Gas exchange

The thin boundary where gases pass between the body and the environment — e.g. the wall of an **alveolus** in the lung.

**Diffusion** — the passive net movement of particles from **high** to **low** concentration.

Diffusion is **passive**: particles move on their own down the **concentration gradient**, so no energy (ATP) is used.

**Large** surface area, **thin** (short diffusion distance), **moist** and **permeable**.

More gas can diffuse across **at the same time**, so exchange is faster.

The wall is only **one cell thick**, giving a **short diffusion distance**, so diffusion is fast.

A thin film of water lets the gases **dissolve** before they cross the membrane.

From the **alveolar air into the blood** (from higher to lower oxygen concentration).

From the **blood into the alveolar air** (from higher to lower carbon dioxide concentration).

Fresh air keeps alveolar **oxygen high** and **carbon dioxide low**, so the gradient stays steep.

It carries blood away and brings fresh blood in, keeping capillary **oxygen low** and **carbon dioxide high**.

Net diffusion **stops** — which is why the gradient must be kept **steep** by ventilation and blood flow.

A tiny **air sac** in the lung where **gas exchange** between air and blood takes place.

**Oxygen** diffuses from the air into the blood; **carbon dioxide** diffuses from the blood into the air.

**Large surface area**, wall **one cell thick** (short diffusion distance), **moist lining**, and a **rich blood supply**.

Many tiny sacs pack a **huge total surface area** into the chest, and surface area controls how fast gases diffuse.

It is a **thin, flat cell** forming most of the alveolar wall, giving a **short diffusion distance** — it is the gas-exchange surface.

It **secretes surfactant** onto the moist alveolar lining.

It **engulfs and digests** pathogens, dust and debris that are breathed in, keeping the surface clean.

A fluid secreted by type II pneumocytes that **lowers the surface tension** of the moist alveolar lining.

It **lowers surface tension**, so alveoli **do not collapse** on exhalation and the lungs are **easier to inflate**.

A wall **one cell thick** gives a **short diffusion distance**, so gases cross the wall quickly.

It keeps a **steep concentration gradient** by carrying gases away, so diffusion stays fast.

**No surfactant** is made → surface tension stays high → **alveoli collapse** → reduced gas exchange.

The movement of **air into and out of the lungs** (breathing) — it keeps fresh air at the gas-exchange surface.

**No** — lungs have no muscle. The **diaphragm** and **intercostal muscles** change the chest's volume to move air.

A sheet of muscle below the lungs. When it **contracts** it **flattens and moves down**, increasing the volume of the thorax.

Between the ribs. The **external intercostals contract** to pull the ribs **up and out**.

**Muscles → volume → pressure → air flow.** Muscles change the volume, which changes the pressure, and air moves down the pressure gradient.

Volume **increases**, so pressure **falls below atmospheric** — and air flows **in**.

Volume **decreases**, so pressure **rises above atmospheric** — and air flows **out**.

Because the pressure inside has **fallen below atmospheric**, and air always moves from **high to low** pressure.

**Passive** — the muscles simply **relax** (no contraction needed); the diaphragm domes up and the ribs drop.

The **diaphragm and external intercostal muscles contracting**.

The **external intercostal muscles**.

Pressure **below** atmospheric = **inhaling** (volume rising); pressure **above** atmospheric = **exhaling** (volume falling).

In **opposite** directions — bigger volume means lower pressure, smaller volume means higher pressure.

An instrument that **measures the air a person breathes in and out**, recording it as a **trace** of lung volume against time.

The volume of air in **one normal, resting breath** — the height of one small wave on the trace.

The **largest volume of air moved in one breath**: IRV + TV + ERV (deepest breath in to fullest breath out).

Air that **always stays in the lungs** and cannot be breathed out — so it never appears on the trace.

Measure the volume from the **top of the deepest breath in** to the **bottom of the fullest breath out**.

**Count the complete waves (breaths) in one minute.**

They keep **inhaled and exhaled air on separate tubes** so the two airstreams do not mix.

It **absorbs the carbon dioxide breathed out**, so the person re-breathes air without a CO₂ build-up.

**Oxygen is used up in respiration** and the **CO₂ is absorbed by the soda lime (not replaced)**, so the total gas in the closed circuit falls.

It **rises** — air is drawn out of the chamber, the drum sinks and the pen goes up.

The waves become **taller (larger tidal volume)** and **closer together (faster rate)**, raising the air inhaled per minute.

**Total lung capacity = vital capacity + residual volume** — the residual volume can never be breathed out.

A lung disease in which the **walls between alveoli are destroyed**, so the sacs merge into **fewer, larger** spaces with a much smaller surface area for gas exchange.

A **reduced surface area** for gas exchange.

Less surface area (and a longer / damaged diffusion path) means oxygen diffuses into the blood **more slowly**.

Elastic recoil lets the lung spring back to push air out. In emphysema it is **lost**, so air is **trapped** and exhaling is hard.

It **reduces the surface area** (fewer, larger sacs) AND it **loses elastic recoil** (air is trapped).

They cannot raise oxygen uptake enough to meet demand, so they become **breathless**, tire quickly and have **limited exercise** capacity.

**Smoking** (cigarette smoke); **air pollution** also contributes.

**Reducing smoking** (anti-smoking laws, stop-smoking support) and **cutting air pollution**.

A larger surface area lets **more oxygen diffuse per breath** — emphysema reduces it, so gas exchange slows.

**Larger** — small sacs merge into fewer, larger spaces (so there is less surface area).

A tiny **air sac** in the lung where gas exchange occurs; its wall is one cell thick, and millions give a large surface area.

'**Fewer, bigger, slower**' — fewer, bigger air sacs make gas exchange slower.

A small **pore** in the leaf surface (mostly the underside) through which **gases enter and leave** the leaf.

The two cells either side of a stoma that change shape to **open or close the pore**, controlling gas exchange and water loss.

The **lower (under) surface** — this reduces water loss while still allowing gas exchange.

A layer of **tall cells packed with chloroplasts** near the upper surface; it carries out **most of the photosynthesis**.

A layer of **loosely-packed cells with large air spaces**; the spaces let **gases diffuse** to and from every cell.

**Thin** = short diffusion distance for gases; **flat and wide** = large surface area for light and gas exchange.

**Diffusion** — from a higher to a lower concentration, with no energy needed.

In through a **stoma** → through the **spongy-mesophyll air spaces** → across the **cell wall and membrane** → into a **chloroplast**.

It is a **transparent, waterproof** coating that **reduces water loss** while still letting light through.

**CO₂ diffuses in**; **O₂ (and water vapour) diffuse out** — through the stomata.

So **CO₂ can reach every cell** and **O₂ can diffuse away** — they are the leaf's internal corridors for gases.

A single layer of **transparent, tightly-packed cells with no chloroplasts**, so **light passes through** to the palisade cells below.