Respiratory

How the Lungs Stay Inflated

Lung tissue is elastic, meaning it naturally wants to collapse like a deflated balloon. Two things keep that from happening:

  • Negative pressure in the pleural cavity — the space around your lungs is kept at a slightly lower pressure than the outside air, which essentially "sucks" the lungs outward and keeps them open

  • Intrapleural fluid — a thin layer of fluid sits between the two membranes surrounding the lungs. The surface tension of this fluid acts like a suction cup, keeping the lungs stuck to the chest wall.

Breathing Mechanics

Breathing in (inspiration): The diaphragm (the dome-shaped muscle under your lungs) and the external intercostal muscles (between your ribs) contract. This makes the chest cavity bigger. When the cavity gets bigger, pressure inside drops, and air rushes in to equalize — like pulling back on a syringe.

Breathing out (expiration): Normally, completely passive — no muscle work needed. The diaphragm and rib muscles relax, the chest cavity shrinks back down, and air gets pushed out.

The Airway — Path Air Takes Into the Lungs

Nose/Mouth → Pharynx (throat) → Larynx (voice box) → Trachea (windpipe) → Bronchi → Bronchioles → Alveoli

  • Trachea — held open by stiff cartilage rings, so it never collapses

  • Bronchi — one enters each lung, then branches into smaller and smaller tubes. The smaller they get, the more smooth muscle and less cartilage they have

  • Bronchioles — the smallest branches, almost entirely made of smooth muscle (which is why they can constrict during an asthma attack)

The airway does three things:

  1. Distributes air throughout the lungs

  2. Warms and humidifies incoming air

  3. Acts as a defense system — the walls are lined with mucus and tiny hair-like cilia that sweep foreign particles and microorganisms up and out (the "mucus escalator"). Smoking destroys these cilia, causing mucus and junk to build up in the airways.

Alveoli — Where Gas Exchange Actually Happens

Alveoli are tiny air sacs at the very end of the bronchioles. Think of a tiny balloon cluster. There are over 300 million of them, giving a total surface area roughly the size of a tennis court.

Two types of cells in alveoli:

  • Type I cells — form the actual wall of the alveoli. Together with the capillary wall, they create an incredibly thin barrier (0.2 micrometers) between air and blood — thin enough for gases to diffuse right through

  • Type II cells — produce surfactant, a soapy substance that coats the inside of each alveolus and reduces the surface tension of water. Without it, the alveoli would collapse every time you exhaled. Premature babies often lack surfactant, which is why they have trouble breathing at birth.

Controlling Airway Size (Bronchodilation vs. Bronchoconstriction)

The smooth muscle in bronchioles can squeeze tight (bronchoconstriction) or relax open (bronchodilation):

  • High CO₂ locally → smooth muscle relaxes → bronchodilation (lets more air in where it's needed)

  • Sympathetic nervous system → bronchodilation (fight or flight — open up airways for more oxygen). This is why asthma inhalers use beta-agonist drugs like Albuterol — they mimic sympathetic stimulation to open airway.s

  • Parasympathetic nervous system → bronchoconstriction

Gas Exchange — How Oxygen and CO₂ Move

Gases move by diffusion — they naturally move from areas of high concentration to low concentration—no energy needed.

  • In the lungs: oxygen concentration is high in air, low in blood → oxygen moves into blood. CO₂ is high in blood, low in air → CO₂ moves out.

  • In body tissues: oxygen is high in blood, low in tissues → oxygen moves into tissues. CO₂ is high in tissues → moves into blood.

How Oxygen is Carried in the Blood

  • 98.5% of oxygen is carried by hemoglobin (Hb) inside red blood cells. Each hemoglobin molecule can carry 4 oxygen molecules.

  • Only 1.5% dissolves directly in the blood.

The Oxygen-Hemoglobin Dissociation Curve shows how well hemoglobin holds onto oxygen depending on oxygen levels:

  • In the lungs (high oxygen) → hemoglobin is nearly 100% loaded with oxygen

  • In active tissues (low oxygen) → hemoglobin releases oxygen to the tissues

Things that make hemoglobin release oxygen MORE easily (shift curve right):

  • High CO₂

  • Lower pH (more acidic)

  • Higher temperature (like in working muscles)

  • DPG (a molecule produced by red blood cells)

  • The combined effect of high CO₂ and low pH is called the Bohr effect — it's the body's way of delivering more oxygen exactly where it's being used the most.

Carbon monoxide (CO) is dangerous because it binds to hemoglobin even more tightly than oxygen, blocking oxygen from attaching. Even 0.1% CO in the air can be lethal.

How CO₂ is Carried in the Blood

Carbon dioxide is transportedin three ways:

  1. Dissolved directly in blood — 10%

  2. Bound to hemoglobin (called carbaminohemoglobin) — 30%

  3. Converted to bicarbonate (HCO₃⁻) — 60% (the main method)

For the bicarbonate method: CO₂ enters red blood cells → an enzyme called carbonic anhydrase rapidly converts it → forms carbonic acid → splits into H⁺ and bicarbonate → bicarbonate dissolves easily in blood and gets carried to the lungs.

How the Brain Controls Breathing

Your brain stem contains respiratory centers that control your breathing automatically:

  • Dorsal Respiratory Group (DRG) — located in the medulla; sets the basic rhythm of breathing by gradually ramping up signals to the breathing muscles

  • Pneumotaxic center — located in the pons; sends "stop" signals to the DRG to prevent you from inhaling forever and limit the length of each breath.

  • Hering-Breuer reflex — stretch receptors in the lungs detect when they are full and send a signal via the vagus nerve to stop inhaling. This prevents the lungs from over-inflating.

Chemoreceptors — How the Body Monitors Air Quality in the Blood

Chemoreceptors are sensors that detect CO₂, O₂, and H⁺ levels and report back to the brain's breathing centers:

Peripheral chemoreceptors (in the aorta and carotid arteries — near the heart and neck):

  • Monitor oxygen, CO₂, and H⁺ in the blood

Central chemoreceptors (in the medulla of the brain):

  • Monitor CO₂ and H⁺ in the fluid surrounding the brain

  • These are the MOST important for controlling breathing rate

The main driver of breathing rate is CO₂ (not oxygen): CO₂ easily crosses into the brain, where it produces H High H⁺ = signals to breathe faster. Oxygen has to drop to very low levels before it significantly increases breathing rate.

Hyperventilation (breathing too fast/deep):

  • Blows off too much CO₂ → blood becomes more alkaline → brain senses no CO₂ threat → causes vasodilation → blood pressure drops → less blood flow to the brain → can cause lightheadedness or fainting. Also shifts the oxygen-hemoglobin curve left, meaning tissues actually get LESS oxygen.

High Altitude Physiology

At high altitude, air pressure is lower, so there is less oxygen available per breath:

Altitude

Hemoglobin Saturation

Sea level

97.5%

10,000 ft

90%

15,000 ft

85%

20,000 ft

67%

23,000 ft

50%

Loss of consciousness occurs around 40–50% saturation. At 23,000 ft (50% saturation) is the maximum ceiling for pilots without pressurized cabins or supplemental oxygen.

Long-term adaptations to high altitude (above 10,000 ft):

  • Kidneys produce more erythropoietin (EPO) → stimulates bone marrow to make more red blood cells → more hemoglobin available to carry oxygen

  • Increased blood volume

  • More DPG produced → shifts the curve right → hemoglobin releases oxygen more easily to tissues

  • More mitochondria and oxidative enzymes in cells → better at using the oxygen they do get

Fetal Respiration

A fetus doesn't use its lungs — gas exchange happens through the placenta. Since the lungs aren't being used, they are collapsed, which creates high resistance to blood flow. The fetal circulation has two shortcuts to bypass the lungs:

  • Foramen ovale — a hole between the right and left atria that lets blood skip the lungs entirely

  • Ductus arteriosus — a connection between the pulmonary artery and aorta that reroutes blood away from the lungs

At birth:

  • Placenta is cut off → baby becomes oxygen-deprived → reflex gasp inflates the alveoli → blood flows into the lungs → pressure in the left atrium rises → foramen ovale snaps shut → ductus arteriosus constricts within minutes

Gills vs. Lungs

Gills (fish):

  • Water flows in one direction over the gills (unidirectional)

  • Use a countercurrent system — blood and water flow in opposite directions, which is incredibly efficient (can extract 50–90% of available oxygen)

  • Downside: water is 100x denser than air, so moving water takes a LOT of energy

Lungs (mammals):

  • Air flows in AND back out the same way (tidal flow) — called a uniform pool design

  • Less efficient — fresh and stale air mix together, so only about 25% of available oxygen is extracted

  • Upside: air has much more oxygen than water, and it takes far less energy to move air than water, so even an inefficient system works well enough