# E1.3 Regulation of Gas Content in Blood

F.6 HKDSE Biology · Topic E1.3 · Lesson 2 · Revision of 1.3.1–1.3.4

The whole topic in one set, with the graph-reading and calculation skills the exam questions test.

## Figure

Left-heart pressures in one cardiac cycle: a valve opens or closes where two curves cross.

## Key points

- **Why regulate?** A **steady supply of oxygen** to body cells for respiration, and a **stable blood pH** so that enzymes work properly.
- **Respiratory centre (呼吸中樞)** in the medulla oblongata keeps breathing going automatically: impulses → intercostal and diaphragm muscles **contract** → inhalation; no impulses → muscles **relax** → exhalation. Rate and depth: **negative feedback**.
- **Heart beat**: **SA node (竇房結)** (pacemaker) generates impulses → both atria contract → **AV node** → **bundle of His** and **Purkinje fibres** → both ventricles contract.
- **Cardiac cycle 0.8 s**: atrial systole 0–0.1 s → ventricular systole 0.1–0.4 s (tricuspid and bicuspid valves close: **1st heart sound**) → diastole 0.4–0.8 s (semilunar valves close: **2nd heart sound**).
- **Cardiac output (心輸出量)** = **stroke volume** (blood pumped by the left ventricle in one beat) × **heart rate** (beats per minute).
- **Nervous control**: cardiovascular centre (medulla oblongata) → more impulses along the **sympathetic nerve** → SA node and cardiac muscle → beats **more frequently and forcefully** → heart rate, stroke volume, cardiac output rise.
- **Exercise**: rate and depth of breathing and cardiac output all **increase** → more oxygen and glucose to skeletal muscles, faster carbon dioxide removal. Breathing stays high afterwards to repay the **oxygen debt (氧債)**: the extra oxygen breaks down lactic acid in the liver.
- **Ventilation rate (換氣率)** = tidal volume (cm^3^/breath) × rate of breathing (breaths/min); depth by **spirometer**, breath volume kit or data logger, heart rate by the **pulse**.
- **Reading graphs**: tidal volume = **peak − trough** of one breath; breathing rate = complete breaths × 60 ÷ time (s); heart rate = 60 ÷ time for one cardiac cycle (s).

## Must know

| | Atrial systole (0–0.1 s) | Ventricular systole (0.1–0.4 s) | Diastole (0.4–0.8 s) |
|---|---|---|---|
| **Atria** | contract | relax | relax |
| **Ventricles** | relax | contract | relax |
| **Tricuspid and bicuspid valves** | open | close (1st heart sound) | open |
| **Semilunar valves** | close | open | close (2nd heart sound) |
| **Blood flow** | atria → ventricles | right ventricle → pulmonary artery; left ventricle → aorta | venae cavae → right atrium; pulmonary veins → left atrium; atria → ventricles |

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

## Common mistakes

- ✗ "The trace rises to 3.0 dm^3^ each breath, so the tidal volume is 3.0 dm^3^." ✓ Tidal volume = **peak − trough** of one breath (3.0 − 2.4 = 0.6 dm^3^); the air below the trough stays in the lungs after a normal breath out.
- ✗ "Muscles pull the valves open, and the heart sounds come from valves opening." ✓ **Pressure differences** push the valves open and shut; both sounds come from valves **closing**: 1st = tricuspid and bicuspid, 2nd = semilunar.
- ✗ "The semilunar valve opens the moment the ventricle starts to contract." ✓ It opens only when the ventricular pressure rises **above the pressure in the aorta or pulmonary artery**, the crossing of the two curves; the bicuspid or tricuspid valve has already closed by then.
- ✗ "Breathing in air with more oxygen makes you breathe faster." ✓ **Carbon dioxide** has the greater effect: more carbon dioxide lowers the blood pH and stimulates the chemoreceptors and respiratory centre; extra oxygen hardly changes the breathing rate.
- ✗ "One cycle takes 0.8 s, so the heart rate is 0.8." ✓ Heart rate = 60 ÷ 0.8 = **75 beats/min**: convert the cycle time to beats per minute and write the unit once (beats/min or beats min^−1^, never beats/min^−1^).

## Quick check

1. A lung-volume trace shows 5 complete breaths in 20 s, rising from 2.5 dm^3^ to 3.1 dm^3^ in each breath. Find the tidal volume, the breathing rate and the ventilation rate.
2. On a pressure graph the left-ventricle curve rises above the aorta curve at 0.15 s. Which valve does what at that moment, and how does the ventricular volume change?
3. One cardiac cycle lasts 0.75 s and the cardiac output is 6 dm^3^/min. Find the heart rate and the stroke volume.
4. In diastole, which valves are open, which are closed, and where does blood flow?
5. One person inhales air with 5% carbon dioxide, another inhales air with 25% oxygen. Whose breathing rate rises? Name the stimulus, the receptors, the centre and the effectors.

## Answers

1. 3.1 − 2.5 = 0.6 dm^3^; 5 × 60 ÷ 20 = 15 breaths/min; 0.6 × 15 = 9 dm^3^/min.
2. The semilunar valve at the base of the aorta opens; blood leaves and the ventricular volume starts to fall.
3. 60 ÷ 0.75 = 80 beats/min; 6000 ÷ 80 = 75 cm^3^/beat.
4. Tricuspid and bicuspid valves open, semilunar valves closed; venae cavae → right atrium, pulmonary veins → left atrium, and atria → ventricles.
5. The 5% carbon dioxide person. Stimulus: fall in blood pH (more carbon dioxide); receptors: chemoreceptors in the aortic bodies, carotid bodies and respiratory centre; centre: respiratory centre in the medulla oblongata; effectors: intercostal and diaphragm muscles.

## Did you know?

A donor heart can be kept beating in a machine outside the body while it is carried to the patient: it needs no nerves, because its own SA node starts every beat.