Circulatory system blood vessels

The adenohypophysis does not have a direct anatomical connection with the hypothalamus therefore, regulation of hormone secretion by way of neuronal signals is not possible. Instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus (see Figure 10.2). Systemic arterial blood is directed first to the hypothalamus. The exchange of materials between the blood and the interstitial fluid of the hypothalamus takes place at the primary capillary plexus. The blood then flows to the adenohypophysis through the hypothalamic-hypophyseal portal veins. Portal veins are blood vessels that connect two capillary beds. The second capillary bed in this system is the secondary capillary plexus located in the adenohypophysis. 

The human heart begins pumping approximately 3 weeks after conception and must continue this activity without interruption all day, every day, for an entire lifetime. In a typical individual, this means the heart pumps over 100,000 times per day and propels about 2000 gallons of blood through almost 65,000 miles of blood vessels. This function of the heart will be discussed here as well as in the following chapter on cardiac output. The function of the blood vessels will be considered in the chapter on the circulatory system. 

The circulatory system is composed of several anatomically and functionally distinct blood vessels including (1) arteries, (2) arterioles, (3) capillaries, and (4) veins. 

Figure 15.1 The circulatory system. Arteries carry blood away from the heart. The smallest arterial vessels, the arterioles, are composed mainly of smooth muscle and are the major resistance vessels in the circuit. The capillaries are the site of exchange between blood and tissues. Veins carry blood back toward the heart. The small veins are the major compliance vessels in the circuit and, under resting conditions, contain 64% of the blood volume.

Therefore, depending upon the degree of constriction of the vascular smooth muscle, these vessels may alter their diameter, and consequently their blood flow, across a very wide range. For this reason, the arterioles are the major resistance vessels in the circulatory system. In fact, the primary function of arterioles is to regulate the distribution of the cardiac output and to determine which tissues receive more blood and which receive less, depending upon the tissue s and the body s needs. 

As mentioned previously, the arterioles are the major resistance vessels in the circulatory system. Because the walls of these vessels contain primarily smooth muscle, they are capable of significant changes in their radius. Therefore, regulation of blood flow to the tissues is carried out by the arterioles. 

The most distensible vessels in the circulatory system are the veins. As with arteries, this feature of the veins also has important physiological implications because it allows them to serve as blood reservoirs. The veins are so distensible that they are capable of holding large volumes of blood at very low pressures. In fact, under resting conditions, 64% of the blood volume is contained within these vessels. 

Wide variations exist from individual to individual with respect to the circulatory system, the size of the respective blood vessels, and their distribution patterns. One might suppose that the main arteries arising out of the aorta, which comes directly from the heart, would always branch in about the same way and follow the same general... 

Two different circulatory systems, the bronchial and the pulmonary, supply the lungs with blood [133], The bronchial circulation is a part of the systemic circulation and is under high pressure. It receives about 1% of the cardiac output and supplies the conducting airways, pulmonary blood vessels and lymph nodes [133], It is important for the distribution of systemically administered drugs to the airways and to the absorption of inhaled drugs from the airways [18]. The pulmonary circulation comprises an extensive low-pressure vascular bed, which receives the entire cardiac output. It perfuses the alveolar capillaries to secure efficient gas exchange and supplies nutrients to the alveolar walls. Anastomoses between bronchial and pulmonary arterial circulations have been found in the walls of medium-sized bronchi and bronchioles [18, 65, 67],... 

Aggregation of blood platelets is the requisite first event for the maintenance of intact circulation in the face of any break in a blood vessel. It is the platelet clump that starts the long and complicated process leading to closure of the broken vessel by an organized blood clot. Though this property of platelets is vital to maintenance of the circulatory system, an excessive tendency to aggregation can also lead to problems. Thus platelet clumps formed in blood vessels in the absence of injury can lead to blockade of blood circulation and subsequent injury. Strokes and some types of myocardial infarcts have thus been associated with platelet clumps. The nonsteroid antiinflammatory... 

Figure 1.5 The blood vessel and nerve supply in the mammary glands of a cow. Circulatory system (arteries, white veins, stippled) h, heart a, abdominal aorta pa, external pudic artery pv, external pudic vein s, subcutaneous abdominal vein c, carotid artery j, jugular vein. Nerves 1, first lumbar nerve 2, second lumbar nerve 3, external spermatic nerve 4, perineal nerve. A and V show blood sampling points for arteriovenous (AV) difference determinations (Mepham, 1987).

These lipids are then packaged into spherical lipoproteins, particles of lipids and proteins, known as chylomicrons, which are secreted into lymphatic vessels and subsequently enter the blood stream. Once in the circulatory system the triacylglycerol components of the chylomicrons are degraded to fatty acids and glycerol by the enzyme lipoprotein lipase, which is attached to the luminal (inner) side of capillary vessels in heart, muscle, adipose (commonly... 

Constriction of blood vessels, arrhythmia, and even aplastic anemia, caused by ethyl alcohol, methylene chloride, and benzene, respectively, are circulatory system diseases that result from misuse of these chemical substances. Methylene chloride, a paint solvent, reacts in the body to produce carbon monoxide, which replaces oxygen in the hemoglobin molecule. This can cause carbon monoxide poisoning, which can be fatal. 

SMAs are also used in another application related to cardiac health problems, as vena-cava filters. In some instances, it is desirable to protect a patient against the possibility that a blood clot formed elsewhere in the body will travel through the circulatory system into the heart, where it may cause a heart attack or stroke. Tiny, umbrella-shaped devices made of SMA materials have proven to he effective in such cases. In these devices, called vena-cava filters, the umbrella portion of the device consists of a mesh of tiny wires made of an SMA material. The device is inserted into the circulatory system in the form of a reduced-size (martensite phase), folded-up umbrella. Once in place, it is opened in such a way that the umbrella fills the vessel leading into the heart. The mesh design allows blood to flow through normally but filters out any blood clots that are carried along with the blood. 

Acute intravasation of barium sulfate into the circulatory system of an adult female patient following a barium enema procedure caused the compound to be deposited in blood vessels throughout the body, including the lungs, and resulted in respiratoryfailure (Cove and Snyder 1974). Acute parenteral administration of barium compounds to animals has been shown to result in paralysis of the respiratory muscles (Roza and Berman 1971). Similar respiratory paralysis is frequently encountered in cases of acute exposure in humans and animals by ingestion or inhalation. Intratracheal administration of barium sulfate into rat lungs produced a mild inflammatory reaction (Huston et al. 1952). Barium sulfate could not be removed by either polymorphonuclear leukocytes or monocytes. A tissue reaction followed however, no fibrosis was observed. Since this mode of entry is similar to inhalation, these results may be significant for cases of inhalation exposure. 

---