Respiratory System Anatomy

Respiratory System

• The trachea is covered with a ciliated mucous membrane, which helps to move mucus and trapped foreign matter to the pharynx.
• The trachea divides into two cartilage-ringed tubes called bronchi, which enter the lungs and spread into treelike smaller tubes called bronchial tubes.
• Each bronchus divides and redivides about 15 times, resulting in thinner walls with less cartilage, eventually becoming a tiny group of tubes called bronchioles.

Bronchioles and Alveoli

• Bronchioles supply air to millions of small sacs called alveoli.
• Alveoli are small, interconnected bubbles, approximately 0.2mm in diameter, with a wall thickness of only 0.4μm.
• At birth, lungs have about 30 million alveoli, which increase to about 300 million by age 8, and then remain relatively constant, but increase in diameter.
• Alveoli play a crucial role in breathing, expanding and contracting during breathing, and facilitating the exchange of O2 and CO2.

Interaction of Blood and Lungs

• Blood is pumped from the heart to the lungs under relatively low pressure, with an average peak blood pressure of about 20mmHg in the main pulmonary artery.
• The lungs offer little resistance to the flow of blood.
• Two general processes are involved in gas exchange in the lungs: ventilation (getting air to the alveolar surface) and perfusion (supplying blood to the alveoli).

Pressure-Airflow-Volume Relationships of the Lungs

• Breathing consists of two phases: inspiration and expiration.
• During inspiration, the diaphragm and intercostal muscles contract, increasing the volume of the thoracic cavity, and the intercostal muscles pull the ribs up, expanding the rib cage and further increasing the volume.
• This increase in volume lowers the air pressure in the alveoli to below atmospheric pressure, causing air to flow into the lungs.
• During expiration, the diaphragm and intercostal muscles relax, returning the thoracic cavity to its original volume, and increasing the air pressure in the lungs, forcing air out.

Physics of the Alveoli

• The alveoli are physically like millions of small interconnected bubbles, with a natural tendency to get smaller due to surface tension.
• The surface tension of the alveoli is decreased by the presence of surfactant, a unique fluid lining the internal surface of the alveoli.
• The pressure difference needed to cause air to flow into or out of the lungs is quite small, with a pressure difference of only a few centimeters of water for normal individuals.

Laws of Gases

• The law of diffusion states that molecules diffuse from a region of higher concentration to a region of lower concentration until the concentration becomes uniform in the two regions.
• The law of diffusion applies to the exchange of O2 and CO2 between alveolar air and pulmonary capillaries.
• Dalton's law of partial pressures states that the total pressure of a mixture of gases in a container is the sum of the partial pressures of the individual gases.
• Henry's law of solubility of gases states that the amount of a gas dissolved in a liquid is directly proportional to the partial pressure of the gas.

Carbon Monoxide (CO) Poisoning

• CO poisoning results in the dissociation of insufficient O2 from Hb for generating energy in body tissues, since it prevents the association of sufficient O2 with Hb in breathing.
• CO molecules attach very securely to Hb, occupying the places normally occupied by O2, and do not easily dissociate in the tissue.
• CO impedes both the dissociation of sufficient O2 from Hb in the oxidation process and the association of sufficient O2 with Hb in the oxygenation process.

Osmosis

• Osmosis is the passage of water from a solution of lower solute concentration to a solution of higher solute concentration through a selectively permeable membrane.
• Osmotic pressure is proportional to the concentration of the solute, inversely proportional to the volume of the solution, and proportional to the absolute temperature.