Capillaries, arterial blood

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. 

Other required substances are transported from the arterial blood through the walls ofthe arterial capillaries into tissues and organs. In contrast, waste products and unrequired substances produced by the organs and tissues are transported to the venous blood through the walls of the venous capillaries, which combine into venules, and then into larger veins. 

The liver contains an enormous number of hepatocytes that perform the various functions noted above. The hepatocytes are contained within minute units known as hepatic lobules, in which the cell layers (which are one or two cells thick) are in contact with networks of minute blood channels - the sinusoids - which ultimately join the venous capillaries. Capillaries carrying blood from the hepatic artery and the portal vein empty separately into the sinusoids. The walls of sinusoids and liver cells are incomplete, and blood is brought into direct contact with the hepatocytes. 

In the early 1600s William Harvey deduced the existence of the microcirculation. Until then it was thought that venous blood and arterial blood made up independent pools. Capillary vessels were first observed later in die century by Marcello Malpighi, verifying the existence of a microcirculation connecting die arterial and venous networks. 

Seventy-five percent of the blood received by the liver is venous and is supplied by the portal vein, which drains the capillary beds of the digestive tract, spleen, pancreas and gallbladder. Arterial blood is provided by the hepatic artery, which originates from the aorta and enters the liver alongside the portal vein. Once the portal vein enters the liver it divides into the right and left main branches and then subdivides to supply the various regions of the liver. The separation of the portal vein to supply the different liver sections provides a convenient means of... 

Vascular disorders A decrease in the arterial blood supply, especially hypoperfusion of the periductular capillary plexus, can be caused by vasculitis, i.v. infusions of cytostatics (e.g. floxuridine) (18, 19, 24, 38) or alcohol into the hepatic artery, postoperative scarry strictures of the hepatic artery, allograft rejection, etc. 

The BBB of the brain has two major components. An endothelial layer lies between the arterial blood in the brain capillaries and the interstitial fluid of the brain. In humans, the surface area of the endothelial layer in the brain is approximately 21 square meters [51]. An epithehal layer lies between venous blood and the cerebrospinal fluid (CSF) in the choroid plexus, and has a surface area of only 0.021 square meters in humans [52]. At the spinal cord, the BBB... 

Because capillary walls are thin (to permit diffusion) the blood that is delivered to them must be delivered under reduced pressure. This is accomplished by the arterioles, which combine relatively muscular walls with a narrow lumen. The arterial blood pressure is a function of cardiac output and the total peripheral vascular resistance, which is primarily a function of the degree of normal tension (tonus) of the smooth muscle cells in the walls of the arterioles. If this tonus increases above the normal range for extended periods of time, hypertension (high blood pressure) will result. This tonus is under the control of the autonomic nervous system and of adrenergic hormones (catecholamines). 

Blood for analysis may be obtained from veins, arteries, or capillaries. Venous blood is usually the specimen of choice, and venipuncture is the method for obtaining this speci-men. In young children and for many point-of-care tests, skin puncture is frequently used to obtain what is mostly capillary blood arterial puncture is used mainly for blood gas analyses. 

Arterialized capillary blood is sometimes an acceptable alternative to arterial blood when blood losses need to he minimized, when an arterial cannula is not available, or to prevent repeated arterial puncture. Freely flowing cutaneous blood originates in the arterioles and corresponds closely to arterial blood in composition. However, arterialized capfl-... 

Transcutaneous PCO2 values can be up to 80% higher than corresponding capillary or arterial blood values of which 40% to 50% of the difference can be explained by the difference in the temperatures of measurement (44 °C versus 37 °C). Guidelines for transcutaneous measurement of PO2 and PCO2 have been published by a committee of the IFCC. ... 

Peripheral venous blood reaches the pulmonary circulation from the right ventricle of the heart and is arterialized in the capillaries of the lungs by uptake of O2 and loss of CO2. Pulmonary venous blood then returns to the left ventricle by way of the left atrium and is pumped through the aorta to the peripheral tissues. In the capillaries of peripheral tissues, the arterial blood releases O2 to the tissue cells and takes up CO2. With return of blood to the lungs, the cycle is completed. 

Arterial blood reaches the pituitary gland via the superior hypophyseal artery, a branch of the internal carotid artery. Venous blood is supplied through a venous portal system that originates in the median eminence of the hypothalamus and ends in sinusoidal capillaries of the pituitary gland. This venous system is known as the hypothalamic-hypophyseal portal system. This system carries neurosecretory hormones from the hypothalamus to the adenohypophysis. These hypothalamic factors stimulate or inhibit the release of hormones from the adenohypophysis. Retrograde flow from the adenohypophysis to the median eminence of the hypothalamus is also believed to occur. With upstream flow, pituitary hormones can reach the hypothalamus and influence hypothalamic function through a short feedback loop. 

Much of the main group metals and non-metals found in vivo are highly solvated by water and produce a pressure within capillaries, arteries, etc. For example, blood plasma approximates to 150 mmol dm 2 sodium chloride. This accounts for a large proportion of the water in humans. Conditions which cause imbalances away from this constant pressure of aquation (called the isotonic pressure) can cause serious cramps in the patient. 

The hypothalamus is a small region of the brain in the ventral aspect of the diencephalon. In the adult human, it is about 2.5 cm in length and weighs about 4 g. Ventromedi-ally, it surrounds the third ventricle and is continuous with the infundibular stalk of the pituitary (hypophysis). This cone-shaped region of the hypothalamus, the median eminence, consists mainly of axonal fibers from hypothalamic neurons, which either terminate in the median eminence or continue down into the posterior lobe of the pituitary, and it is perfused by a capillary network (primary plexus) derived from the carotid arteries. Blood from the primary plexus is transported by portal vessels (hypophyseal portal vessels) to another capillary network (secondary plexus) in the anterior lobe of the pituitary (adenohypophysis) (Figure 31-1). 

The kidney is a highly vascular organ with blood vessels ranging from the very large (renal arteries) to the very small capillaries providing blood to each individual glomerulus. Obstruction of the renal artery will result in an increase in serum creatinine, hematuria, and proteinuria, but obstruction of smaller vessels will only cause infarction of the downstream parenchyma. If this area is small, no change in serum creatinine will occur. 

When arterial blood enters a tissue capillary, it exchanges oxygen and carbon dioxide with its environment, as shown in this diagram. 

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