# Transport in Animals

> Open and closed circulatory systems, the human heart and double circulation, blood vessels, what blood carries, and how blood changes as it moves around the body.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/transport-in-animals/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The giraffe's high-pressure heart
A giraffe's brain sits about 2 metres above its heart. To push blood that high, its heart beats with about twice the pressure of yours. Its heart wall can be over 7 cm thick.

So why doesn't a giraffe faint when it lifts its head after drinking, or burst blood vessels when it bends down? It has valves in its neck veins, a sponge-like net of vessels at the base of the brain that soaks up sudden pressure, and skin on its legs so tight it works like a compression stocking. Every animal's transport system is built for the life it leads.

**In short:** Animals move materials around the body in a circulatory system. Mammals have a closed, double circulation: the heart pumps blood to the lungs and then to the body, arteries carry blood away from the heart, veins bring it back, and capillaries exchange materials with cells.

## Starting point: what you already know

- Large animals have a small surface area to volume ratio, so they need a transport system (see Levels of Organisation).
- Oxygen enters the blood in the lungs. Nutrients enter it in the small intestine (see Nutrient and Gas Exchange).
- Blood is pumped by the heart through blood vessels.

## Open and closed systems

| | Open system | Closed system |
|---|---|---|
| Examples | Insects, crabs, snails | Humans, fish, earthworms, octopus |
| Blood | Pumped into a body cavity and bathes the organs directly | Stays inside blood vessels |
| Pressure | Low | High, so it moves fast |
| Suits | Small animals, or ones like insects that get oxygen by tracheae | Larger, active animals |

## Single and double circulation

A **fish** has **single circulation**: heart, then gills, then body, then back to the heart. Blood passes through the heart **once** per lap.

Mammals and birds have **double circulation**: blood passes through the heart **twice** per lap.

1. **Pulmonary circuit**: right side of the heart pumps blood to the lungs to pick up oxygen.
2. **Systemic circuit**: left side of the heart pumps oxygen-rich blood to the rest of the body.

Returning to the heart after the lungs lets blood be pumped out again at high pressure, which an active, warm-blooded animal needs.

## The human heart

*Diagram: Simple diagram of the heart: right atrium and right ventricle on the left of the picture with deoxygenated blood, left atrium and left ventricle on the right with oxygenated blood and a thicker wall*
*The heart as seen facing a person, so their right side is on your left. The left ventricle (thick red wall) pumps oxygen-rich blood to the whole body. The septum keeps the two sides apart.*

The heart has four chambers. The **atria** (top) receive blood. The **ventricles** (bottom) pump it out. **Valves** stop blood flowing backwards; their closing makes the "lub-dub" sound.

The path of blood: body → **vena cava** → right atrium → right ventricle → **pulmonary artery** → lungs → **pulmonary vein** → left atrium → left ventricle → **aorta** → body.

The **left ventricle** has the thickest wall because it pumps blood all around the body.

## Blood vessels

| | Arteries | Capillaries | Veins |
|---|---|---|---|
| Carry blood | **A**way from the heart | Between arteries and veins, past every cell | Back to the heart |
| Wall | Thick, muscular, elastic | One cell thick | Thin |
| Pressure | High | Low | Lowest |
| Valves | No | No | Yes, stop backflow |

Exchange of materials happens only in **capillaries**, because their walls are so thin.

## What blood carries

| Part | Job |
|---|---|
| **Plasma** (55%) | Yellow liquid carrying glucose, amino acids, $\mathrm{CO_2}$, urea, hormones, salts and heat |
| **Red blood cells** | Carry oxygen joined to haemoglobin |
| **White blood cells** | Fight infection |
| **Platelets** | Help blood clot |

### Worked example: How blood changes on its trip
| Place blood has just left | Oxygen | Carbon dioxide | Glucose | Urea |
|---|---|---|---|---|
| Lungs | High | Low | Same | Same |
| Muscles | Low | High | Lower | Same |
| Small intestine (after a meal) | Low | High | High | Same |
| Liver | Low | High | Controlled | High (urea is made here) |
| Kidneys | Low | High | Same | Low (urea is removed) |

**Mind map**

```text
Transport in animals
  Systems
    Open: insects
    Closed: vertebrates
  Circulation
    Single: fish
    Double: mammals and birds
    Pulmonary and systemic
  Heart
    Atria receive
    Ventricles pump
    Valves stop backflow
    Left ventricle thickest
  Vessels
    Arteries: away, high pressure
    Capillaries: exchange
    Veins: valves, low pressure
  Blood
    Plasma, red and white cells, platelets
```

### Year 11: Investigating blood and the heart
You may compare blood smears under a microscope, dissect a sheep heart, or analyse data on how blood changes in different organs. When analysing, remember:

- The **hepatic portal vein** carries nutrient-rich blood from the intestine **straight to the liver**, which stores extra glucose as glycogen.
- **Oxygen saturation** of haemoglobin in blood leaving the lungs is about 98%. In a working muscle it can drop to 25%.
- **Cardiac output** = heart rate × stroke volume. At rest, $70 \text{ beats/min} \times 70 \text{ mL} \approx 4.9$ L per minute.

Plants and animals both need transport systems, but plants have no pump. Their sap moves by transpiration and pressure flow, and much more slowly.

### Year 12: Circulation in Modules 7 and 8
White blood cells and antibodies travel in the blood (Module 7). Diseases of the circulatory system, such as **atherosclerosis** (fatty plaque narrowing arteries) and heart attack, are major non-infectious diseases in Australia (Module 8). Technologies such as **pacemakers** and **stents** are common HSC examples.

## Quick summary

- Open systems pump blood into a body cavity. Closed systems keep it in vessels.
- Fish have single circulation. Mammals have double circulation: pulmonary and systemic.
- Atria receive blood, ventricles pump it, valves stop backflow. The left ventricle is thickest.
- Arteries carry blood away from the heart at high pressure. Veins return it and have valves. Capillaries exchange materials.
- Plasma carries dissolved substances. Red blood cells carry oxygen.

## Quiz

Try these questions from the chapter quiz.

1. Which animal has an open circulatory system?
   - A. Human
   - B. Fish
   - C. Grasshopper
   - D. Dog
   - Answer: C. Grasshopper. Insects have open systems.

2. Which vessels carry blood away from the heart?
   - A. Arteries
   - B. Veins
   - C. Capillaries
   - D. Lymph vessels
   - Answer: A. Arteries. Arteries carry blood Away.

3. Where are materials exchanged between blood and body cells?
   - A. Arteries
   - B. Veins
   - C. Capillaries
   - D. Aorta
   - Answer: C. Capillaries. Capillary walls are one cell thick.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/transport-in-animals/#quiz