Mean Arterial Pressure

Regardless of whether one is observing the pulmonary circuit or the systemic circuit, the types of vessels and the sequence of blood flow remain consistent across the human body.
Sequence of Vessels from the Heart:
- Elastic Arteries: Upon exiting the heart via the ascending aorta (systemic circuit) or the pulmonary trunk (pulmonary circuit), blood first enters the elastic arteries. These vessels have specific characteristic properties in their walls.
- Muscular Arteries: Elastic arteries diverge into muscular arteries.
- Arterioles: Muscular arteries further branch into arterioles.
- Terminal Arterioles: Arterioles end at terminal arterioles, which directly feed into capillary beds.
- Capillaries: These are the primary sites of gaseous and nutrient exchange. A pericyte is often found associated with these vessels.
- Venules: Capillary beds converge to form venules, such as the postcapillary venule.
- Veins: Venules converge into larger and larger veins, eventually forming the major veins that return blood to the heart.
Specific Return Vessels for the Heart:
- Pulmonary veins return blood from the lungs to the heart (pulmonary circuit).
- The inferior vena cava, superior vena cava, and coronary sinus return blood from systemic tissues to the heart.

Driving Force: The movement of blood in a characteristic direction (from arteries to capillaries to veins and back to the heart) is driven by differences in pressure, specifically pressure gradients.
Systemic Pressures (Aorta/Great Vessels):
- Systolic Pressure: Measured after the left ventricle contracts, approximately $120\,mmHg$ .
- Diastolic Pressure: Measured when the left ventricle relaxes (diastole), approximately $80\,mmHg$ .
Pulmonary Pressures (Pulmonary Trunk):
- Systolic Pressure: Approximately $24\,mmHg$ (referenced via the mnemonic of former Raiders player Charles Woodson's jersey number).
- Diastolic Pressure: Approximately $10\,mmHg$ (referenced via the mnemonic of Seth Roberts, though he no longer plays for the Raiders).

Fundamental Definition of Pressure: Pressure is defined by the collisions of particles (such as blood cells) with the container wall (the blood vessel wall).
Kinetic Energy: Particles in a container have kinetic energy and move in random directions, colliding with each other and the sides of the container.
Factors Increasing Pressure:
1. Increased Heat/Energy: Speeding up molecules gives them more kinetic energy, leading to faster movement and more frequent collisions.
  - Metaphor: Students walking in a classroom until they bump a wall create little pressure. If they are given Red Bull and told to run, they whack the wall with more energy, increasing pressure.
2. Decreasing Volume: Shrinking the container while keeping the number of particles the same increases the collision frequency with the walls.
  - Metaphor: A scene from Star Wars where characters are trapped in a garbage chute; as the walls close in, they get uncomfortable and bang against the walls (higher pressure).
3. Increasing Particle Count: Adding more particles to a fixed volume increases the number of collisions.
  - Metaphor: A classroom filled with too many students leads to people banging against the walls to get out.
Heat and Phase Change: Heating a container (e.g., adding fire under a water container) increases collisions to the point where particles become steam, characterized by very fast-moving particles.

Definition: Mean Arterial Pressure (MAP) is the average arterial pressure throughout the cardiac cycle.
Calculation Logic: MAP is not a simple average because the heart spends more time in diastole than in systole.
- The heart spends approximately $\frac{1}{3}$ of its time in systole.
- The heart spends approximately $\frac{2}{3}$ of its time in diastole.
Calculating MAP for a pressure of $120/80\,mmHg$ :
- Total logic: $120\,mmHg$ (systole) + $80\,mmHg$ (early diastole) + $80\,mmHg$ (late diastole) divided by 3.
- Formula: $\frac{120 + 80 + 80}{3} = 93.33\,mmHg$ .
Alternative Textbook Formula (Pulse Pressure):
- Pulse Pressure: The difference between systolic and diastolic pressure ( $Pulse\,Pressure = Systolic - Diastolic$ ).
- For $120/80\,mmHg$ , the pulse pressure is $40\,mmHg$ .
- Formula: $MAP = Diastolic + \frac{Pulse\,Pressure}{3}$ .
- Calculation: $80 + \frac{40}{3} = 80 + 13.33 = 93.33\,mmHg$ .

Blood only flows from one vessel to the next if the downstream vessel has a lower pressure. Pressure progressively drops as blood moves further from the heart.
Pressure Values by Vessel Type:
- Aorta/Elastic Arteries: $93.33\,mmHg$ (average).
- Muscular Arteries: Approximately $80\,mmHg$ to $60\,mmHg$ as branching goes deeper (e.g., branching from the brachial artery to the bicipital artery).
- Arterioles: Approximately $50\,mmHg$ .
- Capillary Arterial End (Hydrostatic Pressure): Approximately $37\,mmHg$ .
- Capillary Venule End (Hydrostatic Pressure): Approximately $24\,mmHg$ .
- Venules and Veins: Pressures drop further to $20\,mmHg$ , then $10\,mmHg$ .
- Vena Cava: The lowest pressure vessels in the systemic body (IVC, SVC, and coronary sinus).
The Diffusion Analogy: Pressure is similar to diffusion, where particles move from areas of high concentration to low concentration. This movement is dependent on random motion and kinetic energy.

The Lake Metaphor: Flowing from a river (arteriole) into a lake (capillary bed) dramatically increases surface area and resistance. This slows the movement of blood cells significantly, which is essential for exchange (diffusion).
Venous Return Assistance: Because pressures are so low in the veins ( $20-10\,mmHg$ or lower), the body requires assistance to move blood back to the heart:
- Skeletal muscle contraction.
- Valves (to prevent backflow).
- Autonomic tone.
- The Respiratory Pump.
Cardiac Pressure Fluctuations:
- Atrial Diastole: While the atria relax, the pressure is lower than the vena cavae, drawing blood in.
- Ventricular Relaxation: When the ventricles relax, they create a larger volume with the same amount of fluid, causing pressure to drop and creating a vacuum-like effect that draws blood in from the atria.
- Note on Physics: Technically, there is no "sucking force" in physics; rather, it is the creation of a lower-pressure environment that allows higher-pressure blood to flow in.

Graph Axes:
- Y-axis: Blood Pressure (BP) in $mmHg$ (ranging from $0$ up to $120$ , typically marked in increments of $20$ ).
- X-axis: Vessel types (Aorta, Muscular Arteries, Arterioles, Capillaries, Venules, Veins, Vena Cava).
Key Observations from the Graph:
- Aorta: Shows significant oscillation between systolic ( $120$ ) and diastolic ( $80$ ), with the MAP at $93.33$ .
- Pulsation Loss: The difference between systolic and diastolic pressure (the pulsatile nature of flow) gradually disappears as blood moves into the arterioles and capillaries.
- Continuous Flow: By the time blood reaches the capillaries, the flow is steady and continuous, represented by the convergence of the systolic and diastolic pressure lines.
- Steady Decline: The pressure line shows a continuous downward slope from the aorta to the vena cava, ensuring unidirectional flow.