Welcome to the Heart of the Matter!
In this chapter, we are going to explore how your body moves essential "cargo" like oxygen and nutrients to your cells and how it carries away "trash" like carbon dioxide. Imagine your body is a busy city; the blood vessels are the roads, and the heart is the powerful engine keeping everything moving. This system is officially known as the circulatory system.
Don't worry if the diagrams of the heart look confusing at first—we will break them down step-by-step so you can master them for your exams!
1. The General Plan: A Double Circulation
Humans have what is called a double circulatory system. This means that for every one complete journey around the body, the blood passes through the heart twice.
The Pulmonary Circuit: The heart pumps deoxygenated blood to the lungs to pick up oxygen and drop off \( CO_{2} \). The oxygenated blood then returns to the heart.
The Systemic Circuit: The heart pumps this fresh, oxygenated blood to the rest of the body (muscles, brain, liver, etc.) and then returns to the heart once the oxygen has been used up.
Key Takeaway: The double system is efficient because it allows blood to be pumped to the body at a much higher pressure than if it had gone straight from the lungs to the body.
2. The Heart: Your Body's Pump
The heart is a muscular organ made of cardiac muscle. It is divided into four chambers.
Heart Structure
The Upper Chambers (Atria): These are the "loading bays." The right atrium receives deoxygenated blood from the body, and the left atrium receives oxygenated blood from the lungs.
The Lower Chambers (Ventricles): These are the "pumping stations." The right ventricle pumps blood to the lungs, and the left ventricle pumps blood to the entire body.
The Septum: This is the thick muscular wall that separates the left and right sides of the heart. It prevents oxygenated and deoxygenated blood from mixing.
Quick Review Box: Which side is thicker?
The left ventricle has much thicker muscular walls than the right ventricle. Why? Because it has to pump blood all the way to your toes and head against high pressure, while the right ventricle only has to pump blood a short distance to the lungs.
The Valves
Valves are like one-way doors. They ensure that blood flows in the correct direction and prevent it from flowing backwards. There are valves between the atria and ventricles, and also at the exits of the heart leading into the main arteries.
Main Blood Vessels of the Heart
1. Vena Cava: The main vein bringing deoxygenated blood back to the right atrium.
2. Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.
3. Pulmonary Vein: Carries oxygenated blood from the lungs to the left atrium.
4. Aorta: The largest artery in the body; it carries oxygenated blood from the left ventricle to the rest of the body.
Note: For more information on what is inside the blood (like red blood cells and plasma), see the chapter "Blood, tissue fluid and blood groups".
3. Blood Vessels: The Body’s Highways
There are three main types of blood vessels. Each is "built for its job" (structure related to function).
Arteries
Role: Carry blood Away from the heart (usually oxygenated).
Structure: They have thick, muscular, and elastic walls. This is because the blood is under high pressure as it has just been pumped by the heart.
The Pulse: You can feel a pulse in your arteries as the elastic walls stretch and recoil with each heartbeat.
Veins
Role: Carry blood back to the heart (usually deoxygenated).
Structure: They have thinner walls and a larger lumen (the space in the middle). The blood is under low pressure.
Valves: Veins contain valves to keep blood moving toward the heart and prevent it from "pooling" in your legs due to gravity.
Capillaries
Role: These are the tiny vessels where the actual exchange of materials (like oxygen and glucose) happens.
Structure: Their walls are only one cell thick. This provides a very short diffusion distance, making it easy for substances to move in and out of the blood quickly.
Memory Trick:
Artery = Away.
Vein = Visit (the heart).
Capillary = Connects (arteries to veins).
4. Mapping the System: Major Vessels to Organs
In your exam, you may be asked to name the vessels going to specific organs. Most follow a simple naming pattern:
Lungs: Use the word Pulmonary (e.g., Pulmonary Artery).
Liver: Use the word Hepatic (e.g., Hepatic Artery brings blood in; Hepatic Vein takes blood out).
Kidneys: Use the word Renal (e.g., Renal Artery brings blood in; Renal Vein takes blood out).
Digestive System: The Hepatic Portal Vein is a special vessel that carries digested food from the gut directly to the liver.
5. Pulse Rate and Exercise
Your pulse rate is a measure of your heart rate (how many times the heart beats per minute). When you exercise, your pulse rate increases.
Why does heart rate change during exercise?
1. Muscles contract more and require more energy from aerobic respiration.
2. To do this, they need more oxygen and glucose.
3. The heart beats faster and more strongly to deliver these substances and to remove the extra \( CO_{2} \) being produced.
4. The hormone adrenaline can also cause the heart rate to increase (the "fight or flight" response).
Required Practical: Investigate the effect of exercise on the pulse rate
1. Measure the resting pulse rate (usually at the wrist or neck) for one minute.
2. Perform a specific exercise (like star jumps) for a set amount of time (e.g., 2 minutes).
3. Measure the pulse rate immediately after finishing.
4. Continue measuring every minute until the pulse returns to the resting rate (this is called the recovery time).
Key Variables: The independent variable is the intensity or duration of exercise. The dependent variable is the pulse rate.
6. Summary and Key Terms
Double Circulation: Blood goes Heart -> Lungs -> Heart -> Body -> Heart.
Left Ventricle: Thicker muscle because it pumps to the whole body.
Valves: Prevent backflow of blood.
Arteries: Thick walls, high pressure, carry blood away from the heart.
Veins: Thinner walls, valves, carry blood to the heart.
Capillaries: One cell thick for easy diffusion.
Renal/Hepatic: Terms for Kidney/Liver vessels.
Note: To learn about what happens when these vessels get blocked or the heart fails, please refer to the chapter "Heart disease, hypertension and treatments".