Respiratory System Notes

Respiratory System

• Main role: Gas exchange - breathing in oxygen (O2) and getting rid of carbon dioxide (CO2).

Anatomy of the Respiratory System

• Trachea (windpipe):
  • Composed of hyaline cartilage and smooth muscle.
  • Anteriorly: Hyaline cartilage rings.
  • Posteriorly: Smooth muscle.
• Bronchi:
  • Trachea bifurcates into two bronchi (singular: bronchus, plural: bronchi).
  • Supported by cartilaginous rings made of hyaline cartilage.
• Bronchioles:
  • Bronchi split into smaller airways called bronchioles.
  • Enter the lungs.
• Lungs:
  • Divided into different lobes.
  • Bronchioles terminate in clusters of alveoli.
• Alveoli:
  • Clusters of air sacs where gas exchange occurs.
  • Resemble a bunch of grapes.
  • Lined with capillaries.

Gas Exchange

• In Alveoli:
  • Capillaries lining the alveoli facilitate gas exchange.
  • Pulmonary artery (from right ventricle of the heart) brings deoxygenated blood to the lungs.
  • CO_2 is dropped off from the blood into the alveoli to be exhaled.
  • Oxygen (O_2) is picked up from the alveoli into the blood.
  • Pulmonary vein carries oxygenated blood back to the left atrium of the heart.
• In Tissues (e.g., Big Toe):
  • Capillaries deliver oxygen to tissues.
  • Oxygen moves from the capillary into the tissues.
  • CO_2 moves from the tissues into the capillaries.
  • Deoxygenated blood returns to the right atrium of the heart via the superior and inferior vena cava and coronary sinus.
• Gas exchange at the lungs occurs in the opposite direction to gas exchange at the tissues.

Serous Membranes of the Lungs

• Lungs are surrounded by a serous membrane with the same format and nomenclature as other serous membranes.
  • Visceral pleura: Directly on the lung.
  • Parietal pleura: Lines the thoracic cavity.
  • Pleural cavity: Potential space between the visceral and parietal pleura, contains serous fluid.
• Surface Tension:
  • Serous fluid (mainly water) creates surface tension that causes the visceral and parietal pleura to adhere to each other.
  • Allows the lungs to expand and contract with the thoracic cavity.

Breathing Mechanics

• Inhalation:
  • Thoracic cavity expands, causing the lungs to expand, which then allows air to flow in.
  • Expansion of the lungs is not caused by air being breathed in.
• Relationship between Volume and Pressure:
  • Inverse relationship: Increase in volume corresponds to a decrease in pressure, and vice versa.
  • V \propto \frac{1}{P}
• Air Flow:
  • Air flows from areas of higher pressure to areas of lower pressure (down pressure gradients).
  • To inhale, we need to create a pressure gradient by lowering the pressure in the lungs.
• Muscles Involved:
  • Muscles between the ribs (intercostals).
  • Sternocleidomastoid and scalene muscles in the neck.
  • Diaphragm: Main muscle of respiration.
  • Parietal pleura lines the rib cage, while the visceral pleura is directly on the lungs.
• Process:
  • Muscles attached to the ribs pull the rib cage outwards.
  • Diaphragm contracts and moves inferiorly.
  • Expansion of the thoracic cavity pulls the parietal pleura outward, which in turn pulls the visceral pleura and the lungs with it due to surface tension.

Collapsed Lung (Pneumothorax)

• If the parietal pleura is punctured (e.g., by a broken rib), serous fluid leaks out.
• Loss of surface tension causes the visceral and parietal pleura to no longer adhere to each other.
• The lung collapses.
• The rib cage still expands and contracts, but it doesn't pull the lung with it.

Alveolar Cells and Surfactant

• The alveoli are lined with fluid to keep them moist, which cause surface tension that makes the alvoli collapse.
• Alveoli are made of simple squamous epithelium which is one layer and very thin and fragile.
• There are two types of cells that make up the alveoli.
• Capillary beds are made of endothelium containing simple squamous epithelium which allos the movement of gasses.
• Type I Alveolar Cells:
  • Simple squamous epithelial cells (flat cells).
• Type II Alveolar Cells:
  • Release surfactant.
  • Surfactant eliminates surface tension, preventing the alveoli from collapsing.
  • Premature babies have underdeveloped lungs and may not produce enough surfactant, leading to respiratory distress and require surfactant via medication.

Pressure and Air Movement

• Air moves into the lungs when the pressure in the lungs is less than the atmospheric pressure.
• Increasing the volume of the lungs decreases the pressure inside.
• Atmospheric pressure then pushes air into the lungs.

Diaphragm

• Skeletal muscle attached to bone, making it somatically controlled
• Dome-shaped muscle located below the lungs.
• Somewhat voluntary and somewhat involuntary.
• Relaxed Diaphragm (Exhalation):
  • Dome-shaped.
• Contracted Diaphragm (Inhalation):
  • Diaphragm shortens and moves inferiorly.
  • Increases the volume of the thoracic cavity.
  • Decreases the pressure in the thoracic cavity.
  • Air moves into the lungs.

Pulmonary Veins

• Blood is backed up by the left ventricle into the left atrium.
• If pressure increases such as an instance of hypertension then it increases the amount of blood in the left atrium.
• The blood backs up from the left atrium to the pulmonary veins.
• Pulmonary veins then carry the blood back to the left atrium.