The structure and functions of the cardio-respiratory system

Welcome to the Cardio-Respiratory System!

In this chapter, we are going to explore how your body gets the "fuel" (oxygen) it needs to move and how it gets rid of the "exhaust" (carbon dioxide) it creates during exercise. Think of your cardio-respiratory system as a high-speed delivery service. The lungs are the loading dock where oxygen is picked up, and the heart is the powerful pump that sends delivery trucks (blood) all over the body. Understanding this is vital for PE because it explains why we get out of breath and why our hearts beat faster when we play sport!

1. The Respiratory System: The Pathway of Air

When you take a breath in, air travels through a specific "one-way street" to reach your blood. You need to know this order exactly:

1. Mouth/Nose: Air enters here and is warmed and filtered.
2. Trachea: Also known as the windpipe; it’s the main tube leading down.
3. Bronchi: The trachea splits into two tubes (one for each lung).
4. Bronchioles: These are smaller branches that spread out like tree limbs.
5. Alveoli: Tiny air sacs at the very end where the "magic" happens.

Memory Aid: Use the phrase "My Teacher Believes Biology is Awesome" to remember the order: Mouth, Trachea, Bronchi, Bronchioles, Alveoli.

Quick Review: The pathway starts at the mouth/nose and ends at the alveoli. If you can remember it's like a tree getting smaller and smaller, you've got it!

2. Gaseous Exchange: The Big Swap

Gaseous exchange is the process where oxygen moves from the lungs into the blood, and carbon dioxide moves from the blood into the lungs. This happens in the alveoli.

Features of Alveoli that help Gaseous Exchange:

The alveoli are perfectly designed for this job because they have:
• Large surface area: There are millions of them, providing a huge space for gas to move.
• Moist, thin walls: The walls are only one cell thick, so the gases don't have far to travel (a short diffusion pathway).
• Lots of capillaries: They are wrapped in tiny blood vessels.
• Large blood supply: This ensures there is always blood ready to pick up oxygen.

How do the gases move?

Gases move through diffusion. This simply means they move from a high concentration (where there is a lot) to a low concentration (where there is very little).

• Oxygen is high in the alveoli and low in the blood, so it moves into the blood.
• Carbon Dioxide is high in the blood and low in the alveoli, so it moves into the lungs to be breathed out.

Did you know? Oxygen hitch-hikes on a protein in your red blood cells called haemoglobin. When they join together, it’s called oxyhaemoglobin. Haemoglobin also carries some carbon dioxide away from the muscles!

Key Takeaway: Gaseous exchange happens via diffusion in the alveoli because they have thin walls and a massive blood supply.

3. The Mechanics of Breathing

How do we actually get air in and out? It’s all about air pressure. When the space inside your chest gets bigger, the pressure drops, and air rushes in!

Inhaling (Breathing In) at Rest:

1. The diaphragm flattens and moves downwards.
2. The intercostal muscles contract, pulling the rib cage up and out.
3. This makes the chest cavity bigger, so air is sucked in.

Exhaling (Breathing Out) at Rest:

1. The diaphragm relaxes and moves back up into a dome shape.
2. The intercostal muscles relax, and the rib cage drops down and in.
3. This makes the chest cavity smaller, squeezing the air out.

What changes during exercise?

When you are sprinting or playing football, your body needs air faster. To help the lungs expand even more, we use extra muscles:
• For Inspiration (Inhaling): We use the pectorals and sternocleidomastoid (in the neck) to lift the ribs even higher.
• For Expiration (Exhaling): The abdominal muscles contract to pull the rib cage down faster and force air out quicker.

Common Mistake: Don't forget that at rest, exhaling is mostly "passive" (muscles just relax), but during exercise, it becomes "active" because the abdominals help force the air out.

4. Spirometer Traces: Measuring Your Lungs

A spirometer is a machine that measures how much air you breathe. You need to identify these four volumes on a graph:

1. Tidal Volume: The amount of air in a normal breath (at rest).
2. Inspiratory Reserve Volume (IRV): The extra air you can force in after a normal breath (think of taking a deep breath before jumping into a pool).
3. Expiratory Reserve Volume (ERV): The extra air you can force out after a normal breath.
4. Residual Volume: The air that always stays in your lungs so they don't collapse.

During Exercise: Your Tidal Volume increases (breaths get deeper), which means your Reserve Volumes (IRV and ERV) decrease because you are using that "extra" space for your normal exercise breaths.

5. Blood Vessels: Structure and Function

There are three main types of delivery "roads" in your body:

• Arteries: Carry blood Away from the heart. They have thick, elastic walls because the blood is under high pressure.
• Veins: Carry blood back To the heart. They have thinner walls and valves to stop blood flowing backwards.
• Capillaries: The tiny vessels where gaseous exchange happens. Their walls are only one cell thick.

Redistribution of Blood (The Vascular Shunt)

During exercise, your body sends more blood to your working muscles and less to your stomach. This happens through:
1. Vasoconstriction: The blood vessels narrow (get tighter) to reduce blood flow to places like the gut.
2. Vasodilation: The blood vessels widen (get bigger) to increase blood flow to the working muscles.

6. The Heart and the Cardiac Cycle

The heart is split into four chambers: the Atria (top) and the Ventricles (bottom). Remember: Atria are at the top (A for Above) and Ventricles are at the bottom (V for Valley).

The Cardiac Cycle:

• Diastole: The heart is relaxing and filling with blood.
• Systole: The heart is contracting and ejecting (squeezing) blood out.

Pathway of Blood (The Journey):

1. Deoxygenated blood enters the Right Atrium from the body.
2. It goes into the Right Ventricle.
3. It is pumped through the Pulmonary Artery to the lungs (to get oxygen).
4. Oxygenated blood returns via the Pulmonary Vein to the Left Atrium.
5. It goes into the Left Ventricle.
6. It is ejected out of the Aorta to the rest of the body.

Analogy: The Aorta is like the "Main Highway" leaving the heart city, carrying all the fresh supplies to the body.

7. Cardiac Output, Stroke Volume, and Heart Rate

You need to know how these three terms relate to each other using this formula:

\( Q = SV \times HR \)

• Cardiac Output (Q): The total volume of blood pumped out per minute.
• Stroke Volume (SV): The volume of blood pumped out in one beat.
• Heart Rate (HR): How many times the heart beats per minute.

Heart Rate Graphs:

When you look at a heart rate graph, you might see the heart rate start to rise before the exercise even begins. This is called the anticipatory rise, caused by the hormone adrenaline. As exercise intensity increases, the heart rate increases to get more oxygen to the muscles.

Summary Takeaway: To increase your Cardiac Output (Q) during sport, your heart beats faster (HR) and squeezes out more blood per beat (SV).