cardiovascular

Circulation Overview

  • Circulation is divided into two primary types:
    • Pulmonary circulation
    • Involves blood flow to and from the lungs.
    • Systemic circulation
    • Involves blood flow to and from the rest of the body.

Basic Knowledge Recap

  • Familiarity with heart anatomy, valves, and blood types (oxygenated vs. deoxygenated) assumed from lab experience.

Fetal Circulation

  • Fetal circulation has unique characteristics compared to adult circulation.
  • The fetus obtains oxygen from the mother’s blood rather than inhaling air.
Placental Blood Flow
  • Mother’s oxygenated blood enters the fetus through the placenta and flows into:
    • Right atrium
    • Through the tricuspid valve into the right ventricle
    • Blood is then forced through the pulmonic valve into the pulmonary arteries to reach the lungs.
  • Notably:
    • Fetus does not breathe air; lungs are non-functional during this phase.
Bypass Mechanisms in Fetal Circulation
  • The fetal circulatory system includes two main bypass pathways to accommodate non-functioning lungs:
    1. Foramen Ovale
    • An opening that allows blood to flow from the:
      • Right atrium directly to the left atrium.
      • Skips the right ventricle and pulmonic artery thereby bypassing the lungs.
    1. Ductus Arteriosus
    • A vessel that connects:
      • Pulmonic artery directly to the aorta.
      • Prevents blood from reaching the non-functioning lungs, allowing for efficient oxygenation from the mother.
  • The blood pathway is:

    • Mother → Placenta → Right atrium → Foramen ovale → Left atrium → Left ventricle → Aorta → Fetus.
  • In this system, all oxygenation occurs through the mother's bloodstream.
Post-birth Changes
  • Upon the first breath, changes occur:
    • Blood pressure and heart rate in the newborn increase, resulting in:
    • Closure of the foramen ovale due to pressure shifts.
    • The ductus arteriosus eventually atrophies to become the ligamentum arteriosum.
    • The closure of each bypass ensures normal adult circulation.

Heart Function and Regulation

Neural control

  • The heart operates autonomously while also being modulated by the nervous system.
Vagus Nerve
  • One of the key cranial nerves influencing the heart:
    • Function: Regulates the heart's rhythm through autonomic influence (sympathetic and parasympathetic).
    • It connects the brainstem directly to the heart, affecting heart rate and conductance.

Sinoatrial Node and Atrioventricular Node

  • Sinoatrial (SA) Node
    • Located in the right atrium; known as the heart's natural pacemaker.
    • Initiates electrical impulses to stimulate heart contractions.
  • Atrioventricular (AV) Node
    • Receives impulses from the SA Node to coordinate contraction between atria and ventricles.

Electrical Conduction System

  • The heart’s electrical signal pathway includes:
    • SA Node ➝ AV Node ➝ Bundle of His ➝ Right and Left Bundle Branches ➝ Purkinje Fibers.
    • Impulses spread in a coordinated manner to ensure effective heart contractions.
Depolarization and Contraction
  • Once stimulated, the SA node can autonomously depolarize, leading to:
    • Atrial contraction (systole) where blood is forced into the ventricles.
    • Ventricular contraction follows once the signals reach the AV node.

Heart Sounds and Blood Pressure

  • Heart sounds are correlated with specific cardiac events:
    • S1 (first heart sound): Closure of the tricuspid and mitral valves during ventricular contraction.
    • S2 (second heart sound): Closure of the aortic and pulmonary valves during ventricular relaxation.
  • Blood Pressure Measurement:
    • Indicates the force exerted by circulating blood on the walls of blood vessels.
    • Standard reading is represented as a fraction (e.g., 120/80 mmHg):
    • Systolic Pressure (top number)
    • Diastolic Pressure (bottom number)

Capillary Exchange Mechanism

  • Capillaries facilitate the exchange of oxygen and carbon dioxide:
    • Diffusion & Osmosis: Oxygen diffuses from blood to cells, and carbon dioxide moves from cells back into blood via capillaries.
    • Plasma proteins (e.g., albumin) regulate osmotic pressure:
    • Draws water into the circulatory system to balance concentrations inside blood vessels.

Plasma Proteins

  • Major plasma proteins include:
    • Albumin: Regulates osmotic pressure and carrier of various substances in blood.
    • Globulins: Involved in immune functions and transport of substances.
    • Fibrinogen: Key protein for clotting blood.

Summary

  • Understanding fetal and postnatal circulation is crucial in cardiac physiology.
  • The heart operates through a complex electrical conduction system regulated by neural input.
  • Blood pressure and heart sounds provide valuable clinical insights into cardiovascular health.