# E1.3 Regulation of Gas Content in Blood

F.6 HKDSE Biology · Topic E1.3 · Lesson 1

How the body keeps oxygen and carbon dioxide in the blood steady by adjusting breathing and heart beat, and what changes during exercise.

## Key points

- **Why regulate?** Carbon dioxide forms a weak acid in blood, **lowering blood pH** and upsetting **enzyme activities**; the body adjusts the **rate and depth of breathing** and the **rate and strength of heart beat**.
- **Breathing**: voluntary via the **motor area of the cerebrum**; involuntary via the **respiratory centre** in the **medulla oblongata (延⁠髓)** (rest: 18 breaths/min, **tidal volume** 500 cm^3^).
- **Stimulus**: rise in **carbon dioxide concentration** → fall in **blood pH** → **chemoreceptors** → more impulses to the **intercostal and diaphragm muscles** → faster, deeper breathing (**negative feedback**).
- **Myogenic heart beat**: **SA node (竇⁠房⁠結)** (pacemaker) → atria contract → **AV node** (0.1 s delay) → **bundle of His** → **Purkinje fibres (蒲⁠金⁠耶⁠氏⁠纖⁠維)** → ventricles contract from the apex up.
- **Cardiac cycle** (0.8 s): **atrial systole** 0–0.1 s → **ventricular systole** 0.1–0.4 s (**'lub'** = tricuspid and bicuspid valves close) → **diastole** 0.4–0.8 s (**'dub'** = semilunar valves close).
- **Cardiac output (心⁠輸⁠出⁠量)** = stroke volume × heart rate (70 mL/beat × 72 beats/min = 5040 mL/min at rest); stroke volume depends on **venous return** and **strength of contraction**.
- **Cardiovascular centre**: input from chemoreceptors, **baroreceptors** (blood pressure), **stretch receptors** (limb movements). **Sympathetic nerve** (**noradrenaline**) stimulates, **vagus nerve** (**acetylcholine**) inhibits the SA node and cardiac muscle (**antagonistic**); **adrenaline** (hormone) also raises cardiac output.
- **Exercise**: muscles need more oxygen and glucose, make more carbon dioxide and **lactic acid**. **Ventilation rate (換⁠氣⁠率)** = tidal volume × rate of breathing rises; it stays high afterwards to repay the **oxygen debt**.
- **Cardiac output rises**: sympathetic nerve, adrenaline, larger **venous return**; blood diverted to muscles and skin (heat loss). Training: thicker cardiac muscle wall, larger stroke volume, lower resting heart rate.

## Must know

| | Control of breathing | Control of heart beat |
|---|---|---|
| **Stimulus** | rise in carbon dioxide concentration in blood → fall in blood pH | fall in blood pH; change in blood pressure; limb movements |
| **Receptors** | chemoreceptors in the aortic bodies, carotid bodies and respiratory centre | chemoreceptors (aortic, carotid bodies); baroreceptors (aorta, carotid arteries); stretch receptors (muscles, tendons) |
| **Centre** | respiratory centre in the medulla oblongata | cardiovascular centre in the medulla oblongata |
| **Nerves** | more nerve impulses to the intercostal and diaphragm muscles | sympathetic nerve (noradrenaline, stimulates) or vagus nerve (acetylcholine, inhibits) → SA node and cardiac muscle; adrenal glands add adrenaline |
| **Response** | rate and depth of breathing ↑ → ventilation rate ↑ | heart rate and stroke volume change → cardiac output changes |

$\text{cardiac output} = \text{stroke volume} \times \text{heart rate}$ (mL/min = mL/beat × beats/min). $\text{ventilation rate} = \text{tidal volume} \times \text{rate of breathing}$ (cm^3^/min = cm^3^/breath × breaths/min). Both are volumes per minute: always write the unit.

## Common mistakes

- ✗ "A fall in oxygen concentration is the stimulus for faster breathing." ✓ The main stimulus is a **rise in carbon dioxide concentration** in blood (a fall in blood pH), detected by chemoreceptors.
- ✗ "'Lub' is the semilunar valves closing, pushed shut by muscles." ✓ 'Lub' = **tricuspid and bicuspid valves** close at the start of ventricular systole; 'dub' = **semilunar valves** close at the start of diastole. **Pressure differences** open and close the valves.
- ✗ "During exercise the vagus nerve becomes more active and releases adrenaline." ✓ The **sympathetic nerve** becomes more active and releases **noradrenaline** at the SA node; the vagus nerve releases acetylcholine and slows the heart. Adrenaline is a **hormone** from the adrenal glands, carried in the blood.
- ✗ "The heart stops beating if the nerves to it are cut." ✓ Cardiac muscle is **myogenic**: the SA node generates the impulses itself. The sympathetic and vagus nerves and adrenaline only change the rate and strength of the beat.
- ✗ "Oxygen debt is the shortage of oxygen during exercise." ✓ Oxygen debt is the **extra oxygen taken in after exercise** to break down lactic acid or convert it to glucose or glycogen in the liver; that is why breathing stays fast afterwards.

## Quick check

1. A student's stroke volume is 75 mL and her heart rate is 80 beats/min. Calculate her cardiac output, with the unit.
2. Put these in the order the electrical impulses travel: AV node, Purkinje fibres, SA node, bundle of His, ventricles, atria.
3. Which valves close to make the first heart sound, and at what time in a 0.8 s cardiac cycle does this happen?
4. You hold your breath. Name the stimulus, the receptors, the centre and the effectors that make you breathe faster and more deeply afterwards.
5. Give two reasons why venous return increases during exercise.

## Answers

1. 75 mL/beat × 80 beats/min = 6000 mL/min.
2. SA node → atria → AV node → bundle of His → Purkinje fibres → ventricles.
3. Tricuspid and bicuspid valves; at 0.1 s, the start of ventricular systole.
4. Rise in carbon dioxide concentration (fall in blood pH); chemoreceptors in the aortic bodies and carotid bodies (and the respiratory centre); respiratory centre in the medulla oblongata; intercostal and diaphragm muscles.
5. Contracting skeletal muscles squeeze the veins next to them; deeper breathing makes the thoracic pressure more negative, drawing blood towards the heart.

## Did you know?

Left to itself the SA node fires about 100 times a minute; your resting heart rate is only about 70 because the vagus nerve keeps braking it.