Ganong's Review of Medical Physiology, 23rd Edition

Blood as a Circulatory
Fluid & the Dynamics
of Blood & Lymph Flow
OBJECTIVES
After studying this chapter, you should be able to:
■ Describe the components of blood and lymph, their origins, and the role of hemoglobin
in transporting oxygen in red blood cells.
■ Understand the molecular basis of blood groups and the reasons for transfusion
reactions.
■ Delineate the process of hemostasis that restricts blood loss when vessels are
damaged, and the adverse consequences of intravascular thrombosis.
■ Identify the types of blood and lymphatic vessels that make up the circulatory
system and the regulation and function of their primary constituent cell types.
■ Describe how physical principles dictate the flow of blood and lymph around the
body.
■ Understand the basis of methods used to measure blood flow and blood pressure
in various vascular segments.
■ Understand the basis of disease states where components of the blood and vasculature
are abnormal, dysregulated, or both.
INTRODUCTION
The circulatory system supplies O 2 and substances absorbed
from the gastrointestinal tract to the tissues, returns CO 2 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. The
blood, the carrier of these substances, is pumped through a
closed system of blood vessels by the heart. From the left ventricle,
blood is pumped through the arteries and arterioles to the
capillaries, where it equilibrates with the interstitial fluid. The
capillaries drain through venules into the veins and back to the
right atrium. Some tissue fluids enter another system of closed
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 that function
in general to maintain adequate capillary blood flow when possible
in all organs, but particularly in the heart and brain.
C H A P T E R
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Blood flows through the circulation primarily because of
the forward motion imparted to it by the pumping of the
heart, although in the case of the systemic circulation, 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 to a minor
degree on the viscosity of the blood but mostly on the diameter
of the vessels, principally the arterioles. The blood flow
to each tissue is regulated by local chemical and general neural
and humoral mechanisms that dilate or constrict the vessels
of the tissue. All the blood flows through the lungs, but
the systemic circulation is made up of numerous different
circuits in parallel (Figure 32–1). The arrangement permits
wide variations in regional blood flow without changing
total systemic flow.
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