Serum Cholesterol, Atherosclerosis, and “Heart Attacks” Help

Introduction to Serum Cholesterol, Atherosclerosis, and “Heart Attacks”

Having discussed the heart as a pump and the blood vessels as tubes for carrying flowing blood, it is now time to consider the blood itself. Blood is a red, sticky, connective tissue that occupies about 5–6 liters ( LEE -ters) of volume in an average-sized adult.

Plasma

At first, it may seem odd to classify blood as a connective tissue. After all, it contains no connective tissue fibers (such as collagen or elastic fibers) that directly strap body parts together. Nevertheless, blood is considered a special connective tissue, because it contains plasma ( PLAZ -muh). Plasma is the clear, watery, intercellular substance found “between” (inter-) the “cells” (cellul) of the bloodstream. Because it circulates throughout the body within the bloodstream, plasma acts like a functional connective tissue rather than a structural one. Recall that hormones (Chapter 15), for instance, are First Messengers secreted by the endocrine glands and carried throughout the body within the bloodstream. It is the plasma, not the blood cells per se, that does this functional connecting or carrying of the hormones towards their various target cells.

The plasma is basically a saline ( SAY -leen) or “salt”-containing solution consisting of 0.9% NaCl (sodium chloride) solute dissolved in water solvent. In addition to carrying hormones, nutrients such as glucose and O ₂ , and waste products like CO ₂ , the blood also contains a number of important plasma proteins. Some of these plasma proteins are critical for blood clotting. Still others play a role in such processes as body defense from foreign invaders.

Formed Elements In The Blood

Besides the plasma, there are also many formed elements within the bloodstream. Formed elements are cells and cell fragments within the blood that have a definite shape or form.

When a sample of human blood is stained and viewed under a compound microscope (Figure 16.7), several major types of formed elements can be identified. Most numerous among these are the erythrocytes (air- RITH -roh- sights ) or “red” (erythr) blood “cells” (cytes). Erythrocytes (red blood cells) are anucleate ( ay-NEW -klee-aht); that is, they are “without” (a-) any nucleus. They are also quite special in that they are shaped like biconcave (buy- KAHN -cave) discs, being “caved-in” on “both” (bi-) sides. Viewed from the side, this makes them look like red hourglasses! The red color is mainly due to the presence of hemoglobin ( HEE -moh- glohb -in). Hemoglobin is a reddish-colored, “globe”-shaped (glob), “protein substance” (-in) found within the cytoplasm of the red “blood” (hem) cells. There may be as many as 10,000 hemoglobin molecules present within a single erythrocyte. The main job of these thousands of hemoglobin molecules is carrying oxygen (O ₂ ) molecules through the bloodstream, and to the tissue cells.

Fig. 16.7 The major formed elements of the blood.

Leukocytes ( LEW -koh- sights ) are the “white” (leuk) blood “cells” (cytes). These cells typically have a large, purplish-staining nucleus, but they are named for the clear, whitish appearance of their cytoplasm. (This is in marked contrast, of course, to the red, hemoglobin-rich cytoplasm of the erythrocytes.) The leukocytes have significant roles in protecting the body from various foreign invaders.

The third common formed element are the platelets ( PLAY -teh- lets ) or thrombocytes ( THRAHM -buh- sights ). The platelet name comes from their shape – “little plate”-like fragments of disintegrated bone marrow cells. The thrombocyte name derives from their function – “clotting” (thromb) “cells.” About 1/3 the size of an erythrocyte, the platelets (thrombocytes) are scattered here and there in small groups, throughout the plasma. These purplish-colored, plate-like cell fragments have very sticky surfaces. Thus, whenever a blood vessel ruptures and hemorrhages, the platelets soon collect around the open hole and stick to a network of thin, “fiber”-like strands, composed of the protein, fibrin ( FEYE -brin). As the platelets collect and stick to one another, a “clot” or thrombus ( THRAHM -bus) is soon formed. The hole is blocked, hemorrhaging stops, and wound healing follows.

When a person has a so-called “heart attack,” it usually involves a problem with abnormal clotting or an occlusion (uh- KLEW -zhun) – “shutting up” – of the blood vessels serving the myocardium. The site of the troublesome occlusion (shutting up) is somewhere within the coronary ( KOR -uh- nair -ee) arteries. These vessels are named for their resemblance to a “crown” (coron) encircling the top of the heart, just below the auricles ( OR -ih- kls ) or “little ear”-like flaps. The right and left coronary arteries look like a red-colored crown slipped down over the ears of a real prince (Figure 16.8, A)!

Fig. 16.8 Atherosclerosis and occlusion of the coronary arteries.