Blood Flow and Circulation

Blood Flow and Pressure

• The blood flow system starts with the heart and the largest artery, the aorta, which branches into smaller arteries, such as the brachial artery, and eventually reaches the capillary bed.
• The blood then flows from the capillary bed into venules and veins, collecting into a large vein called the vena cava.

Blood Pressure

• Blood pressure can be represented as a number, measured in millimeters of mercury (mmHg).
• The average pressure in the aorta is around 95 mmHg.
• The pressure falls dramatically as it flows through the system, reaching around 5 mmHg at the end.

Pressure at Different Points

• The pressure at the beginning of the system (aorta) is around 95 mmHg.
• The pressure at the brachial artery is around 90 mmHg.
• The pressure falls to around 80 mmHg as it enters the arteriole.
• The pressure drops dramatically to around 30 mmHg as it enters the capillary bed.
• The pressure is around 20 mmHg in the venule and 10 mmHg in the vein.

Resistance

• The total body resistance is the key question to be answered.
• The equation to solve for total body resistance is: ΔP = Q × R, where ΔP is the change in pressure, Q is the flow (blood flow), and R is the resistance.
• The flow (Q) can be calculated by knowing the stroke volume and heart rate.
• For a 70 kg person, the stroke volume is around 70 mL, and with a heart rate of 70 beats per minute, the flow is around 5 liters per minute.
• The resistance (R) is proportional to 1/R⁴, where R is the radius of the vessel.

Solving for Total Body Resistance

• Given the system, with the heart pumping blood into the aorta, the pressure at the beginning (95 mmHg) and end (5 mmHg), and the flow (5 liters per minute), the total body resistance can be calculated.
• The calculation yields a value of 18 mmHg·min/L for the total body resistance.

Learn about blood pressure, resistance, and flow as we explore the circulatory system, starting with the heart, aorta, and brachial artery.