General Anatomy

From the time blood leaves the heart to the time it returns, it usually passes through one capillary bed. In pathways called [1], however, it passes through two capillary beds in series, and in an [2], it passes directly from an artery to a vein without passing through any capillaries.

A blood vessel has an internal lining called the [3] with a simple squamous epithelium called the [4]; a middle [5] of muscle and elastic tissue; and a loosely organized tunica adventitia on the outside. The large arteries near the heart are highly elastic, whereas the smallest arteries, called [6], have only a few layers of smooth muscle cells. Capillaries are arranged in beds of usually 10–100, all supplied by a single [7]. Most capillaries are of a continuous type with only narrow gaps called [8] between the endothelial cells. Some, however, are [9] capillaries with patches of large pores through the endothelial cells. Veins have relatively low blood pressure and, unlike arteries, many of them are equipped with [10] to ensure a one-way flow of blood back to the heart.

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Blood Flow, Pressure, and Resistance

Blood flow is proportional to the [11] between two points, divided by [12]. When measuring blood pressure in an artery, two pressures are recorded–a peak, or [13], pressure when the ventricles contract and a low, or [14], pressure when the ventricles relax. [15] is a condition of chronic, abnormally high resting blood pressure. It creates a risk of weakened, bulging arteries called [16], which may eventually hemorrhage. Although peripheral resistance to flow is determined partly by vessel length and the thickness, or [17], of the blood, the main variable that can be adjusted from moment to moment to alter blood flow is vessel [18]. Blood flow is proportional to this value raised to the [19] power; therefore a doubling of this value can produce a 16-fold increase in blood flow.

The ability of a tissue to regulate its own blood flow is called [20]. This can be achieved by vasomotion or by [21], the growth of new blood vessels. Flow is also subject to remote control by the nervous and endocrine systems. A [22] is an autonomic reflex in response to fluctuations in blood pressure above the heart. This mechanism is very important in quickly adjusting blood pressure to changes in posture, as when you get out of bed. A [23] is an autonomic reflex in response to changes in blood chemistry, especially its pH and O₂ and CO₂ content. These changes in blood chemistry are detected by chemoreceptors located in the [24] bodies. One hormone that strongly stimulates vasoconstriction and raises blood pressure is [25], made by the action of kidney and lung enzymes on a blood protein from the liver.

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Capillary Exchange

Capillaries are the "business end" of the circulatory system, because it is only here that substances can be exchanged between the bloodstream and the [26]. Materials can escape the bloodstream by way of diffusion, filtration, or [27], the transport of pinocytotic vesicles across the endothelial cells. The loss of fluid from the capillaries is partially balanced by [28], the osmotic reuptake of fluid from the tissue spaces. The main force driving fluid out of the capillaries is the physical, or [29], pressures of the blood and tissue fluid. The main force causing reuptake of fluid is the [30] (two words) pressure of the blood, which is due mainly to the albumin in the plasma. This is opposed by osmotic pressure of the tissue fluid, but the difference between the two, called [31], favors capillary reabsorption. The difference between the net outward forces and net inward forces at the arterial end of a capillary is called the [32]. As water leaves the bloodstream, it carries many dissolved solutes along with it. This process is called [33].

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Venous Return and Circulatory Shock

The main factor forcing blood back to the heart is the pressure [34] from the venules to the right atrium. This is assisted, however, by the [35] pump, the action of muscle contractions on the veins of the extremities; the [36] pump, the action of cyclic pressure differences between the abdominal and thoracic cavities caused by breathing; [37], the tendency of the expanding right atrium to suck blood from the venae cavae during ventricular systole; and [38], the main force draining blood from the head and neck back to the heart. Physical activity promotes venous return, whereas inactivity promotes [39], the accumulation of blood in the lower parts of the body. When the heart does not pump enough blood to meet the body's metabolic needs, [40] occurs. This is sometimes due to weakness or incapacity of the heart itself, but more often it is because not enough blood is returning to the heart. The latter is called [41] shock. If it is due to fluid loss from the body, it is [42] shock, whereas if it is due to accumulation of blood in the lower regions of the body, it is [43] shock. A potentially deadly form of 43 is [44] shock, which results when a sensitized individual is exposed to an antigen source such as bee venom or horse serum.

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Special Circulatory Routes

Unlike most other arteries, cerebral arteries [45] when the systemic blood pressure drops, thus ensuring stability of the cerebral blood flow at about 750-900 mL/min. If the mean arterial blood pressure in the cerebrum drops below 60 mmHg, syncope, or [46], is likely to occur. The death of brain tissue due to interruption of blood flow is called a stroke, or [47]. It may result from blockage of a blood vessel or rupture of a cerebral aneurysm. An early warning sign of an impending stroke is a [48], in which brief cerebral ischemia produces dizziness, weakness, or other sensations.

In the muscular system, muscle contraction obstructs blood flow. Consequently, [49] exercises produce fatigue more rapidly than intermittent isotonic contractions. In the lungs, blood pressure is so low that the alveolar capillaries are engaged almost entirely in [50], thus preventing pulmonary congestion with fluid. Unlike arteries elsewhere, pulmonary arteries [51] in response to hypoxia; this minimizes wasteful blood flow to areas of the lung that are poorly [52].

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Anatomy of the Pulmonary Circuit

The pulmonary circuit, which serves only to rid the blood of [53] and pick up [54], begins with a large artery called the [55] arising from the right ventricle. A pair of right and left [56] directs the blood from here to the lungs, and two [57] on each side return oxygenated blood to the left atrium of the heart.

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Anatomy of the Systemic Arteries

The systemic circulation begins with the [58], which arises from the left ventricle. After giving off the coronary arteries, this vessel forms an inverted U called the [59] above the heart. This U gives off three major arteries whose branches supply the head, neck, and upper extremities: first the [60] artery, then the [61], and finally the [62]. The 58 then turns downward and passes through the thoracic and abdominal cavities, giving off arteries to their viscera.

The [63] arteries ascend the neck and then branch near the angle of the mandible; the external branch supplies the facial muscles and skin, and the internal branch enters the cranial cavity to perfuse the brain. A pair of [64] arteries ascend the back of the neck through the transverse foramina of the cervical vertebrae. These, too, enter the cranial cavity to perfuse the brain. They anastomose along the anterior side of the brainstem to form a basilar artery, then this divides to produce an arterial loop, the [65], around the pituitary gland. Anterior, middle, and posterior [66] arteries distribute blood from here throughout the cerebrum.

A [67] artery travels under each clavicle toward the arm, becomes the [68] artery in the region of the armpit, and then continues as the [69] artery along the humerus. Near the elbow, this divides into a [70] artery on the lateral side of the forearm and a [71] artery on the medial side. These eventually anastomose at deep and superficial [72], which give off arteries to the hand and fingers.

In the thoracic cavity, the aorta gives off [73] arteries which provide a systemic perfusion of the lungs; these supply the larger lung tissues with oxygen and nutrients and do not reach to the alveoli as the pulmonary arteries do. Among other thoracic vessels, the aorta also gives off posterior [74] arteries between the ribs and superior [75] arteries to the diaphragm.

Below the diaphragm, the abdominal aorta gives off a stubby [76] trunk, which immediately divides into a complex network of arteries supplying the stomach, pancreas, spleen, duodenum, and liver; superior and inferior [77] arteries to the intestines; [78] arteries to the kidneys; and long, slender, winding [79] arteries that lead to the testes of the male or ovaries of the female. At its inferior end, the aorta forks into a right and left [80], which supply the pelvic region and lower extremities.

Some of the immediate branches of 80 include the superior and inferior [81] arteries, which supply the urinary bladder; superior and inferior [82] arteries, which supply the muscles of the buttocks; and the internal [83] artery, which supplies the genitals. The 80 artery then continues as the [84] artery in the femoral triangle. Below the knee, the two main arteries are the anterior and posterior [85] arteries. Their branches anastomose at the [86] arch, which gives off digital arteries to the toes.

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Anatomy of the Systemic Veins

The smaller divisions of an artery are called its branches, whereas the smaller divisions of a vein are called [87] because their bloodstreams flow together like the smaller divisions that feed a river. Blood that has circulated through the brain collects in large spaces called the superior and inferior sagittal [88], among others. From here, blood flows down the neck in the internal and external [89] veins. They empty into the subclavian vein, which continues into the [90] vein and finally the [91], which empties into the heart.

Veins of the upper extremity have names similar to the arteries. The major deep veins of the forearm are the [92] vein on the lateral side and [93] vein on the medial side. They unite at the elbow to form the [94] vein of the arm proper. This continues as the [95] vein in the armpit and [96] vein inferior to the clavicle. The major superficial veins of the upper extremity, visible through the skin, are the [97] vein on the lateral side and [98] vein on the medial side. Both of these empty into the axillary vein.

Venous drainage of the thoracic organs is mainly by way of the [99] on the right and the smaller artery with a longer name, the [100], on the left. The 100 vein empties into the 99 vein, and this empties into the superior vena cava.

The inferior vena cava (IVC) begins with the union of the right and left [101] from the legs. It its ascent through the abdominal cavity, the IVC receives the [102] veins from the kidneys, [103] veins from the liver, and [104] veins from the diaphragm. A special venous pathway, the [105], carries blood from the intestines via the inferior and superior [106] veins to the liver. The vein that enters the inferior surface of the liver is the [107].

Anterior and posterior tibial veins drain the foot and leg, then unite at the back of the knee to form the [108] vein. This continues into the thigh as the [109] vein. The two major superficial veins of the lower extremity are the great and small [110] veins. The small one empties into the 108 vein, whereas the great one, the longest vein in the body, empties into the 109 vein. The union of the great 110 and 109 veins forms the external iliac vein. This unites with the internal iliac to form the [111].

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