Animal Support, Transport, Defense, and Reproduction

This is Unit Thirteen of Biology, Part Two: gas exchange. Part One dealt with holding a body up and moving it; this part is the first of the transport problems. The through-line is that no animal moves gases by pumping them — every exchange in this part is diffusion down a partial-pressure gradient, and all the anatomy exists to keep that gradient steep. By the end you will be able to name the three requirements of a gas-exchange surface; say why countercurrent exchange beats parallel flow; say what makes the oxygen-haemoglobin curve S-shaped; say what a rightward shift of that curve means; and say what actually drives the breathing reflex.

Two entries setting up everything that follows: the gradient gases move down, and the three properties a surface needs before any of it can happen.

Down the gradient.

Bio 3141 Partial Pressure and the Diffusion Gradient

A gas's partial pressure is the portion of total pressure it contributes, and gas diffuses across a surface from higher to lower partial pressure until equalised. Oxygen's higher partial pressure in inhaled air than in blood drives it into the bloodstream. Each gas follows its own gradient — oxygen and carbon dioxide diffuse in opposite directions at the same time.

Thin, moist, large.

Bio 3143 Gas Exchange Surface Requirements

An effective gas-exchange surface is thin, moist and large relative to body size, since diffusion works only across short wet distances and exchange scales with area. Lung alveoli multiply surface area through millions of small sacs rather than one large chamber. A dry surface stops working — gases must dissolve in moisture before they can diffuse across.

Five entries, four strategies. Exchange straight across the skin, gills in water, air tubes reaching tissue directly, and two quite different lungs — each carrying its own ceiling on body size or habitat.

Straight across the skin.

Bio 3145 Integumentary Gas Exchange

Integumentary gas exchange occurs directly across an animal's moist skin, working only for small or thin-bodied animals whose surface-to-volume ratio keeps diffusion distances short. An earthworm exchanges gases across its moist skin alone, with no lungs or gills. A dried-out surface halts exchange entirely, which limits this strategy to consistently moist environments.

Against the flow.

Bio 3147 Gills and Countercurrent Exchange

Gills exchange gases across thin, blood-filled filaments bathed by water, and many fish maximise uptake through countercurrent exchange, with blood and water flowing in opposite directions to sustain the gradient along the whole filament. Measuring oxygen extraction along a perfused gill confirms uptake stays high across its entire length. Parallel flow would let the gradient equalise partway through, wasting much of the surface.

Air straight to the cells.

Bio 3149 Tracheal System in Insects

An insect's tracheal system delivers air directly to tissues through branching tubes opening at external spiracles, exchanging gases without relying on blood at all. Fine tracheole branches reach individual muscle cells, bypassing the circulatory system for oxygen delivery. A tracheal system still limits body size, since diffusion through air-filled tubes only reaches so far into a large body.

Half a tennis court.

Bio 3151 Mammalian Lung Structure

A mammalian lung branches from the trachea through bronchi and bronchioles into millions of alveoli, thin-walled sacs surrounded by capillaries, supplying the area the surface requirements demand. Alveolar surface area in human lungs is roughly seventy-five square metres, about half a tennis court. Alveoli hold no muscle; airflow depends on pressure changes elsewhere in the chest.

One direction only.

Bio 3153 Avian Flow-Through Lungs and Air Sacs

Bird lungs connect to air sacs that keep air flowing through in one direction on both inhalation and exhalation, unlike a mammal's in-and-out flow. One-directional airflow lets the lung contact fresh air continuously, supporting flight at low-oxygen altitude. It needs two full breathing cycles to move one volume of air completely through.

Three entries. How a mammal actually gets air in, the two properties that decide how hard breathing is, and what goes wrong when airflow and blood flow to the same region stop matching.

Pulled in by pressure.

Bio 3155 Negative-Pressure Breathing Mechanics

Mammalian inhalation works by negative pressure: the diaphragm contracts and flattens, the chest cavity expands, and falling internal pressure draws air in. Diaphragm relaxation during exhalation shrinks the cavity, raising pressure and pushing air back out. The lungs do not pull themselves open — they expand passively because the pressure around them drops first.

Why breathing is hard.

Bio 3157 Compliance and Resistance in the Lungs

Lung compliance measures how easily lung tissue expands under a pressure change, while airway resistance measures how much the airways impede airflow, and the two together determine breathing effort. Stiffened, low-compliance tissue needs more pressure change for the same inhaled volume. Low compliance and high resistance are distinct problems with different causes, even though both raise effort.

Air and blood, out of step.

Bio 3159 V/Q Mismatch Mechanism

Ventilation-perfusion mismatch occurs when airflow and blood flow to the same lung region fall out of proportion, reducing that region's exchange efficiency. A region with blood flow but little airflow cannot oxygenate that blood effectively. The mismatch can run either direction — too little airflow for the blood, or too little blood for the airflow.

Four entries. The protein that carries oxygen and the cooperative trick that makes it good at both loading and unloading, the curve that behaviour produces, what shifts the curve, and the quite different route carbon dioxide takes back.

Four sites, cooperating.

Bio 3161 Hemoglobin Structure and Cooperative Binding

Haemoglobin, a four-subunit protein inside erythrocytes, binds up to four oxygen molecules cooperatively, meaning each bound oxygen makes the next site bind more readily. That cooperation lets haemoglobin load oxygen rapidly at the lungs and still unload it efficiently at low-oxygen tissue. Cooperative binding is a property of the whole four-subunit protein, not of any single site acting alone.

The S-shape.

Bio 3163 The Oxygen-Hemoglobin Dissociation Curve

The oxygen-haemoglobin dissociation curve plots saturation against oxygen partial pressure, forming an S-shape produced directly by cooperative binding. The curve's steep midsection lets a small drop in partial pressure release a large share of bound oxygen at active tissue. A curve shifted rightward means haemoglobin releases oxygen more readily — not that it carries less overall.

Shifting it where needed.

Bio 3165 The Bohr Effect

The Bohr effect describes how rising carbon dioxide and falling pH at active tissue shift haemoglobin's dissociation curve rightward, releasing oxygen more readily exactly where it is needed. Exercising muscle's acidic, CO2-rich environment triggers greater release than resting tissue does. The Bohr effect adjusts local release — it does not change uptake at the lungs.

The return trip.

Bio 3167 CO2 Transport: Bicarbonate and Carbaminohemoglobin

Blood carries carbon dioxide mostly as bicarbonate ion, formed with help from carbonic anhydrase, with smaller amounts dissolved directly in plasma or bound to haemoglobin as carbaminohaemoglobin. At the lungs, bicarbonate formation reverses, releasing carbon dioxide gas that diffuses out on exhalation. Most carbon dioxide travels as bicarbonate rather than bound to haemoglobin, unlike oxygen's more direct route.

Three entries. A widespread assumption about what makes you take a breath, the chemoreceptor mechanism that answers it, and the chapter's walkthrough of one full exchange cycle.

The assumption.

Bio 3169 "Breathing Is Driven by Low Oxygen"

Assuming falling oxygen is what triggers each breath overstates a real signal — the primary drive tracks rising carbon dioxide and falling pH instead. Holding one's breath becomes urgent from rising carbon dioxide discomfort well before oxygen levels drop critically low. The chemoreceptor mechanism this overlooks comes next.

What actually triggers it.

Bio 3171 Rising CO2 and Falling pH Drive the Breathing Reflex

Central chemoreceptors detect rising carbon dioxide and falling pH in cerebrospinal fluid and signal the brainstem to raise breathing rate before oxygen drops critically. A rebreathing bag raises inhaled carbon dioxide and quickly drives faster breathing despite ample oxygen. The low-oxygen framing ignores that oxygen sensors serve only as backup.

One breath, all the way round.

Bio 3175 Tracing Gas Exchange Across the Pulmonary-Systemic Loop, Worked

One breath during moderate exercise. Diaphragm contraction lowers chest-cavity pressure, drawing air into the alveoli, where oxygen's higher partial pressure drives it across the alveolar-capillary surface; haemoglobin binds it cooperatively, reaching near-full saturation. At active muscle the Bohr effect applies — higher carbon dioxide and lower pH shift the curve rightward, prompting release where demand is highest. The carbon dioxide produced converts mostly to bicarbonate for the return trip, reversing at the lungs and leaving on exhalation. Assuming both gases travel the same way is the error: oxygen rides haemoglobin, carbon dioxide rides as an ion.

Five things to carry out of this part. A gas-exchange surface has to be thin, moist and large relative to body size — thin because diffusion only crosses short distances, moist because gases must dissolve before they can cross at all, and large because exchange scales with area. Countercurrent exchange beats parallel flow because running blood and water in opposite directions keeps a gradient along the gill's whole length, where parallel flow lets the two equalise partway through and wastes the rest of the surface. The oxygen-haemoglobin curve is S-shaped because binding is cooperative: each oxygen that binds one of haemoglobin's four subunits makes the next site bind more readily, which produces a steep midsection where a small pressure drop releases a lot of oxygen. A rightward shift means haemoglobin releases oxygen more readily, not that it carries less — and the Bohr effect is what shifts it, at tissue that is acidic and CO2-rich, which is to say tissue that is working. And the breathing reflex is driven by rising carbon dioxide and falling pH detected by central chemoreceptors, not by falling oxygen; oxygen sensors exist, but they are the backup. Part Three takes up circulation and osmoregulation — the blood that carried everything in this part, and what the body does with the waste.