbon dioxide)

Acidosis (low pH)

26

What is the response of peripheral chemoreceptors to decreases in the partial pressure of O2?

  • Decreases in pO2 activate vasomotor center that produce vasoconstriction → increase total peripheral resistance and increase arterial pressure
  • Decreases in pO2 <60 mmHg cause hyperventilation

27

Which is more powerful: chemoreceptors or baroreceptors

Baroreceptors

  • Chemoreceptor reflex is not a powerful arterial pressure controller in the normal BP range
    • Stimulated when the pressure falls below 80 mmHg
  • Chemoreceptors play a role only during severe hypoxia (e.g. hemorrhagic hypotension)

28

What are the two chemoreceptor mechanisms for increasing the heart rate?

Decreased O2, increased CO2, and decreased pH

  • Decrease parasympathetic stimulation of the heart
  • Increase sympathetic stimulation of the heart (also increases stroke volume)

 

In addition, there is increased sympathetic stimulation of blood vessels to increase vasoconstriction

29

What happens in the chemoreceptor reflex when breathing has been stopped (asphyxia)?

  • Triggers chemoreceptor pathways 
  • Excitatory signals to the CNS and NTS (nucleus tractus solitarii)
  • Efferent vagal activity increases to the heart → bradycardia
  • Sympathetic activity to the kidney increases → renal constriction and activation of RAAS
  • Respiratory rate and depth increased → increased tidal volume

30

What are the chemoreceptors in the medulla responsible for?

  • Control breathing 
    • Increases in pCO2 and [H+] stimulate breathing
    • Decreases in pCO2 and [H+] inhibit breathing
  • Resulting hyperventilation then returns the arterial pCO2 toward normal

31

What happens if a decrease in PO2 or a decrease in pH is the primary insult?

  • Bradycardia occurs when ventilation is fixed or prevented
  • When we do breathe, mechanism is different
    • The effects of breathing overcome the intrinsic cardiovasculare response, producing tachycardia

32

What is the Renin-Angiotensin System?

Renin-angiotensin-aldosterone system plays important role in the regulation of arterial blood pressure and blood volume

33

What triggers the RAAS and in what context is it used?

  • Triggered by a decrease in renal perfusion pressure
  • Relatively slow, hormonal mechanisms and is used in long-term blood pressure regulation by adjustment of blood volume and vascular resistance

34

What happens in the event of a CNS ischemic response?

  • Increased PCO2 stimulates vasomotor center
  • Increased sympathetic ouflow to heart and blood vessels
    • Increased HR, SV, and peripheral vasoconstriction
    • Increased systemic arerial pressur
  • Increased parasympathetic outflow to heart - decreased heart rate

Same thing as the Cushing response to increased cranial pressure

35

When is vasopressin (ADH) released an what is its function?

  • Released in response to hemorrhage (decreased blood volume)
  • Released in response to activated low-pressure baroreceptors
  • Causes vasoconstriction (mediated by V1 receptors)
  • Increases water reabsorption and ultimately increases blood volume and interstitial fluid volume

36

What effect does increased extracellular fluid volume have on the arterial pressure?

Increases aterial pressure

  • Increased cardiac output has both a direct effect by increasing arterial pressure and indirect effect by increasing total peripheral resistance

37

When are natriuretic peptides (ANP, BNP) released and what is their function?

  • Released from the atria in response to increased atrial pressure
  • Cause vasorelaxation and decreased TPR
  • Cause increased excretion of Na+ and water by the kidneys - reduces blood volume
  • Inhibit renin secretion

38

What is the relationship between the various mechanisms that work to control arterial blood pressure?

Mutiple mechanisms work over different time scales to correct arterial blood pressure back to normal

  • Some of the regulating factors act very quickly
  • Others act more slowly
  • The strength of the reflex response varies widley

39

Which mechanism for controlling arterial blood pressure responds the quickest?

Baroreceptors

40

Which is the slowest of the mechanisms for controlling arterial pressure?

Renal mediated fluid retention

Also has the greatest gain

41

Which system for controlling arterial blood pressure is the least powerful?

Renin-angiotensin system

  • Small, but still very important
  • Keeps MAP in normal range

42

Which system for control of the arterial blood pressure is the most powerful?

Renal-mediated fluid retention