Introduction to the Heart and Circulation

Welcome! In this chapter, we are going to explore the ultimate engine of your body: the heart. Its job is to pump blood around your body to deliver oxygen and nutrients to your cells and carry away waste. It’s part of the circulatory system, which is like a massive motorway network inside you. Don't worry if it seems like a lot of names to remember—we will break it down piece by piece!

Note: This chapter focuses on the heart's structure and how we calculate its performance. For more details on the blood itself or the different types of blood vessels, check out the "Blood and blood vessels" chapter.

The Structure of the Heart

The heart is a muscular pump divided into two sides: the left side and the right side. A helpful way to remember which side does what is the mnemonic LORD: Left Oxygenated, Right Deoxygenated.

1. The Four Chambers

The heart has four rooms, or chambers:

  • Atria (Singular: Atrium): These are the top chambers. They receive blood coming into the heart.
  • Ventricles: These are the bottom chambers. They are much stronger because they have to pump blood out of the heart.

2. Major Blood Vessels

There are four main vessels you need to know that connect directly to the heart:

  1. Vena Cava: Brings deoxygenated blood from the body into the right atrium.
  2. Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.
  3. Pulmonary Vein: Brings oxygenated blood from the lungs into the left atrium.
  4. Aorta: Carries oxygenated blood from the left ventricle to the rest of the body.

Top Tip: Remember that Arteries go Away from the heart, and Veins go In (the "in" is in the word "vein").

3. Valves

The heart has valves between the chambers and the main arteries. Their job is simple but vital: to prevent the backflow of blood. They make sure blood only moves in one direction.

4. Wall Thickness

You might notice on a diagram that the walls of the heart are not the same thickness everywhere:

  • The atria have thin walls because they only need to push blood a short distance down into the ventricles.
  • The left ventricle has a much thicker muscular wall than the right ventricle. Why? Because the right side only pumps blood to the lungs (which are right next door), but the left side must pump blood at high pressure to the entire body.

Quick Review: The heart is a double pump. The right side pumps to the lungs; the left side pumps to the body. The left ventricle is the thickest because it does the most work!

The Circulatory System

Humans have a double circulatory system. This means the blood passes through the heart twice for every one complete circuit of the body.

  1. The First Circuit: The heart pumps deoxygenated blood to the lungs to pick up oxygen. The oxygenated blood then returns to the heart.
  2. The Second Circuit: The heart pumps that oxygenated blood all around the body to deliver the oxygen to cells. The now deoxygenated blood returns to the heart to start again.

This system is very efficient because it allows the blood to be pumped to the body at a much higher pressure than if it went straight from the lungs to the body.

Calculating Cardiac Output

In your exam, you may be asked to calculate how hard the heart is working. We use a formula to find the cardiac output.

Key Terms to Know:

  • Heart Rate: The number of times the heart beats in one minute (measured in beats per minute, or bpm).
  • Stroke Volume: The volume of blood pumped out of one ventricle in one single beat (measured in \( \text{cm}^3 \)).
  • Cardiac Output: The total volume of blood pumped by a ventricle every minute (measured in \( \text{cm}^3 \text{/min} \)).

The Formula:

\( \text{cardiac output} = \text{stroke volume} \times \text{heart rate} \)

Example Calculation:

Question: A student has a heart rate of \( 70\text{ bpm} \) and a stroke volume of \( 60\text{ cm}^3 \). Calculate their cardiac output.

1. Write down the formula: \( \text{cardiac output} = \text{stroke volume} \times \text{heart rate} \)
2. Substitute the numbers: \( \text{cardiac output} = 60 \times 70 \)
3. Final answer: \( \text{cardiac output} = 4200\text{ cm}^3 \text{/min} \)

Did you know? When you exercise, your muscles need more oxygen for respiration. To provide this, both your heart rate and your stroke volume increase, which significantly boosts your total cardiac output!

Common Mistakes to Avoid

  • Mixing up the sides: Remember that diagrams of the heart are usually drawn as if you are looking at someone else. This means the Left side of the heart is on the Right side of the paper!
  • Units: Always check your units for cardiac output. It is usually \( \text{cm}^3 \text{/min} \). If a question gives you the volume in Litres, you may need to convert it (\( 1\text{ Litre} = 1000\text{ cm}^3 \)).
  • Vessel names: Make sure you don't swap the Aorta and the Pulmonary Artery. The Aorta is the big one that goes to the body!

Summary Checklist

- Can you label the four chambers and four main vessels of the heart?
- Can you explain why the left ventricle wall is thicker than the right?
- Do you know the function of the heart valves?
- Can you use the formula to calculate cardiac output, stroke volume, or heart rate?