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Learning Objectives

Learning Objectives

By the end of the chapter, the student will be able to:

  • Explain the baroreceptor reflex: anatomical features and mechanisms.

  • Distinguish between short- and long-term mechanisms in blood pressure and blood volume regulation.

  • Describe the renin-angiotensin II-aldosterone system in regulating arterial pressure.

  • Detail the mechanisms of the Bainbridge reflex, respiratory sinus arrhythmia, and the peripheral chemoreceptor reflex.

  • Provide Fick’s equation for diffusion.

  • Explicate Starling forces and how they influence fluid movement across the capillary walls.

  • Identify the conditions that affect the filtration and reabsorption forces.

  • List the causes of edema.

  • Describe the lymphatic system.

  • Identify the mechanisms that control blood flow: local and general.

  • Differentiate between active hyperemia, reactive hyperemia, and autoregulation.

  • Describe the regulation of blood flow in various organs: heart, kidney, muscle, skin, and so on.

  • Describe the cardiovascular consequences of hemorrhage on cardiac output, central venous pressure, and arterial pressure and explain the compensatory mechanisms activated by these changes.

  • Describe the cardiovascular responses to hemorrhage and change in posture.

  • Define circulatory shock, briefly explain its stages and various types and the basis of their management.

Introduction

Maintaining a constant blood flow to all of the body tissues is an essential function of the cardiovascular system to sustain tissue homeostasis. Blood flow delivers oxygen and nutrients to the tissues and removes CO2 and waste products. The blood pressure gradient between the arterial and venous sides is essential for blood flow. This gradient is represented in the mean arterial pressure (MAP), which is considered to be the driving force for blood flow. In this chapter, the mechanisms that maintain MAP at a constant level (at approximately 100 mmHg) will be explained. The exchange of fluid, O2, CO2, and other substances occurs in the microcirculation. The gas and substance exchange transpires mainly by simple diffusion, while the fluid exchange occurs across the capillary walls by osmosis. The regulation of osmosis and fluid exchange is driven by hydrostatic and osmotic pressures that are called Starling pressures or forces. These forces and how they regulate fluid transport across the capillary beds will also be explained.

Finally, the regulation of various circulations in different organs will be discussed. The integration of these mechanisms during exercise, hemorrhage, and change of posture will be briefly illustrated.

Regulation of Mean Arterial Blood Pressure

In general, MAP is regulated by cardiac output (CO) and total peripheral resistance (TPR) as depicted in the following equation:

MAP = CO × TPR

 

where,

  • MAP = mean arterial pressure (mmHg)

  • CO = cardiac output (mL/min)

  • TPR = total peripheral resistance (mmHg/mL/min)

This is the same relationship of Ohm’s law that regulates resistance and blood flow. CO and TPR are not independent from each other in this equation. To illustrate, if the CO is ...

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