Welcome to the Cardiovascular System at Rest!
In this chapter, we are going to explore how your heart and blood vessels work when you are just chilling out—sitting at your desk, lying on the sofa, or reading these notes. Even when you aren't moving, your body is incredibly busy! We will look at how the heart pumps, how we measure its performance, and the "electrical wiring" that keeps it beating perfectly. Understanding this at rest provides the foundation for knowing how the body adapts when we start to exercise later in the course.
1. The Big Three: HR, SV, and Cardiac Output
To understand the heart, we need to know three key measurements. Think of these as the "performance stats" for your heart.
Heart Rate (HR)
Heart Rate is the number of times your heart beats in one minute. For a typical healthy adult at rest, this is usually around 70 to 72 beats per minute (bpm). If you are very fit (like an endurance athlete), your resting heart rate might be much lower, sometimes even below 50 bpm!
Stroke Volume (SV)
Stroke Volume is the amount of blood pumped out of the left ventricle of the heart in just one beat. At rest, the average stroke volume is approximately 70ml.
Think of this like a single squirt from a water pistol—it’s the volume of one "shot."
Cardiac Output (Q)
Cardiac Output (represented by the letter Q) is the total volume of blood pumped out of the left ventricle in one minute. At rest, for an average adult, this is about 5 litres per minute (5 L/min).
The Relationship and Calculation
There is a simple mathematical relationship between these three. If you know how many times the heart beats (HR) and how much blood it pushes out each time (SV), you can find the total for the minute (Q).
The Formula:
\( Q = HR \times SV \)
Example Calculation at Rest:
If a student has a resting HR of 70 bpm and a resting SV of 70ml:
\( 70 \text{ beats/min} \times 70 \text{ ml/beat} = 4,900 \text{ ml/min} \) (which is 4.9 Litres per minute).
Quick Review Box:
HR: Beats per minute (Avg ~72)
SV: Volume per beat (Avg ~70ml)
Q: Volume per minute (Avg ~5L)
Summary Takeaway: Cardiac Output is simply Heart Rate multiplied by Stroke Volume. At rest, your body aims to keep this at about 5 Litres per minute to keep your organs happy!
2. The Cardiac Cycle: Diastole and Systole
The Cardiac Cycle is the sequence of events that happens during one single heartbeat. It is split into two main phases: filling and emptying.
Diastole (The "Filling" Phase)
During Diastole, the heart muscle relaxes. This allows the chambers of the heart to fill up with blood. Imagine it like letting go of a squeezed sponge so it can soak up water.
Systole (The "Contraction" Phase)
During Systole, the heart muscle contracts. This squeezes the blood out of the heart and into the arteries to be sent around the body. Imagine this as squeezing that soaked sponge to push the water out.
Common Mistake to Avoid: Don't mix these up! Remember that Diastole is for Dumping blood into the heart (filling), and Systole is for Squeezing it out.
Summary Takeaway: The heart works in a "relax and fill" (Diastole) then "contract and pump" (Systole) rhythm. One cycle of Diastole and Systole equals one heartbeat.
3. The Conduction System: The Heart’s Electrical Wiring
How does the heart know when to beat? It has its own built-in electrical system! This ensures the top of the heart contracts first, followed by the bottom, so blood flows in the right direction.
Step-by-Step Electrical Path
1. SA Node (Sino-atrial Node): Located in the right atrium, this is known as the pacemaker. It sends out an electrical impulse that starts the heartbeat. It makes the atria (the top chambers) contract.
2. AV Node (Atrio-ventricular Node): This node receives the signal from the SA node. It acts like a "gatekeeper" and delays the signal for about 0.1 seconds.
Why? To allow the atria to finish emptying their blood into the ventricles before the ventricles contract!
3. Bundle of His: The signal travels down this bundle of nerve fibers located in the center wall (septum) of the heart.
4. Purkyne Fibers: These fibers wrap around the bottom of the ventricles (the bottom chambers). When the signal reaches them, they cause the ventricles to contract from the bottom up, pushing blood out to the lungs and body.
Memory Aid (Mnemonic):
Some Athletes Always Beat Precords
(SA Node -> AV Node -> Bundle of His -> Purkyne Fibers)
Did you know?
The SA node is so powerful that even if the heart is removed from the body (like in surgery), it can continue to beat on its own for a short time because it generates its own electricity!
Summary Takeaway: The electrical impulse starts at the SA node (pacemaker), pauses at the AV node to let blood through, then travels down the Bundle of His to the Purkyne fibers to trigger the main pump (systole).
Final Quick Review for the Chapter
Don't worry if this seems tricky at first! Just remember the main "story":
1. The heart has a rhythm of filling (Diastole) and squeezing (Systole).
2. This rhythm is controlled by an electrical path: SA Node -> AV Node -> Bundle of His -> Purkyne Fibers.
3. We measure how well this is working by looking at Heart Rate, Stroke Volume, and the total Cardiac Output (\( Q = HR \times SV \)).
4. At rest, your heart is efficient and steady, keeping you alive and well while using very little energy.