Blood Pressure Regulation Mechanisms, Types of Blood Vessels😍

Blood Pressure (BP) Regulation Mechanisms

Speed of Action:

Short term/Rapid (seconds-minutes):

• Baroreceptor > Chemoreceptor > CNS Ischemic Response.

Intermediate (minutes-hours):

• Renin - Angiotensin II (vasoconstriction)

• Capillary fluid shift

• Stress relaxation

Long-term (hours-days):

• ADH, ANP, Aldosterone → Pressure diuresis

• RAAS

Potency:

• RAAS >>

• CNS Ischemic Response
• Last Ditch Response

Chemical Regulation of BP

Natriuretic peptide

Baroreceptor Reflex: Marey's Law

• Mnemonic:
• Marey → high BP
• Bp kudiyapo (↑↑BP) Baril (Baroreceptor) Poi
• Nattampidich (NTS) vannu
• Sis (CIC) Para (Parasympathetic) vachu
• Amma Kavalam (CVLM) nokki adichu → “Veedu Mudipikkathe” (VMC) Roadilirakki Vittu (RVLM)

• First line to regulate BP.

• Negative Feedback system.

• Activation: Increase in blood pressure (stretch).

Marey's Law:

• ↑ BP leads to reflex ↓ HR 
• BP x HR = constant

• Sister (CIC) Mary () → Para

Baroreceptors

• Carotid sinus:
• Small dilations in internal carotid artery.
• Afferent nerve: CN 9 (Hering nerve).

• Aortic arch:
• Wall of aorta
• Afferent nerve: CN 10 (Cyon's nerve).

Most Sensitive Stimulus: 

• Distension
• ↑↑ Pulse pressure > ↑↑ MAP
• 70 - 140 

• Response: 
• Up to MAP of 180 

Centre (Nucleus Tractus Solitarius in medulla):

• BP =
• Pulse pressure >> MAP

Clinical Aspects:

• Occlusion of common Carotid artery:
• ↓ Blood flow to internal carotid artery → ↓ Pressure
• Baroreflex inhibited → ↑ BP & ↑ HR (↑ Sympathetic activation).

• Clamping above carotid sinus:
• ↑ Pressure
• Baroreflex activated → ↓BP & ↓HR (↑ Sympathetic inhibition).

• Supine → On Standing immediately
• If SNS underactivity → Fall due to postural hypotension.
• Normal:
1. ↓ BP (Baroreflex inhibited)
2. Sympathetic Nervous System (+) → Release of norepinephrine →
3. ↑ HR, Vasoconstriction (via α-receptors) → ↑ Total peripheral resistance.

• Impact of Lying Down
• Lying down → ↑ Venous return → Stored in Pulmonary veins → Pulmonary congestion → Breathlessness (Orthopnea)

• Carotid Sinus Massaging/ Czermak Hering Test:
• Activates PNS →Vagus → Ach release → M2 receptors
• Baroreflex → ↓HR.
• Used in treatment of PSVT
• Mark gave her ring → carotid kiss

Other Baroreceptors:

• Atrial & Pulmonary artery baroreceptors
• Low pressure receptor (volume sensing receptor).

2 Types of atrial stretch receptors

Chemoreceptor Reflex:

• Second line for BP control.

Location

• Carotid bodies: At carotid bifurcation

• Aortic bodies: Near arch of aorta (usually >2)

• Actual chemoreceptors:
• Type I Glomus cells

• Type II: Glia-like supporting cells

Mechanism

• Glomus cells have oxygen-sensitive K⁺ channels

Stimulated by (in arterial blood)

• ↓ PO₂ (Hypoxia) → Primary/Direct stimulus

• ↑ PCO₂ (Hypercapnea) → Most sensitive

• ↑ H⁺ (Acidosis)

• Hypotension

• Most potent stimulus: Cyanide poisoning (histotoxic hypoxia)

MOA – Glomus Cells

1. ↓ PO₂

2. O₂-sensitive K⁺ channels close

3. Depolarization

4. Ca²⁺ channels open

5. Ca²⁺ influx

6. Dopamine exocytosis

7. Stimulates respiratory centre

• Afferents, Center, Efferents: Same as Baroreceptors.

Response:

• Chemoreceptor activation
• Positive signals to both 
• Vasomotor Center (RVLM)
• Strong sympathetic activation → ↑ BP
• Cardiac Inhibitory Center (CIC)
• Parasympathetic activation → ↓ HR
• Mild activation → ↓HR
• Severe activation → ↑HR
• indirectly via ↑ ventilation

Overall Effect:

• Always ↑BP

CNS Ischemic Response (CIR):

• 2nd most Potent.

• “Last line of control” / "Last Ditch Resort" / "Last Ditch Response"

Activation: 

• Severe ↓↓ in BP
• < 50 mmHg 
• CNS hypoxia/ischemia

Response: 

• Activation of RVLM (SNS)
• Most powerful response
• Massive sympathetic activation
• Increase MAP (≥ 250 mmHg)

Fluctuations in Blood Pressure:

Progressive blood loss

• Mayer waves/vasomotor waves:
• Slow, rhythmic BP fluctuations
• Due to interplay of Baro/Chemo/CIR.
• Interval:
• 26s (animals)
• 7-10s (humans)

Cough and Sneeze

Valsalva Manoeuvre

• Valsalva Manoeuvre
• Forceful expiration against closed glottis
• ↑ Intrathoracic pressure

Intrathoracic Pressure Changes

Types of Blood Vessels

WindKessel Effect:

• Elastic property of large arteries → Allows vessels to distend during systole → Stores energy during systole → Releases energy as kinetic energy during diastole → Helps push blood to the periphery.

Vena Cava

• Has max diameter

• Exhibits minimum BP

William Syndrome

• Chromosome 7 micromutation

• Elastin Mutation
• Results in Williams syndrome

• Overfriendly

• HyperCalcemia

• Elfian facies

• Leads to Supravalvular aortic stenosis
• Differential BP
• Right arm BP > Left arm Bp > Lower Limb

---

ㅤ	Williams syndrome	Marfan syndrome
Mutations	Elastin Mutation	Fibrillin Mutation
Leads to	Supravalvular aortic stenosis	Dilatation of aortic root ↳ Rupture → Death

ㅤ	ㅤ
Supravalvular AS	• Vitamin D toxicity • William syndrome
Supravalvular PS	• Noonan syndrome

ㅤ	Seen in
GNAS	• Mccune Albright • Cardiac Myxoma
GNAS 1	• Pseudohypoparathyroid/ Albright Hereditary Osteodystrophy
GNAQ	• Sturge Weber (Sporadic)

Types of Blood Flow

Feature	Laminar flow	Turbulent flow
Pattern	Streamlined	Disordered
Velocity	• Highest at center • Lowest at walls	Velocity in multiple directions
Cell position	Cells in center	Random distribution
Vessel condition	Normal vessels	Compressed / obstructed vessels
Sound	Silent	Noisy ↳ Murmurs, Korotkoff sounds
Reynolds Number (Re)	< 2000: Always laminar	> 3000: Always turbulent

Determined by Reynolds Number (Re): 

• Re = (Density x Diameter x Velocity) / Viscosity

• Velocity:
• Most important determinant
• due to Critical Velocity

Poiseuille's Equation (Hagen-Poiseuille Equation)

• Calculates Vessel resistance

• R = 8 x Viscosity x Length / (π x Radius⁴)

ㅤ	Resistance	Flow
Directly proportional	Blood viscosity & vessel length	Radius (R⁴)
Inversely proportional	Radius (R⁴)	Length of vessel

Poiseuille's Law (Formula for Blood Flow)

• F = ΔP x r⁴ x π / 8hl
• F: Rate of blood flow
• ΔP: Pressure difference between vessel ends
• r: Radius of the vessel
• l: Length of the vessel
• h: Viscosity of the blood

Blood Flow & Oxygen Consumption in Organs

Maximum and Minimum Values Table

• Liver → large organ → so max blood flow, ↑↑ % of total CO

• Highest O2 consumption/g → Heart

• Highest rate of blood flow/g → Kidney