How Respiratory physiology Works

Breathing begins with the diaphragm, a dome-shaped muscle beneath your lungs that contracts and flattens downward. Simultaneously, the external intercostal muscles between your ribs contract, lifting the ribcage up and out. These movements expand the chest cavity volume, which causes the pressure inside your lungs to drop below atmospheric pressure—typically by about 1-3 mmHg.

This pressure difference is what actually pulls air into your airways, much like how sucking on a straw creates low pressure that draws liquid upward. Air rushes through your nose or mouth, down the trachea, and through progressively smaller bronchi and bronchioles until it reaches approximately 300 million tiny air sacs called alveoli. At rest, an adult moves about 500 mL of air with each breath, though only about 350 mL actually reaches the alveoli where gas exchange occurs—the rest fills the "dead space" of airways.

Exhalation is mostly passive at rest: the diaphragm relaxes and returns to its dome shape, the ribcage falls inward due to gravity and elastic recoil, and lung tissue springs back like a stretched rubber band. This compression increases pressure inside the lungs above atmospheric pressure, forcing air out. During exercise or forced exhalation, internal intercostal muscles and abdominal muscles actively contract to push air out faster and more completely.

The alveolar-capillary membrane where gas exchange occurs is remarkably thin—only about 0.5 micrometers thick, roughly 1/200th the width of a human hair. This membrane consists of the alveolar epithelium, a shared basement membrane, and the capillary endothelium. Approximately 280 billion capillaries surround the alveoli, creating a gas exchange surface area roughly the size of a tennis court compressed into your chest.

Diffusion happens because gas molecules naturally move from areas of high partial pressure to low partial pressure. Incoming air in the alveoli has an oxygen partial pressure of about 100 mmHg, while blood arriving from the body has only 40 mmHg—this 60 mmHg gradient drives oxygen into the blood. Simultaneously, carbon dioxide moves in the opposite direction: venous blood arrives with a CO2 partial pressure of 46 mmHg, while alveolar air contains only 40 mmHg, so carbon dioxide diffuses out to be exhaled.

The entire exchange takes less than one second, even though red blood cells spend only about 0.75 seconds passing through each alveolar capillary. Carbon dioxide diffuses about 20 times faster than oxygen through biological membranes because of its higher solubility, which is why oxygen transfer is typically the limiting factor. Diseases like emphysema or pulmonary fibrosis impair diffusion by either destroying alveolar surface area or thickening the membrane barrier.

While a small amount of oxygen dissolves directly in blood plasma (about 3 mL per liter), this would be catastrophically insufficient for survival. Instead, hemoglobin molecules inside red blood cells do the heavy lifting—each hemoglobin protein contains four iron atoms that can each bind one oxygen molecule. One liter of blood can carry about 200 mL of oxygen when fully saturated, roughly 65 times more than plasma alone could manage.

Hemoglobin exhibits cooperative binding: once the first oxygen molecule attaches, the protein changes shape slightly, making it easier for the next three oxygen molecules to bind. This creates an S-shaped oxygen-hemoglobin dissociation curve. In the lungs where oxygen partial pressure is high (100 mmHg), hemoglobin becomes 97-98% saturated. In metabolically active tissues where oxygen pressure drops to 40 mmHg or lower, hemoglobin releases its oxygen cargo—a mechanism so elegant it automatically delivers more oxygen to tissues that need it most.

Carbon dioxide transport is more complex and uses three pathways. About 7% dissolves directly in plasma, 23% binds to hemoglobin at different sites than oxygen (forming carbaminohemoglobin), and a remarkable 70% is converted to bicarbonate ions by the enzyme carbonic anhydrase inside red blood cells. This bicarbonate system not only transports CO2 efficiently but also serves as the blood's primary pH buffer, maintaining the narrow pH range (7.35-7.45) that our enzymes require to function.

The entire respiratory system exists to serve cellular respiration, the process where mitochondria—the "powerhouses" inside nearly every cell—use oxygen to convert glucose and other nutrients into ATP, the universal energy currency of life. Specifically, oxygen acts as the final electron acceptor in the electron transport chain, a series of protein complexes embedded in the inner mitochondrial membrane. As electrons pass through this chain, they release energy that pumps hydrogen ions across the membrane, creating a gradient that drives ATP synthesis.

Without oxygen as the terminal electron acceptor, this chain backs up and stops, much like a factory assembly line halts when the end product cannot be removed. Cells can temporarily produce ATP through glycolysis alone, breaking glucose into lactate without oxygen, but this yields only 2 ATP molecules per glucose compared to the 30-32 ATP produced through complete aerobic respiration. This explains why oxygen deprivation (hypoxia) rapidly impairs organ function, particularly in energy-demanding tissues like the brain and heart.

Different tissues consume oxygen at vastly different rates depending on their metabolic activity. At rest, your brain uses about 20% of your total oxygen consumption despite being only 2% of body weight. During intense exercise, skeletal muscles can increase their oxygen consumption by more than 20-fold. The waste product of this metabolism is carbon dioxide, which diffuses out of mitochondria and cells into the bloodstream to be carried back to the lungs for elimination.

Your breathing rate and depth are not consciously controlled—you breathe automatically even while asleep, thanks to respiratory centers in the medulla oblongata and pons of the brainstem. These centers generate rhythmic nerve signals to the diaphragm and intercostal muscles, establishing a baseline breathing pattern of about 12-20 breaths per minute at rest. However, this rhythm constantly adjusts in response to your body's changing needs through an elegant feedback system.

Central chemoreceptors in the brainstem are exquisitely sensitive to hydrogen ion concentration in the cerebrospinal fluid, which reflects blood carbon dioxide levels. When CO2 rises—even by just 5 mmHg—it crosses into the cerebrospinal fluid and forms carbonic acid, increasing acidity. This triggers the respiratory centers to increase breathing rate and depth within seconds. Peripheral chemoreceptors in the carotid arteries and aorta provide backup monitoring, responding to low oxygen levels (below 60 mmHg), high CO2, and increased acidity.

This system prioritizes carbon dioxide removal over oxygen delivery, which is why you feel breathless when CO2 accumulates, not when oxygen runs low. During exercise, metabolically active muscles produce more CO2, triggering deeper and faster breathing that can increase ventilation from 6 liters per minute at rest to over 100 liters per minute during maximum exertion. Additional inputs from stretch receptors in the lungs prevent over-inflation, while signals from muscles and joints during movement preemptively increase breathing in anticipation of metabolic demands.