Chapter 9 Fetal Circulation and Respiratory System Review

Fetal Circulation Dynamics and Vascular Shunts

• Systemic Vascular Resistance (SVR) and Pulmonary Vascular Resistance (PVR) in Utero:
  • Systemic Vascular Resistance (SVR) is defined as how difficult it is for blood to flow through the body's blood vessels (high resistance indicates blood has difficulty flowing, whereas low resistance indicates blood flows easily).
  • In utero fetal SVR is low because the placenta provides oxygen and functions as a large-volume, low-resistance vascular system allowing blood to flow easily through it into the baby.
  • Pulmonary Vascular Resistance (PVR) is defined as how difficult it is for blood to flow through the lungs' blood vessels.
  • Fetal lungs in utero have high PVR because they are not breathing air and have a low partial pressure of oxygen ($PaO_2$).
• Fetal Shunts and Circulatory Path:
  • Circulation in the fetus fundamentally differs from the circulation of the neonate after birth due to three important bypass pathways (shunts).
  • Umbilical Vein: Returns oxygenated, oxygen-rich blood from the placenta to the fetus.
  • Ductus Venosus: A specialized shortcut and connected blood-flow section branching from the umbilical vein that passes through the liver area. It carries most blood past the liver without entering it, joining directly with the Inferior Vena Cava (IVC).
  • Inferior Vena Cava (IVC): A large vein that receives oxygenated blood from the ductus venosus and carries it into the right atrium of the heart.
  • Right Atrium (RA): The upper-right heart chamber receiving incoming venous blood.
  • Foramen Ovale: An opening between the right atrium and left atrium (LA, upper-left heart chamber). It shunts oxygenated blood from the right atrium directly into the left atrium, allowing blood to move into the systemic circulation while skipping the non-aerated lungs.
  • Ductus Arteriosus: A blood vessel connecting the pulmonary artery (which carries blood away from the right side of the heart) directly to the aorta (the main artery carrying blood from the heart to the body). It shunts blood away from the high-resistance fetal pulmonary circulation into the low-resistance aorta.

Gas Exchange and Respiration Physiology

• Primary Function of the Respiratory System:
  • The primary job of the respiratory system is gas exchange, which involves the continuous absorption of oxygen ($O_2$) into the body and the continuous elimination of carbon dioxide ($CO_2$).
• External Respiration:
  • External respiration is the gas exchange occurring between alveolar gas and pulmonary capillary blood at the alveolar level.
  • Oxygen moves passively from the alveoli into the blood, while carbon dioxide moves passively from the blood into the alveoli.
  • Occurs by gases diffusing across the alveolar-capillary membrane due to pressure gradients.
  • Diffusion: The natural process where gases move from an area of higher pressure to an area of lower pressure.
  • Alveolar-Capillary Membrane: The thin wall separating the alveolar air space from the blood in the capillaries.
• Internal Respiration:
  • Internal respiration is the exchange of gases between blood and tissues at the cellular level.
  • Oxygen moves from the systemic capillary blood into the cells of the body, while carbon dioxide moves from the body cells into the blood.

Embryonic Development, Surfactant, and Postnatal Adaptation

• Embryonic Period:
  • Occurs during the first $8\text{ weeks}$ of pregnancy, during which the major organ systems of the baby begin developing.
• Pulmonary Surfactant Function and Timing:
  • Surfactant decreases surface tension inside the alveoli, keeping the alveoli open and preventing them from collapsing.
  • Production of surfactant begins around $24\text{ to }28\text{ weeks}$ of gestation.
  • Premature infants may lack sufficient surfactant, leading to Respiratory Distress Syndrome (RDS).
• Postnatal Physiological Events:
  • After birth, air enters the lungs as the baby takes its initial breath.
  • The lungs gain physical room to expand within the thoracic cavity.
  • Surfactant functions to open the alveoli and keep them open.
  • The baby's diaphragm contracts and moves downward to facilitate inhalation.
• Alveolar Structural States:
  • Alveoli Cluster: Aggregations of many small open spaces grouped together, where each individual hole represents a separate alveolus with its own open lumen space.
  • Single Alveolus: An individual small open air space.
  • Collapsed Alveolus: State where the alveolar walls fold inward, leaving little to no open space for air.

Respiratory Mechanics and Thoracic Movements

• Rib Movements During Inhalation:
  • During inhalation, the ribs move upward and outward, enlarging the chest cavity to accommodate expanding lungs.
  • Bucket-Handle Movement: Upward and outward movement of the ribs that increases lateral chest space, widening the thorax from side to side.
  • Pump-Handle Movement: Upward and forward movement of the upper ribs and sternum, increasing the anterior-posterior (AP) diameter and enlarging space from front to back.
  • Specific Actions by Rib Pair:
  • Rib Pair $1$: Raises slightly and pulls the sternum upward.
  • Rib Pairs $2\text{ to }7$: Perform both pump-handle and bucket-handle movements.
  • Rib Pairs $8\text{ to }10$: Move mainly outward to increase side-to-side lateral dimension.
• Respiratory Muscle Roles and Dynamics:
  • Diaphragm: Main muscle of respiration, responsible for performing approximately $75\%$ of the total work of breathing.
  • Inhalation Mechanics: The brain initiates breathing by sending signals through the phrenic nerve to the diaphragm; the diaphragm contracts and moves downward, while external intercostal muscles lift the ribs upward and outward. This creates negative pressure that pulls air into the lungs.
  • Accessory Muscles of Inhalation: Scalene and sternocleidomastoid muscles assist breathing when breathing becomes difficult or when increased ventilation is needed.
  • Normal Exhalation Mechanics: A passive process where respiratory muscles relax, the diaphragm moves upward, and air flows out naturally (passive exhalation).
  • Forced Exhalation Mechanics: An active process where abdominal muscles and internal intercostal muscles contract to pull the ribs down and push air out faster.

Diaphragmatic Innervation and Clinical Correlates

• Phrenic Nerve Anatomy:
  • Cervical spinal nerve levels $C3$, $C4$, and $C5$ ($C3\text{--}C5$) join together to form the phrenic nerves.
  • Each phrenic nerve provides motor control to one side (hemidiaphragm) of the diaphragm.
  • The brain transmits breathing signals through the phrenic nerves to trigger diaphragmatic contraction.
• Non-Respiratory Abdominal Compression Functions:
  • Prolonged diaphragmatic contraction occurring concurrently with abdominal muscle contraction compresses the abdominal cavity.
  • This coordinated contraction creates intra-abdominal pressure necessary for coughing, vomiting, bowel movements (defecation), and childbirth.
• Spinal Cord Injury (SCI) Implications:
  • A spinal cord injury occurring at or above the third cervical vertebra ($C3$) blocks brain signals from reaching the phrenic nerves.
  • Results in diaphragmatic paralysis (inability of the diaphragm to contract normally) and loss of nervous control of respiratory muscles.
  • Blocks independent breathing capability, making mechanical ventilation necessary for survival.

Pulmonary Circulation Architecture and Membrane Functions

• Hemodynamics and Blood Flow:
  • Oxygen-poor blood is delivered to the lungs from the right side of the heart via the pulmonary arteries.
  • At rest, the pulmonary circulation is supplied with blood at a flow rate equal to the entire blood volume each minute.
  • Oxygenated blood is delivered back to the left side of the heart via pulmonary veins.
• Capillary Distribution:
  • Pulmonary capillaries represent the smallest blood vessels and form an extensive network wrapping around the alveoli.
  • Capillaries cover approximately $90\%$ of the total alveolar surface area.
• Functions of the Alveolar-Capillary (A/C) Membrane:
  • Serves as a physical barrier separating air spaces in the lungs from capillary blood while enabling rapid passive exchange of $O_2$ into blood and $CO_2$ into alveoli.
  • Regulates fluid exchange within the lung to prevent fluid accumulation inside the alveoli.
  • Contributes to systemic homeostatic processes including the production, chemical processing, and clearance of specific chemicals and filtering small blood clots from circulation.

Lung Segmentation, Conducting Airways, and Dead Space

• Gross Anatomical Organization:
  • Right Lung: Composed of $3\text{ lobes}$ and $10\text{ bronchopulmonary segments}$.
  • Left Lung: Composed of $2\text{ lobes}$ and $8\text{ to }10\text{ bronchopulmonary segments}$ (smaller in volume because the heart occupies space on the left side of the thoracic cavity).
  • Each bronchopulmonary segment is supplied by its own segmental bronchus.
• Conducting Airways and Anatomic Dead Space:
  • Airways divide progressively down to terminal bronchioles, which are the final airways that only conduct air without participating in gas exchange.
  • Anatomic Dead Space: Refers to all conducting airways—including the nose, mouth, trachea, bronchi, and terminal bronchioles—that move air but contain no alveoli.
  • Anatomic dead space volume equals approximately $2\,mL/kg$ of lean body weight, which averages around $150\,mL$ in a normal adult.
  • Gas exchange begins immediately distal to the terminal bronchioles (at the respiratory bronchioles).

Alveolar Histology, Cell Types, and Defense Systems

• Alveolar Structure:
  • Alveoli are sac-like growths situated on respiratory bronchioles, alveolar ducts, and alveolar sacs, whose primary function is gas exchange.
• Type I Pneumocytes:
  • Very flat and extremely thin epithelial cells specialized for enabling gas exchange.
  • Cover approximately $93\%$ of the total alveolar surface area.
  • Form tight intercellular junctions that limit substances and fluids from entering the alveolar space.
• Type II Pneumocytes:
  • Cuboidal-shaped cells that are approximately twice as numerous as Type I pneumocytes.
  • Manufacture, store, and secrete pulmonary surfactant.
  • Surfactant reduces surface tension, prevents alveolar collapse, increases lung compliance (ease of lung expansion), and decreases the overall work of breathing.
  • Act as progenitor stem cells: when Type I cells are damaged, Type II cells divide and differentiate into new Type I pneumocytes.
• Alveolar Macrophages:
  • Immune defense cells residing in the alveoli that engulf and clean up micro-organisms, dust particles, and foreign debris.
• Club Cells (formerly Clara Cells):
  • Specialized cells found mainly in the bronchioles that protect the airways and produce surfactant-like material.