Ramchandra: The Cardiovascular Control System

Question 1

What is the role of the cardiovascular system?

Answer 1

• Maintain Blood Pressure
• Regulate Cardiac Output
• Maintain Blood Flow To Other Organ Systems

Effectively it is the means to distribute oxygenated blood around the body.

Question 2

What are the two main afferent receptors?

Answer 2

• The Arterial Baroreceptors - Found in the Aorta and Pulmonary Trunk.
• The Cardiopulmonary Receptors - Found in large systemic veins, in pulmonary vessels, and in the walls of the right atrium and ventricles of the heart.

Question 3

What are the two efferent neuron types from the brain?

Answer 3

• Vagal Efferents
• Sympathetic Efferents

Question 4

What is the solitary tract nucleus?

Answer 4

The solitary tract is a compact fibre bundle that extends longitudinally through the posterolateral region of the medulla. It acts to convey afferent information from stretch receptors and chemoreceptors in the walls of the cardiovascular, respiratory and intestinal tracts.

• Lets of Efferent Fibres that are distributed to - and control - organ systems.

Question 5

What factors cause the release of renin?

Answer 5

• Decreased Perfusion Pressure
• Increased Sympathetic Drive to the Kidney
• Decreased Salt Content in the Tubular Fluid

Renin functions as the rate-limiting step in the production of Angiotensin II

Question 6

What is the function of Angiotensin II?

Answer 6

• Arteriole Vasoconstriction.
• Aldosterone Release - Both of which act to increase tubular reabsorption of NaCl and therefore H₂O.
• Increase Sympathetic Drive.
• Increased ADH release.

Question 7

How does NaCl and H₂O feedback on the RAAS system?

Answer 7

1. Effective Circulating Blood Volume Increases.
2. Perfusion of the Juxtaglomerular Apparatus Increases.
3. Inhibition of Renin Release.

Question 8

What causes the release of ADH?

Answer 8

• Angiotensin II Formation.
• Increased Osmolarity of the Blood.

ADH released from the Posterior Pituitary.

Question 9

Where are the Primary Systemic Arterial Baroreceptors found?

Answer 9

• Carotid Sinus.
• Aortic Arch.

Question 10

At normal arterial pressure, what is the pattern of carotid sinus nerve impulse firing?

Answer 10

At the start of cardiac cycle, there is an increase in firing which gradually slows. However, the carotid sinus nerves still fire.

Question 11

What happens to the carotid sinus nerve impulse as the arterial pressure is elevated?

Answer 11

There is an increase in the firing rate of the carotid sinus.

Question 12

What happens to the carotid sinus nerve impulses as the arterial pressure decreases?

Answer 12

There is a decrease in the firing rate of the carotid sinus.

Question 13

What is the relationship between baroreceptor nerve impulse frequency and sympathetic drive?

Answer 13

A decrease in the frequency of baroreceptor nerve impulse firing leads to increased sympathetic firing.

For example…

1. Decreased Arterial Pressure
2. Reduced Carotid Sinus Nerve Impulse Frequency
3. Increased Sympathetic Firing
4. Accelerated HR and Increased Contractility.

Question 14

Why does hypertension lead to reduced baroreceptor reflex efficacy?

Answer 14

When you have high blood pressure chronically, your body reset it’s new ‘normal’ to this increased rate. this leaves the patient predisposed to maintaining a higher than normal BP.

Hypertension then means that either change in pressure…

• Doesn’t Change Sinus Activity Enough

or

• It Doesn’t Change Cardiac Output Enough.

Question 15

What are the characteristics of the carotid sinus baroreceptors?

Answer 15

It has a…

• Threshold: Below 30 - 50 mmHg, a decrease in pressure does not alter the firing rate.
• Saturation: An increase in pressure above 150 - 180 mmHg also has no further effect on firing frequency.
• Rate Sensitivity: For a given mean pressure the rate of firing is greater for pulsatile pressure than for steady pressure. Has implications if you have left ventricular assist device which doesn’t have pulsatile heartbeats.

Question 16

When arterial pressure falls what does the systemic arterial baroreceptors cause?

Answer 16

• Increased HR and Cardiac Inotropic State.
• Graded constriction of precapillary resistance vessels in skeletal muscle, splanchnic, cutaneous and renal circulations - not cerebral or coronary circulations.
• Venoconstriction.
• Increased catecholamine secretion by the adrenal medulla.
• Increase in circulating levels of ADH, AII and other hormones.

Question 17

When does coronary circulation increase?

Answer 17

When the heart is working harder. This is because cardiac muscle output has increased and therefore more energy is required.

Question 18

What are the cardiac afferent receptors?

Answer 18

These are found in the heart and sense changes in cardiac volume. They are either called “a” or “b” receptors and fire off during atrial or ventricular contraction.

Myelinated vagal afferents of the cardiopulmonary receptors

Act to relay information about the stretch / volume of the muscle fibres during atrial or ventricular contraction.

Question 19

What is the relationship between the left atrial pressure and the vagal nerve activity?

Answer 19

Linear - As the pressure increases the nerve activity also increases.

Question 20

What is the efferent response to right atrial receptors?

Answer 20

Activation of the myelinated RA receptors leads to…

• Increased HR
• Reduced Renal Sympathetic Stimulation

Increased aggregate activation of cardiac receptors qualitatively similar systemic arterial baroreceptor reflex response.

Question 21

What is the role of osmoreceptors?

Answer 21

These are found in the supraoptic and paraventricular nuclei of the hypothalamus and act to sense changes in effective plasma osmolality by altering their volume. This modulates the synthesis and release of ADH by the posterior pituitary gland. Reduced ADH leads to net water excretion and vice versa.

Low ADH = dilute urine

also a potent vasoconstrictor ‘vasopressin’

Question 22

What is the role of the peripheral chemoreceptors?

Answer 22

Decreased PaO₂ , stimulates an increase in sympathetic drive to the heart and vessels. When oxygen delivery is compromised for instance by reduced arterial pressure chemoreceptor activation causes vasoconstriction of resistance vessels and decr HR and contributes to restoration of blood pressure.

Question 23

What is the initial processing region where the information from the cardiac and baroreceptors are sent?

Answer 23

The nucleus tractus solitarius (NTS) .

Question 24

What is the neural pathway in the brainstem for the arterial barocreceptors?

Answer 24

Enters the NTS and then passes to the CVLM. This CVLM has an inhibitory connection with the RVLM. The RVLM provides sympathetic nervous activity to the blood vessels.

Therefore increased firing from the arterial baroreceptors leads to an increased inhibitory effect on the RVLM and therefore reduced sympathetic drive.

Question 25

What is the pathway of the cardiopulmonary receptors in the brainstem?

Answer 25

Firstly information travels to the NTS before being relayed to the paraventricular nucleus. The PVN sends information to the posterior pituitary leading to changes in ASH secretion and to the RVLM to change kidney sympathetic drive.

Question 26

How was it proved that higher centres controlled cardiovascular homeostasis?

Answer 26

Transverse sections through regions above the facial colliculus lead to preserved arterial pressure.

However, bilateral ablation of the ventrolateral medulla leads to a profound fall in arterial pressure.