Circulatory system capillaries

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 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.

The velocity of blood flow through capillaries is slow compared to the rest of the circulatory system because of the very large total cross-sectional surface area of the capillaries. Although each individual capillary has a diameter of... 

An interesting phenomenon in the circulatory system is that, even though capillaries have numerous pores in their walls, all of the fluid does not leak out of them into the interstitial space. If a balloon filled with water had multiple pin pricks in it, all of the water would clearly leak out. What prevents this from happening in the capillaries The Starling Principle describes the process by which plasma is held within the vascular compartment. 

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],... 

The circulatory system moves materials (and heat) from one organ to another. It is centred on the heart which pumps blood through arteries to capillaries, where exchange occurs before the blood returns to the heart via the veins (Figure 1.16). During its passage through the tissues, there... 

Amino acids, sugars, and minerals pass through the small intestine into the circulatory system, where they are mixed with blood. The primary reactor organs in processing blood are muscle and the kidneys. The fluid flows in nearly total recycle through arteries and veins, which are basically the pipes in the system, and capillaries, where most of the transfer to and from the reactors and separators occurs. 

Structurally, the skin consists of the outermost epidermis which is essentially composed of dead squamous cells sloughed off from the underlying dermis (Figure 7.1). The dermis lies on top of subdermal layers, which further down gives access to nerve endings and capillaries of the circulatory system. 

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... 

To estimate inhalation contact exposure, some assumptions must be made which err on the side of conservatism and which should be modified as more complete data become available. It is necessary to know the droplet size spectrum of the spray because the diameter of the droplet influences its movement down the respiratory system (11). The functional unit of the lung is the alveolus, which is the terminal branch in the system. It is presumed that pesticide particles which are soluble in respiratory tract fluid and are 5p or less in diameter will reach the alveolus where they will be readily absorbed through the cells of the alveolar membrane into the pulmonary capillary beds and hence into the circulatory system. A recent review by Lippmann at al. (12) discusses in depth the deposition, retention and clearance of inhaled particles. 

The term blood-brain barrier (BBB) refers to the special obstacle that drugs encounter when trying to enter the brain from the circulatory system. The difference between the brain and other tissues and organs is that the capillaries in the brain do not have pores for the free flow of small molecules in the interstitial fluid of the brain. To enter the interstitial fluid, all molecules must cross a membrane. This design is a protective measure to defend the brain from unwanted and potentially hazardous xenobiotics. Traditionally, drugs that target the brain or central nervous system (CNS) cross the BBB by passive diffusion. Transport by carrier proteins across the BBB is becoming better understood but remains an area of active research. 

The apical side of the cell faces the lumen this side is also the mucosal side in many tissues, The basal side faces the interstitial fluid and the capillaries this side is also called the serosal side. The sides of the cell facing adjacent cells are called the lateral sides. Nutrients entering and exiting the basal and lateral membranes (basolateral membrane) can freely enter and exit the permeable membranes of nearby capillaries. Thus, the Na secreted by the basolateral membrane can enter the circulatory system. Potassium leaking from the capillaries can be taken up by the Na,K-ATPase of the basolateral membrane. The cells are connected to each other by impermeable junctions called tight junctions. 

FIGURE 2.37 Thoracic duct and other vessels of the thorax. Lymphatic capillaries are most numerous just beneath body surfaces, such as the skin and the mucus membranes of the gastrointestinal and respiratory tracts. The mucus membrane of the gastrointestinal tract is called the gut mucosa. The general function of these capillaries is to absorb interstitial fluid that has leaked from the circulatory system and to return it to the bloodstream. The function of the l)miphatic capillaries that end in the lacteals of the small intestine is to transport absorbed dietary lipids. These capillaries coalesce and eventually deliver their contents to the thoracic duct. The lymph collected from other parts of the body, as indicated by the "collecting trunk," also is transferred to the thoracic duct. [Redrawn with permission, from "Grant s Atlas of Anatomy," Williams Wilkins Co., Baltimore, 1978.]... 

Water, salt, and blood pressure are related. The blood volume is closely related to the blood pressure. A loss in blood volume can occur with water deficiency or because of extensive bleeding. The lack of enough blood to fill up the vessels of the circulatory system leads to a drop in blood pressure. A severe drop in blood pressure results in the inability of the heart to pump vital nutrients to the brain and other tissues. A loss in blood volume can also result from sodium deficiency. The concentrations of sodium and its counterion chloride must be maintained to maintain the osmotic strength of the blood plasma. Osmotic strength is expressed by the term osmolality. Osmolality is equal to the sum of the molarities of the separate particles (ions or molecules) in a liquid. For example, a solution of 1 mole of NaCl in 1 liter has an osmolality of 2.0 osmol/liter. Na and Cl ions dissociate completely in solution. Osmotic pressure develops when two solutions of differing osmolalities are placed in contact with each other but separated by a semiperme-able membrane. The walls of capillaries are semipermeable membranes. The renal... 

Sublingual medications are administered under the tongue. Buccal medications are administered between the cheek and the gum. Both routes absorb medication quickly into the circulatory system because there is a vast network of capillaries beneath the thin layer of epithelium tissue in those areas. 

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