Introduction to Heart Control and Homeostasis
Welcome to one of the most vital chapters in your Biology A Level! We are exploring how your body stays "balanced" even when you are sprinting for a bus or sitting in a freezing classroom. This balance is called homeostasis. We will look at how your heart knows when to beat faster, how your brain acts as a thermostat, and why your body is a master of "negative feedback."
1. Myogenic Control of the Heart
In your earlier studies, you learned about the cardiac cycle. Now, we look at what actually triggers those beats. Unlike other muscles that need a signal from a nerve to move, the heart is myogenic. This means the signal for a heartbeat starts within the heart muscle itself.
The Electrical Pathway
The heart has a built-in "pacemaker" system. Here is the step-by-step sequence you need to know:
1. Sinoatrial Node (SAN): Located in the right atrium, the SAN sends out a wave of electrical excitation. This causes the atria to contract.
2. Atrioventricular Node (AVN): The electrical wave reaches the AVN. There is a short delay here. Why? To allow the atria to finish emptying blood into the ventricles before they contract.
3. Bundle of His: The signal then travels down this group of specialized muscle fibers in the septum (the middle wall of the heart).
4. Purkyne Fibres: The signal spreads through these fibers into the walls of the ventricles, causing them to contract from the apex (the bottom) upwards. This squeezes blood out into the arteries efficiently.
Electrocardiograms (ECGs)
An ECG is a recording of this electrical activity. Doctors look for specific peaks:
• The P wave: Atrial systole (contraction).
• The QRS complex: Ventricular systole (contraction).
• The T wave: Diastole (the heart relaxing and recovering).
Calculating Cardiac Output
You might be asked to calculate how much blood your heart pumps per minute. Use this formula:
\(Cardiac Output = Stroke Volume \times Heart Rate\)
Quick Tip: Stroke volume is the amount of blood pumped in one beat, and Heart Rate is beats per minute (bpm).
2. Modifying the Heart Rate
While the heart is myogenic, it doesn't always beat at the same speed. If you are scared or exercising, your brain takes over the "fine-tuning."
The Medulla Oblongata
The medulla oblongata in your brain is the control center for both heart rate and ventilation (breathing). It sends signals via the autonomic nervous system:
• Sympathetic Nerve: Speeds up the heart rate (think "S" for "Speed").
• Vagus Nerve (Parasympathetic): Slows the heart rate down.
Adrenaline
During a "fight or flight" response, the adrenal glands release adrenaline. This hormone travels in the blood and acts directly on the SAN to increase the heart rate, preparing your body for action.
3. Homeostasis and Negative Feedback
Homeostasis is the maintenance of a stable internal environment (like blood glucose, water levels, and temperature). It relies on negative feedback.
How Negative Feedback Works
Imagine a central heating system. If the room gets too cold, the thermostat turns the heater on. Once the room reaches the right temperature, the thermostat turns the heater off.
In biology:
1. A receptor detects a change (a stimulus).
2. A control center (usually the brain) processes the information.
3. An effector (a muscle or gland) carries out a response to reverse the change and return levels to the set point.
4. Thermoregulation
Maintaining a core body temperature of around \(37^{\circ}C\) is vital because it is the optimum temperature for enzyme activity. If you get too hot, enzymes denature; too cold, and reactions happen too slowly.
The Hypothalamus
The hypothalamus in the brain acts as the body's thermostat. It receives data from:
• Central receptors: Monitoring the temperature of the blood flowing through the brain.
• Peripheral receptors: Monitoring the temperature of the skin.
How the Body Cools Down
• Sweating: Water evaporates from the skin, using heat energy from the body (latent heat of evaporation).
• Vasodilation: Arterioles near the skin surface widen. More blood flows through capillaries near the surface, so more heat is lost by radiation.
How the Body Warms Up
• Vasoconstriction: Arterioles near the skin surface narrow, keeping blood deeper in the body to reduce heat loss.
• Shivering: Muscles contract rapidly, which requires respiration. Respiration is an exothermic process that releases heat energy.
• Erector Pili Muscles: These tiny muscles in the skin contract, making hairs stand up to trap a layer of insulating air (though this is more effective in furry animals than humans!).
Summary: Key Takeaways
• The heart's rhythm is myogenic, starting at the SAN and traveling to the AVN, Bundle of His, and Purkyne fibres.
• The medulla oblongata controls the heart and breathing rates via nerves and adrenaline.
• Homeostasis uses negative feedback to keep the internal environment constant.
• The hypothalamus controls temperature by triggering vasodilation/vasoconstriction, sweating, or shivering.
Note: For more on how hormones work at the cellular level or how the kidney maintains water balance, see the chapters on "The Kidney and Osmoregulation" and "Neurones and Synapses."