Blood Vessel Structure and Rate of Flow

• Arteries carry blood away from the heart to the organs of the body.
• Because this blood has just been powerfully pumped by the ventricles, it travels at very high pressure.
• To withstand and maintain this high pressure without bursting, arteries have very thick walls relative to their lumen (the central hole through which blood flows).
• The walls contain thick layers of muscle to provide strength, and elastic fibres that stretch as a surge of blood passes through.
• Between heartbeats, these fibres spring back (a process called elastic recoil) to smooth out the pulsating flow and maintain the pressure.
• Arteries do NOT contain valves (except at the exit of the heart) because the high pressure of the blood is enough to keep it moving in one direction.

Veins: Returning Blood to the Heart

Imagine trying to push water up a tube from your toes to your chest against the pull of gravity—it naturally wants to fall backward.

• Veins transport blood back towards the heart.
• By the time blood reaches the veins, it has passed through narrow capillary beds, so it is travelling at much lower pressure.
• To help blood flow easily at this low pressure, veins have a large, wide lumen which reduces resistance.
• Because the pressure is low, vein walls do not need to be strong, so they are much thinner than artery walls, with less muscle and fewer elastic fibres.
• Most veins carry deoxygenated blood (except the pulmonary vein).
• To prevent the backflow of blood due to gravity, veins contain pocket valves that snap shut if blood tries to flow backward.
• Skeletal muscles surrounding the veins contract to squeeze the vessels, pushing the blood through the valves and back to the heart.

Capillaries: Exchange at the Cellular Level

To supply every single cell in your body with oxygen, you need a delivery network so vast and microscopic that cells have to squeeze through one by one.

• Capillaries are the smallest blood vessels, branching out to form extensive capillary beds that reach every tissue.
• Their walls are strictly one cell thick, which creates a very short diffusion pathway to increase the rate at which oxygen and glucose diffuse into cells, and carbon dioxide diffuses out.
• The lumen is incredibly narrow (about 5–10 $\mu$m), which forces red blood cells to travel in single file.
• This slows down the blood flow, providing more time for diffusion, and pushes the cells close to the wall, further reducing the distance oxygen must travel.
• The walls are also permeable, allowing blood plasma to leak out through tiny gaps to form tissue fluid which bathes the surrounding cells.

Calculating the Rate of Blood Flow

Doctors monitor how quickly blood moves through your vessels to check the health of your cardiovascular system and ensure your organs receive enough oxygen.

• The rate of blood flow measures the volume of blood passing through a vessel per unit of time.
• A related measure is cardiac output, which is the total volume of blood the heart pumps per minute.

The Equation:

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

Worked Example 1: Rate of Blood Flow

Question: 2400 ml of blood passes through an artery in 4 minutes. Calculate the rate of flow.

• Step 1: Write the formula and identify your values.

  • Volume = 2400 ml
  • Time = 4 min

• Step 2: Substitute the values into the equation.

$$\text{Rate} = \frac{2400}{4}$$

• Step 3: Calculate the answer and add the correct units.
  • Answer: 600 ml/min

Worked Example 2: Cardiac Output

Question: A person has a heart rate of 75 bpm and a stroke volume of 70 ml. Calculate their cardiac output.

• Step 1: Write the specific cardiac output formula.

$$\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$$

• Step 2: Substitute the values into the equation.

$$\text{Cardiac Output} = 75 \times 70$$

• Step 3: Calculate the answer and add the correct units.
  • Answer: 5250 ml/min