Chapter 19: The Circulatory System I—Blood
Blood has always fascinated people, at least in part because if significant
quantities are lost a person quickly dies. Even though it is one of the most accessible
tissues, its components remained a mystery until microscopes were invented. Indeed,
many secrets of the function of the blood have been revealed only in recent decades. The
study of blood is hematology.
Functions of the Circulatory System
The circulatory system consists of the heart, blood vessels, and the blood.
Functions of the circulatory system include the following.
Blood carries oxygen to the tissues and carries carbon dioxide wastes back to the
lungs. It also transports nutrients, metabolic wastes, and hormones.
The blood contains white blood cells that destroy microorganisms and cancer cells,
and produce antibodies that help fight infections. Platelets clot the blood if vessels have
been injured to minimize blood loss.
Capillaries help to stabilize fluid distribution in the body. The blood contributes to
optimal pH by buffering acids and bases. Shifts in blood flow regulate body temperature.
Components and General Properties of Blood
Blood is a liquid connective tissue with two components—plasma and formed
elements. The plasma is the clear extracellular matrix. Formed elements comprise cells
and cell fragments: the red blood cells, white blood cells, and platelets.
Plasma is a complex soup of water, proteins, nutrients, electrolytes, hormones,
and gases. Serum is what remains when solids are removed. The most abundant plasma
solute (by weight) is proteins, of which there are three main categories: albumin,
globulins, and fibrinogen. Albumin is the most abundant and acts to transport solutes,
buffer pH, and contributes to viscosity and osmolarity. Globulins play roles in transport,
clotting, and immunity. Fibrinogen is the precursor of fibrin, a clotting protein. The liver
produces all of the major blood proteins except gamma globulins, antibodies which come
from plasma cells (descendants of B lymphocytes).