3.2.5. Reflex Control

Question 1

What is Homeostasis?

Answer 1

Homeostasis is the “tendency toward a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processess”.

Question 2

What is negative feedback?

Answer 2

Negative feedback is when a control system exerts a corrective process on a variable

Question 3

Where are high pressure baroreceptors located in our vasculature?

Answer 3

High pressure baroreceptors are located in the carotid sinus (specifically in the internal common carotid) and in the aortic arch

Question 4

How do our high pressure baroreceptors function?

Answer 4

1. these channels open and close with the stretching of the membrane, allowing depolarization of the nerve endings and activation of the CNS
2. increasing pressure results in increasing nerve activity
   
   1. the frequency of activation increases, not the amplitude
   2. impulses are triggered by pressures near the baroreceptor, not the total arteriolar pressure

Question 5

How do baroreceptors influence our nervous system?

Answer 5

The nerve impulses then modulate autonomic activity (specifically in the medulla to activate the glossopharyngeal or the vagus nerve)

Question 6

What effects do the baroreceptors have on our body? Which set responds to which stimuli?

Answer 6

1. results in changes in the vasoconstriction, cardiac output, and total blood volume
   
   1. Aortic arch transmits via vagus nerve to solitary nucleus of medulla (responds only to increase in blood pressure)
   2. Carotid sinus (dilated region at carotid bifurcation) transmits via glossopharyngeal nerve to solitary nucleus of medulla (responds to decreases and increases in blood pressure).

Question 7

What happens when an increase in pressure is sensed by the baroreceptors?

Answer 7

1. Increased arterial pressure on each system:
   
   1. Decreases sympathetic stimulation
      
      1. which controls NE release to stimulate a1 receptors
   2. Increases parasympathetic stimulation
      
      1. lengthens phase 4 of action potentials; increasing AP length and depolarization rate

Question 8

What is the point of the baroreceptor reflex?

Answer 8

All responses are related to maintaining P = (1/C) * V [P = pressure, C = contractility, V = volume]

Question 9

When standing, what role do the baroreceptors play?

Answer 9

1. Standing results in pooling of the blood in the lower extremities
2. this results in a drop of arterial pressure at the carotid baroreceptors and triggers the reflex response
   
   1. Sympathetic response kicks in
   2. increases HR and BP
   3. will level out once BP normalizes
3. In orthostatic hypotension, this reflex response is not present because the baroreceptor reflex is not able to compensate appropriately; results in a markedly decreased BP

Question 10

What is the valsalva maneuver and what are its effects?

Answer 10

1. Valsalva: “forcible exhalation effort against 1) a closed glottis or 2) closed mouth and nostrils
2. Results in an increase in BP and activation of the reflex response
   
   1. “reproducible and consistent changes in BP and HR”
3. Initial drop and then increased in HR (can correct arrhythmias in some cases)

Question 11

How fast are CNS reflexes in general?

Answer 11

Generally speaking, CNS-based reflexes have fast response times (seconds)

Question 12

How does hypotension involve the baroreceptors? What is the chain of events caused by the reflex?

Answer 12

1. Hypotension = decreased arterial pressure ⇒ decreased stretch ⇒ decreased afferent baroreceptor firing ⇒ increased efferent sympathetic firing and decreased efferent parasympathetic stimulation ⇒ vasoconstriction, increased heart rate, increased contractility, and increased blood pressure. (Important in the response to severe hemorrhage.)

Question 13

How does the carotid sinus massage involve the baroreceptors?

Answer 13

Carotid massage = increased pressure on carotid sinus ⇒ increased stretch ⇒ increased afferent baroreceptor firing ⇒ increased AV node refractory period ⇒ decreased heart rate

Question 14

How does the baroreceptor reflex contribute to Cushing’s?

Answer 14

Contribute to Cushing reaction (triad of hypertension, bradycardia, and respiratory depression) = increased intracranial pressure constricts arterioles ⇒ cerebral ischemia and reflex sympathetic increases in perfusion pressure (hypertension) ⇒ increased stretch ⇒ reflex baroreceptor induced-bradycardia

Question 15

What are the chemoreceptors and where are they?

Answer 15

O2 and pH receptors in the aortic and carotid bodies affect reflex responses to maintain appropriate O2 sats

Question 16

What are the two main locations for chemoreceptors in our vasculature and what do they respond to?

Answer 16

Peripheral = carotid and aortic bodies are stimulated by decreased PO2 (< 60 mmHg), increased PCO2, and decreased pH of blood

Central = stimulated by changes in pH and PCO2 of brain interstitial fluid, which in turn are influenced by arterial CO2. Do not directly respond to PO2.

Question 17

What is the diving reflex?

Answer 17

considered a “hold over reflex”

cold water on the face slows HR and causes vasoconstriction to limit blood flow to the extremities

Question 18

What are some examples of endocrine reflexes? Do they act in short term or long term adjustments of the body?

Answer 18

Renin-Angiotensin-Aldosterone System

Atrial stretch and ANP

Ventricular stretch and BNP

Endocrine reflexes are the primary long-term method of adjustments (hours to days)

Question 19

Describe (briefly) the Renin-Aldosterone-Angiotensin System (RAAS) and its effect on the body. What does it respond to?

Answer 19

1. The RAA system responds to changes in blood pressure (usually decreases)
   
   1. Decreased BP in the kidney results in renin secretion
   2. Renin cleaves angiotensinogen (released from the liver) in the blood stream to form angiotensin I
   3. Angiotensin I is converted to angiotensin II in the lungs by the Angiotensin Converting Enzyme (ACE) in the lungs
   4. Angiotensin II acts on the adrenal gland to secrete aldosterone
   5. Aldosterone causes increased Na and water retention in the kidneys
   6. Results in an increased blood volume and, therefore, an increased BP

Question 20

Where does Atrial Natriuretic Peptide (ANP) come from, and what does it do?

Answer 20

Atrial stretch yields ANP

Released from atrial myocytes in response to increased blood volume and atrial pressure. Causes vasodilation and decreased Na+ reabsorption at the renal collecting tube. Constricts efferent renal arterioles and dilates afferent arterioles via cGMP, promoting diuresis and contributing to “aldosterone escape” mechanism.

Question 21

Where does B-type Natriuretic Peptide (BNP) come from and what are its effects?

Answer 21

1. Ventricular stretch yields BNP
   
   1. Released from ventricular myocytes in response to increased tension. Similar physiologic action to ANP, but with a longer half-life. BNP blood test is used for diagnosing heart failure (very good negative predictive value). Available in recombinant form (nesiritide) for treatment of heart failure.

Question 22

What effects do ANP and BNP share. What else do they ahve in common?

Answer 22

Both are peptide hormones with guanylate cyclase linked receptors

increase vasodilation

increase excretion of Na and water

decrease aldosterone secretion (and therefore Na and water retention)

Question 23

Why is BNP a valuable test?

Answer 23

BNP is a good, non-specific, diagnostic tool

tells you something is wrong with the heart, but not what is wrong with the heart

Question 24

What are the net effects of BNP and ANP release into the body?

Answer 24

Net effect is reduced blood flow, BP, and cardiac workload