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After studying this chapter, you should be able to:

  • Describe the components of blood and lymph and the role of hemoglobin in transporting oxygen.

  • Understand blood groups and the reasons for transfusion reactions.

  • Delineate the process of hemostasis and the adverse consequences of intravascular thrombosis.

  • Identify the types of blood and lymphatic vessels that make up the circulatory system and the regulation of their constituent cell types.

  • Describe how physical principles dictate the flow of blood and lymph.

  • Understand the basis of methods used to measure blood flow and blood pressure.

  • Understand the basis of disease states where components of the blood and vasculature are abnormal.


The circulatory system supplies inspired O2 as well as substances absorbed from the gastrointestinal tract to the tissues, returns CO2 to the lungs and other products of metabolism to the kidneys, functions in the regulation of body temperature, and distributes hormones and other agents that regulate cell function, all via the blood. Blood is pumped through a closed system by the heart. From the left ventricle, blood is pumped through the arteries and arterioles to the capillaries, where it equilibrates with interstitial fluid. The capillaries drain through venules into the veins and back to the right atrium. Some tissue fluids enter another system of vessels, the lymphatics, which drain lymph via the thoracic duct and the right lymphatic duct into the venous system. The circulation is controlled by multiple regulatory systems to maintain adequate capillary blood flow when possible in all organs, but particularly in the heart and brain.

Blood flows through the circulation primarily because of the forward motion imparted to it by the heart, although diastolic recoil of the walls of the arteries, compression of the veins by skeletal muscles during exercise, and the negative pressure in the thorax during inspiration also move the blood forward. The resistance to flow depends mostly on the diameter of the vessels, and principally that of the arterioles. The blood flow to each tissue is regulated by mechanisms that dilate or constrict its vessels. All of the blood flows through the lungs, but the systemic circulation is made up of numerous different circuits in parallel (Figure 31–1). The arrangement permits wide variations in regional blood flow.


Diagram of the circulation in the adult.


Blood consists of a protein-rich fluid known as plasma, in which are suspended cellular elements: white blood cells, red blood cells, and platelets. The normal total circulating blood volume is about 8% of the body weight (5600 mL in a 70-kg man). About 55% of this volume is plasma.


In the adult, red blood cells, many white blood cells, ...

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