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Inputs to the VMC

A. Inhibitory inputs

From cortex via hypothalamus b. Lungs c. Baroreceptors

Excitatory inputs

From cortex via hypothalamus b. Pain pathways c. Chemo receptors

Direct stimulation of VMC by hypoxia and hypercapni


  1. NTS= Nucleus of tractus solitarius
  2. CVLM = Caudal ventrolateral medulla
  3. IVLM = Intermediate ventrolateral medulla
  4. RVLM = Rostral ventrolateral medulla which is the VMC
  5. CIC = Cardio inhibitory centre
  6. IML = Inter mediolateral horn
  7. The above structures are bilaterally present. For clarity, structures on only one side are shown.
    Related to
    1. Inotropic                             = Force
    2. Chronotropic                        = HR
    3. Dromotropic                         = Conduction velocity
    4. Bathmotropic                        = Excitability
    5. Lusiotropic                            = Relaxation time.(Phospholamban inhibit the sarcoplasmic reticulum calcium pump

(SERCA) is lost by blocking it beta-adrenergic agonist epinephrine enhance the rate of cardiac myocyte relaxation. )

Resting Vagal Tone –
HR is low due to this, cutting vagus inc. HR to 100/min
Regulation of Arterial B.P

1. Rapidly acting regulating Mechanism: within seconds, Approximately corrects the two-third fall in B.P, 

  1. Baroreflex
  2. Chemoreflex
  3. CNS ischemic reflex

2. Intermediate Acting Regulatory Mechanism: take min - hours

  1. Capillary fluid shift mechanism
  2. Stress relaxation and reverse stress relaxation Mechanism
3. Long term regulatory Mechanism: take days
  1. Direct :Renal fluid mechanism
  2. Indirect: Hormonal regulation  (RAS, Aldosterone)

Baroreceptor Mechanism

i. Arterial baroreceptors:  The arch of the aorta, and the carotid sinuses of internal carotid arteries and Pulmonary trunk

ii. Low pressure baroreceptors:  Atriocaval receptor, Pulm. venoarterial receptors, Atrial receptors & Ventricular receptors

iii. Innervation of barroreceptors: Carotid baroreceptor -Glossopharengeal and others by Vagus nerve

iv. Set point and tonic activation: Most sensitive to MAP. Baroreceptors are tonically active at (MAP) 60 TO 180 mm Hg. Below 60 mm Hg they do not act. Act by inhibiting VMC & stimulating CVC. They have a value called baroreceptor set point where they tend to maintain the arterial MAP. Baroreceptor set point is not fixed, in chronic hypertension it increases in chronic hypotension decreases. When they or their nerve supply cut BP Increases called Neurogenic or experimental hypertension.

Chemoreceptor reflex

a. Act eithin 40-60 mm Hg
b. Corrects approx. 2/3 rd of further fall in B.P. act

CNS Ischaemic response

The arterial pressure elevation in response to cerebral ischaemia (severely decreased blood flow to VMC,
direct VMC stimulation)
Operates between 15-50 mm Hg of Mean BP.

Emergency pressure control system.

Also called the last ditch stand pressure control mechanism.
Intermediate Mechanisms (30% To 15% Correction)

a. They begin to act within a few minute and reach full function within a few hours.

b. They primarily correct any alteration in b.p. by altering blood volume.

Capillary Fluid Shift Mechanism

a. Increased BP leads to increased filtration at the arterial end.

b. As a result,circulating blood volume decreases and BP comes back to normal.

Stress Relaxation

a. When there is increased BP it causes relaxation of blood vessels by local vascular tone adjustment.

b. Therefore, cardic output decreases and BP. falls back to normal.

Reverse Stress Relaxation

a. Fall in BP decreases perfusion pressure in blood storage organs causing constriction of blood vessels limitations

Effect Of Exercise On Blood Pressure (B.P)

a. Isometric Contraction: In this the systolic and the diastolic blood pressure increases in all grades of exercise.

b. Isotonic Contraction:

i. Mild: systolic blood pressure increases but there is no change or a slightly increase in the diastolic blood pressure.
Moderate: the systolic blood pressure increases but there is no change or a slight decrease in diastolic blood   pressure.

Severe : Systolic increase but diastolic dec due to local metabolites. Pulse pressure widens

Atrial Stretch receptors


e. Reflexes:-

  1. Bainbridge reflex
  2. Bezold – Jarisch reflex
  3. Cushing’s reflex Or the C.N.S. ischaemic response or Last Ditch effort
Bainbridge reflex: - 
  1. Infusion of blood or saline causes increase in heart rate (if the initial heart rate is low)
  2. Receptors involved: Atrial stretch receptors
  3. Bezold – Jarisch reflex:- (Coronary chemoreflex)
  4. Injections of veratridine ,serotonin , capsaicin etc into the coronary arteries supplying the left ventricle    causes apnoea followed by rapid breathing , ↓ in BP and ↓ in heart rate.
  5. Receptors involved: Left ventricular (C fibre endings)
  6. Cushing’s reflex:- ( C.N.S. Ischaemic response)
  7. Increase in intracranial pressure causes hypoxia and hypercapnia in medulla, which directly stimulates the V.M.C.This results in an increase in B.P.
  8. The  ↑ in B.P. through the baroreceptor mechanisms causes reflex Bradycardia.
    MAREY’S LAW: blood pressure is inversely proportional to heart rate.Due to baroreceptor reflex. Eg Shock : low BP but Inc.HR & Cushing’s reflex

Effects of chemoreceptor stimulation of VMC

Stimulation of chemoreceptor →    Stimulates VMC →    in H.R in C.O

 [Note that the effect of hypoxia on heart rate is an increase (because of hyperapnoea) and a reflex decrease

because of stimulation of VMC. Therefore, its effect on heart rate is variable.

Blood Pressure Waves: -

In direct record (intra arterial) of B.P., many types of waves can be seen;

  1. Cardiac waves - These are the waves because of the systolic rise and diastolic fall
  2. Traube – Hering (T-H) Waves – These are the fluctuation in the B.P., synchronous with respiration.
  3. Mayer Waves – These are seen in conditions like hypotension. The wave pattern is 1 per 20-40 second

The wave patter is 1 per 20 – 40 seconds


Valsalva manoeuvre:-

This is forced expiration against a closed glottis. It is one of the tests used for assessing the baroreceptor responses. Characteristic changes in heart rate and BP are seen during the various phases of the Valsalva manoeuvre:

i. At the beginning of the manoeuvre:                          in BP

ii. During the maneuver:                                             in B.P

                                                                                in H.R.

iii. Immediately after the end of the manoeuvre            in B.P.

                                                                                in H.R.

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