Veins vena cava

Osmolarity of the nutrient admixtures and thereby the infusion route is determined by the type and amount of the components mixed. In general the admixtures are hyperosmolar and to be administered via a central venous catheter in a big vein (vena cava superior or vena subclavia). Only admixtures with a maximum osmolarity of900 mOsm/ L can be administered via a peripheral vein and only for a limited period of time [69]. Lmig term parenteral nutrition can be also administered via a port (see Sect. 13.10.3) especially when patients are treated at home. Because of the high probability of incompatibilities nutrition admixtures should always be administered via a separate line and Y-site infusion should be avoided. 

The superior vena cava (SVC) is the primary drainage vein for blood return from the head, neck, and upper extremities. It is a relatively thin-walled vein that is particularly vulnerable to obstruction from adjacent tumor invasion or thrombosis. The obstruction leads to elevated venous pressure, although collateral veins partially compensate. This is one reason for the relatively slow onset of the classic symptoms of SVCS. In fact, 75% of patients have signs and symptoms for more than 1 week before seeking medical attention.15... 

PN can be administered via a smaller peripheral vein (e.g., cephalic or basilic vein) or via a larger central vein (e.g., superior vena cava). Peripheral PN (PPN) is infused via a peripheral vein and generally is reserved for short-term administration (up to 7 days) when central venous access is not available. PN formulations are hypertonic, and infusion via a peripheral vein can cause thrombophlebitis. Factors that increase the risk of phlebitis include high solution osmolarity, extreme pH, rapid infusion rate, vein properties, catheter material, and infusion time via the same vein.20 The osmolarity of PPN admixtures should be limited to 900 mOsm/L or less to minimize the risk of phlebitis. The approximate osmolarity of a PN admixture can be calculated from the osmolarities of individual components ... 

Central PN refers to the administration of PN via a large central vein, and the catheter tip must be positioned in the vena cava. Central PN allows the infusion of a highly concentrated, hypertonic nutrient admixture. The typical osmolarity of a central PN admixture is about 1500 to 2000 mOsm/L. Central veins have much higher blood flow, and the PN admixture is diluted rapidly on infusion, so phlebitis is usually not a concern. Patients who require PN administration for longer periods of time (greater than 7 days) should receive central PN. One limitation of central PN is the need for placement of a central venous catheter and an x-ray to confirm placement of the catheter tip. Central venous catheter placement may be associated with complications, including pneumothorax, arterial injury, air embolus, venous thrombosis, infection, chylothorax, and brachial plexus injury.1,20... 

The semilunar valves separate the ventricles from their associated arteries. The pulmonary valve is found between the right ventricle and the pulmonary artery and the aortic valve is found between the left ventricle and the aorta. These valves prevent backward flow of blood from the pulmonary artery or the aorta into their preceding ventricles when the ventricles relax. The semilunar valves also have three cusps. There are no valves between the venae cavae or the pulmonary veins and the atria into which they deliver blood. The closure of the valves causes the "lub-dub" associated with the heart beat. Tire first heart sound, or the "lub," occurs when the ventricles contract and the AV valves close. The second heart sound, or the "dub," occurs when the ventricles relax and the semilunar valves close. 

Figure 5.2 Cross section through bLood vessels (a) an artery. Sizes of arteries vary from 25 mm diameter with 2 mm wall thickness in the aorta to 20 pm diameter with a 15 pm wall thickness in the smallest arterioles, (b) a vein. Note there are no elastic lamellae in veins so tension is maintained by elastic fibres which are arranged within the media. Typically, veins are approximately 5 mm in diameter but with a very thin wall, for example 0.5 mm. The vena cava is the largest vein at 30 mm diameter...

Blood is supplied to the kidneys via the renal vein, a branch of the descending vena cava, at relatively high pressure to ensure rapid filtration of plasma across the membranes of the blood vessels in the glomeruli and the epithelial cells of the Bowman s capsule. The net filtration pressure of about 5-6 kPa, is the difference between the blood pressure forcing plasma water across the filtration barrier and the opposing osmotic and... 

Classically the liver has been divided into hexagonal lobules centred around the terminal hepatic venules. Blood enters the liver through the portal tracts that are situated at the corners of the hexagon. The portal tracts are triads of a portal vein, an hepatic artery, and a common hepatic bile duct. The vast expanse of hepatic tissue, mostly consisting of parenchymal cells (PC) or hepatocytes, is serviced via terminal branches of the portal vein and hepatic artery, which enters the tissue at intervals. The hepatocytes are organized into cords of cells radially disposed about the central hepatic venule. Between these cords are vascular sinusoids that transport the blood to the central hepatic venules. The blood is collected through the hepatic venules into the hepatic vein which exits the liver into the inferior vena cava (Figure 4.1). 

First level of thoracic vessels Aorta, pulmonary trunk, ductus arteriosus, atria, bronchus, pulmonary vein and arteries, and vena cava (see Note 11). 

The hepatic first-pass effect can be avoided to a great extent by use of sublingual tablets and transdermal preparations and to a lesser extent by use of rectal suppositories. Sublingual absorption provides direct access to systemic—not portal—veins. The transdermal route offers the same advantage. Drugs absorbed from suppositories in the lower rectum enter vessels that drain into the inferior vena cava, thus bypassing the liver. However, suppositories tend to move upward in the rectum into a region where veins that lead to the liver predominate. Thus, only about 50% of a rectal dose can be assumed to bypass the liver. 

A possible explanation for the differences observed in rat could be the use of animals under basal conditions vs electrically-driven systems and/or different receptor densities for different strains. Certainly, histamine H3-receptors are not homogeneously distributed along the rat vascular system, as they were found in some districts, like pulmonary microvessels (Danko et al., 1994), portal vein (Smit et al., 1997) or tail artery (Godlewski et al., 1997b), but not in vena cava (Schneider et al., 1991). 

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