31. Central factors of the cardiovascular regulation

Question 1

Regulation of circulation, and the satisfaction of the demand for material exchange is based on two principles:

Answer 1

• Regulation of circulation, and the satisfaction of the demand for material exchange is based on two principles:
  
  • The demands of material transport to and from tissues should be met.
  • The collapse of normal circulation should be avoided by compensating for the inproportional blood supply to different organs during different physiological activities.
• The continuous change of the intensity of tissue activities requires that the blood is directed towards more active tissues.
  
  • This demand is met by the central regulation of the circulation.
  • The major element of this central mechanism is the adjustment of the actual state of vessel contraction on the arterial (resistance) and venous (capacitance) segment of circulation by the CNS.

Question 2

Short-run

• Sympathetic effects:

Answer 2

Sympathetic effects:

• Arteries and veins are under the influence of the sympathetic tone, which exerts vasoconstrictive effects and at the same time under the influence of the continuous local NO production that continuously dilates the vessels. The balance of these two effects results in an actual diameter of the vessel.
  
  • Under resting circumstances this diameter is much less than the maximal possible diameter of that vessel.
  • The CNS controls the diameter of the vessels via the vasomotor center in the medulla.
• The sympathetic activation is mediated by the thoracolumbar segments of the spinal cord.
  
  • Cardiovascular regulatory nuclei is situated close to the respiratory centers of the reticular formation.
  • The anterolateral portion (pressor area) of the cardiovascular center exerts spontaneously activity and positively influences (positive chronotrop and dromotrop effects) the cardiac function and causes increase of the peripheral resistance by actively contracting vessels.
  • The caudomedial portion (depressor area) has no spontaneous activity. These exert negative chrono- and dromotrop effects on the heart (by n. vagus) and causes the decrease of the peripheral resistance.
    
    • Mainly due to inhibitory interneurons that decrease the activity of the pressor areas. Therefore vasodilation is normally the consequence of the depressed sympathetic activity, and not the direct result of the parasympathetic stimulation.

Question 3

Short-run

• Parasympathetic effects:

Answer 3

• Most vessels get sympathetic neural supply (except of skin and skeletal resistance vessels)
• The withdrawal of the sympathetic activity is the physiological mediator of the vasodilator influence.
• Direct parasympathetic stimulation is found in the:
  
  • Corpus cavernosus in penis o Uterus
  • Pancreas
• Indirect cholinergic vasodilation occurs in the salivary glands
  
  • Mediated by the release of brandykinin as a response to ACh stimulation of the muscarinic choline receptors of the acini. The released bradykinin acts on paracrine ways and causes vasodilation.

Question 4

Short-run

• Humoral factors

Answer 4

• Beside neural control, the endocrine system also oversees the regulation of the distribution of blood among organs.
• Hormones of the adrenal medulla play a role in the regulation of circulation under increased activity only (e.g. stress)
• Epinephrine:
  
  • Small dose: beta-adrenergic vasodilation in the skeletal muscle, alpha-adrenergic vasoconstriction in skin and in sphlanchnic areas
  • High dose: general alpha-adrenergic vasoconstriction
• Norepinephrine: alpha-adrenergic vasoconstriction
• Other hormonal influences act via the modulation of the pressor/depressor centers.

Question 5

Short-run

• Reflex mechanisms

Answer 5

• Baroreceptor mechanism:
  
  • Found in many places in the circulatory bed, and allow the very fast adjustment of blood pressure to the needs of the body.
  • Baroreceptor functions in the range of 40-170 mmHg of mean arterial blood pressure.
  • Increased vlood pressure decreases the sympathetic influence on the heart (drop of heart rate and contractility) and it increases the parasympathetic discharge (heart rate drops).
  • Below 50 mmHg mean arterial blood pressure: the sympathetic activity reaches its maximum, while the parasympathetic effects cease  protection against hypotension.
  • Above 170 mmHg mean pressure: the sympathetic discharge ceases and the parasympathetic influence is maximal  protection against hypertension.
• Volume sensors:
  
  • Found in the lung and in the capacitance system of vessels.
  • Centripetal fibers from these sensors lead to the cardiovascular center in the medulla.
• Atrial natriuretic factor (ANF) is produced in the atria as a response to increased EC volume.
• Antigiuretic hormone (ADH): Atrial stretch reflexly elicits the inhibition of the neurohypophysael let down of this hormone.
• Brainbridge reflex: Increase of the heart rate occurs upon stretching the atrium only if the heart rate was low previously –> correcting it back to normal

Question 6

Short-run

• Effects of pO₂ and pCO₂

Answer 6

• Signals from the central and peripheral gas sensors supply information to the cardiovascular center of the medulla too (i.e. not only to the respiratory center).
• Also, within the medulla direct neuronal connection of the cardiovascular and respiratory centers are justified.

Question 7

Long-run

Answer 7

• CNS: changes in the vasoconstrictor tone
• Extended emotional effects
• Adaptation to climate
• Long run change in the oxygen supply
• Adjustment of the circulating blood volume, redistribution of the blood etc.