Medical brain notes 27

  VEINS   Veins  carry blood from capillaries back to the heart;   the smaller veins are called  venules . The same three tissue layers are present in veins as in the walls of arteries, but there are some differences when com-pared to the arterial layers. The inner layer of veins is smooth endothelium, but at intervals this lining is folded to form  valves  (see Fig. 13–1). Valves prevent backflow of blood and are most numerous in veins of the legs, where blood must often return to the heart against the force of gravity.   The middle layer of veins is a thin layer of smooth muscle. It is thin because veins do not regulate blood pressure and blood flow into capillaries as arteries do. Veins can constrict extensively, however, and this function becomes very important in certain situations such as severe hemorrhage. The outer layer of veins is also thin; not as much fibrous connective tissue is nec-essary because blood pressure in veins is very low. ...

  CAPILLARIES   Capillaries  carry blood from arterioles to venules.   Their walls are only one cell in thickness; capillaries are actually the extension of the endothelium, the simple squamous lining, of arteries and veins (see Fig. 13–1). Some tissues do not have capillaries; these are the epidermis, cartilage, and the lens and cornea of the eye.   Most tissues, however, have extensive capillary networks. The quantity or volume of capillary networks in an organ reflects the metabolic activity of the organ. The functioning of the kidneys, for example, depends upon a good blood supply. Fig. 18–2 shows a vascular cast of a kidney; you can see how dense the vessels are, most of which are capillaries. In contrast, a tendon such as the Achilles tendon at the heel or the patellar tendon at the knee would have far fewer vessels, because fibrous connective tissue is far less metabolically active. Blood flow into capillary networks is regulated by smooth muscle cells ca...

  PATHWAYS OF CIRCULATION   The two major pathways of circulation are pulmonary and systemic. Pulmonary circulation begins at the right ventricle, and systemic circulation begins at the left ventricle. Hepatic portal circulation is a special segment of systemic circulation that will be covered separately. Fetal circulation involves pathways that are present only before birth and will also be discussed separately.     PULMONARY CIRCULATION   The right ventricle pumps blood into the pulmonary artery (or trunk), which divides into the right and left pulmonary arteries, one going to each lung. Within the lungs each artery branches extensively into smaller arteries and arterioles, then to capillaries. The pul-monary capillaries surround the alveoli of the lungs; it is here that exchanges of oxygen and carbon dioxide take place. The capillaries unite to form venules, which merge into veins, and finally into the two pul-monary veins from each lung that return blood to ...

  VELOCITY OF BLOOD FLOW   The velocity, or speed, with which blood flows differs in the various parts of the vascular system. Velocity is inversely related (meaning as one value goes up, the other goes down) to the cross-sectional area of the particular segment of the vascular system. Refer to Fig. 13–9 as you read the following. The aorta receives all the blood from the left ventricle, its cross-sectional area is small, about 3 cm 2  (1 sq. inch), and the blood moves very rapidly, at least 30 cm per second (about 12 inches). Each time the aorta or any artery branches, the total cross-sectional area becomes larger, and the speed of blood flow decreases. Think of a river that begins in a narrow bed and is flowing rapidly. If the river bed widens, the water spreads out to fill it and flows more slowly. If the river were to narrow again, the water would flow faster. This is just what happens in the vascular system.   The capillaries in total have the greatest cross-sec...

  BLOOD PRESSURE   Blood pressure  is the force the blood exerts against   the walls of the blood vessels. Filtration in capillaries depends upon blood pressure; filtration brings nutri-ents to tissues, and as you will see, is the first step in the formation of urine. Blood pressure is one of the “vital signs” often measured, and indeed a normal blood pressure is essential to life.   The pumping of the ventricles creates blood pres-sure, which is measured in mmHg (millimeters of mercury). When a systemic blood pressure reading is taken, two numbers are obtained: systolic and dias-tolic, as in 110/70 mmHg.  Systolic  pressure is always the higher of the two and represents the blood pres-sure when the left ventricle is contracting. The lower number is the  diastolic  pressure, when the left ventri-cle is relaxed and does not exert force. Diastolic pres-sure is maintained by the arteries and arterioles and is discussed in a later section.  ...