Dark Banquet - Bill Schutt [39]
Okay, back to tissues.
The next requirement for a tissue is that its cells work together in some particular function. For example, nervous tissue is composed of neurons and a support team of glial cells—each contributing in specific ways to the functioning of the nervous system.*54
Connective tissue is characterized by being composed of relatively few cells surrounded by a significant amount of noncellular matrix. As a result, connective tissue cells are generally not in contact with each other. Think of them as bricks in a wall, with the matrix being the mortar that surrounds them and binds them to each other. The matrix is also what gives connective tissues their physical properties. For example, the hardness of bone comes from calcified bone matrix, not from the bone cells (osteocytes) themselves.†55
In blood, the matrix is called blood plasma and it’s neither solid nor gel—it’s a liquid (composed primarily of water)—and this property is just as important functionally as hardness, flexibility, and strength are for the other types of connective tissue. The reason is that plasma acts as the transport medium for blood cells, as well as tiny cell fragments called platelets that are involved in blood clotting. Additionally, many other important substances are carried within the plasma in a dissolved state, including nutrients, vitamins, hormones, waste products, gases, and ions.
Pumped out of the heart by the muscle-bound left ventricle, the force exerted by the blood on the inside of the vessels it passes through is known as blood pressure. When the left ventricle contracts, expelling its arterial blood, the blood pressure increases. This produces the higher number (known as the systolic pressure) in a typical blood pressure measurement. As the left ventricle empties, relaxes, and begins to fill again, the blood pressure drops, producing the lower diastolic pressure.*56
As the arterial blood nears its destination, it moves from arteries to smaller arterioles and finally to miles and miles of microscopically tiny (and thin) capillaries. These mini-vessels form dense, netlike beds around organs and other structures. During this passage to smaller and smaller blood vessels, the blood pressure drops significantly. To understand how this pressure drop takes place, visualize the water traveling through a garden hose. Now imagine that the far end of that hose begins to split into smaller and smaller tubes, each of those tubes splitting again and again—until the end of the hose has been divided into a million tiny branches. The pressure of the water in any one of these branches would be far less than the original pressure. Basically, this is because of the tremendous increase in the total combined area inside the millions of branching tubes (as compared to the area inside the original hose). This is also the reason why the water pressure drops when everybody decides to take a shower at the same time.
But not only are these low-pressure capillaries incredibly small, their walls are so thin that once the blood gets to its destination, nutrients and oxygen contained within the blood plasma simply diffuse through the capillary walls to supply the surrounding tissues and their cells.*57
Metabolic waste products and carbon dioxide move in the exact opposite direction (from the tissues into the blood) via the same process. Once within the capillaries, they start their journey back to the heart, passing through increasingly larger vessels (venules leading into veins) before entering the heart’s right atrium. Unfortunately, this low-pressure blood sometimes has a hard time returning from places like the legs and feet—since it must overcome the considerable force of gravity. Under normal conditions, venous return is aided by a series of one-way