The Heart, Double Circulation and the Pacemaker

Ventricle Walls and Pressure

You can easily burst a thin water balloon, but a thick rubber tire can withstand immense force.

• The atria have thin walls because they only need to pump blood a very short distance into the ventricles below.
• The ventricles have thicker walls to pump blood out of the heart, but the left ventricle has a significantly thicker muscular wall than the right.
• This is a vital causal mechanism: a thicker muscular wall produces a more powerful contraction, which generates a higher pressure.
• This high pressure is required to pump blood across a greater distance (the entire body) and overcome higher resistance.
• In contrast, the right ventricle pumps at a lower pressure to prevent damage to the delicate, thin-walled capillaries in the lungs.
• Note that both ventricles hold the exact same volume of blood; they do NOT hold different amounts despite the difference in wall thickness.

Major Blood Vessels and the Pathway of Blood

Is it true that all arteries carry oxygen-rich blood?

• Following a drop of blood through the heart reveals a strict step-by-step pathway.
• Deoxygenated blood from the body enters the right atrium via the vena cava.
• It flows into the right ventricle, which pumps it to the lungs via the pulmonary artery (this is a vital exception; it does NOT carry oxygenated blood like other arteries).
• After gas exchange, oxygenated blood returns to the left atrium via the pulmonary vein (it does NOT carry deoxygenated blood like other veins).
• It flows into the left ventricle, which pumps it out to the whole body via the aorta.
• Valves are located between the atria and ventricles, and at the exits of the vessels. Their sole function is to prevent backflow, ensuring blood does NOT flow backwards.
• The coronary arteries branch off the aorta and spread over the outside of the heart, supplying the cardiac muscle with oxygenated blood for aerobic respiration.
• Narrowing of the lumen (central channel) in these arteries restricts blood flow, which can cause a heart attack.

Controlling the Heart Rate

Understanding how your heart controls its own rhythm explains why some people need electronic devices implanted in their chest.

• A healthy resting heart rate is controlled by a natural pacemaker.
• This is a specialized group of cells located entirely in the wall of the right atrium.
• These cells produce small electrical impulses (they do NOT use nerve impulses) that spread through the heart muscle, causing it to contract.
• If the natural pacemaker stops working properly, an arrhythmia (irregular heart rhythm) can develop.
• An artificial pacemaker is a battery-operated electronic device implanted under the skin and connected to the heart by wires.
• It sends out electrical impulses to correct irregularities in the heart rate, such as ensuring the heart beats frequently enough if it drops too low.

The Lungs and Gas Exchange

You can fit a pair of lungs inside your ribcage, but their internal surface area is approximately $70\text{--}100\text{ m}^2$.

• The lungs are located in the thorax (upper chest) and are responsible for gas exchange.
• Ventilation is the mechanical process of breathing air in and out; it is NOT the same thing as respiration (a cellular chemical reaction).
• Air travels down the trachea (windpipe), which is held open by C-shaped rings of cartilage. This structure ensures the airway does NOT collapse during pressure changes.
• The air splits into two bronchi, divides into smaller bronchioles, and finally ends in millions of tiny air sacs called alveoli.
• The alveoli are perfectly adapted for rapid gas exchange:
  • Massive surface area: Millions of spherical alveoli provide an enormous area for diffusion.
  • Thin walls: The walls of the alveoli and capillaries are only one cell thick, providing a short diffusion path.
  • Rich blood supply: A dense network of capillaries constantly removes oxygenated blood, maintaining a steep concentration gradient.
  • Moist lining: Allows oxygen and carbon dioxide to dissolve so they can diffuse easily.

Calculating Blood Flow

Doctors use specific formulas to monitor how well your cardiovascular system is functioning.

• The rate of blood flow measures how much blood passes a point in a given time.

$$\text{Rate of blood flow} = \frac{\text{Volume of blood}}{\text{Time taken}}$$

Worked Example:

If $2400\text{ ml}$ of blood flows through a blood vessel in $4\text{ minutes}$, what is the rate of blood flow?

Step 1: Identify the known values.

• Volume = $2400\text{ ml}$
• Time = $4\text{ minutes}$

Step 2: Substitute into the equation.

• $\text{Rate of blood flow} = \frac{2400}{4}$

Step 3: Calculate the final answer with units.

• $\text{Rate of blood flow} = 600\text{ ml/min}$