521

CHAPTER

326

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 hemo-

globin 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 vascu-

lature 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, func-

tions 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 ventri-

cle, 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 circu-

lation is controlled by multiple regulatory systems that function

in general to maintain adequate capillary blood flow when pos-

sible 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 pumping of the

heart, although in the case of the systemic circulation, dia-

stolic 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 diam-

eter of the vessels, principally the arterioles. The blood flow

to each tissue is regulated by local chemical and general neu-

ral and humoral mechanisms that dilate or constrict the ves-

sels 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.