From the time blood leaves the heart to the time it returns, it usually passes through one capillary bed. However, in pathways called [1] portal systems , it passes through two capillary beds in series, and in a/an [2] arteriovenous anastomosis (shunt) it passes directly from an artery to a vein without passing through any capillaries.

A blood vessel has an internal lining called the [3] tunica interna , with a simple squamous epithelium called the [4] endothelium ; a middle [5] tunica media of muscle and elastic tissue; and a loosely organized tunica externa (or tunica adventitia) on the outside. The large arteries near the heart are highly elastic, whereas the smallest arteries, called [6] arterioles , have only a few layers of smooth muscle cells. Capillaries are arranged in beds of usually 10–100, all supplied by a single [7] metarteriole . Most capillaries are continuous, with only narrow gaps called [8] intercellular clefts between the endothelial cells. However, some are [9] fenestrated 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] venous valves to ensure a one-way flow of blood back to the heart.

Blood flow is proportional to the [11] pressure difference between two points, divided by [12] resistance . When measuring blood pressure in an artery, two pressures are recorded—a peak, or [13] systolic , pressure when the ventricles contract and a low, or [14] diastolic , pressure when the ventricles relax. [15] Hypertension is a condition of chronic, abnormally high resting blood pressure. It creates a risk of weakened, bulging arteries called [16] aneurysms , which may eventually hemorrhage. Although peripheral resistance to flow is determined partly by vessel length and the thickness, or [17] viscosity , of the blood, the main variable that can be adjusted from moment to moment to alter blood flow is vessel [18] radius . Blood flow is proportional to this value raised to the [19] fourth 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] autoregulation . This can be achieved by vasomotion or by [21] angiogenesis , the growth of new blood vessels. Flow is also subject to remote control by the nervous and endocrine systems. A [22] baroreflex 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] chemoreflex is an autonomic reflex in response to changes in blood chemistry, especially pH and O₂ and CO₂ content. These changes in blood chemistry are detected by chemoreceptors located in the [24; two words] aortic and carotid bodies. One hormone that strongly stimulates vasoconstriction and raises blood pressure is [25] angiotensin II , made by the action of kidney and lung enzymes on a blood protein from the liver.

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

The main factor forcing blood back to the heart is the pressure [34] gradient from the venules to the right atrium. This is assisted, however, by the [35] skeletal muscle pump, the action of muscle contractions on the veins of the extremities; the [36] thoracic (respiratory pump, the action of cyclic pressure differences between the abdominal and thoracic cavities caused by breathing; [37] cardiac suction , the tendency of the expanding right atrium to suck blood from the venae cavae during ventricular systole; and [38] gravity , the main force draining blood from the head and neck back to the heart. Physical activity promotes venous return, whereas inactivity promotes [39] venous pooling , 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] circulatory shock 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, a condition called [41] low venous return shock. If it is due to fluid loss from the body, it is [42] hypovolemic shock, whereas if it is due to accumulation of blood in the lower regions of the body, it is [43] venous pooling, or vascular shock. A potentially deadly form of 43 is [44] anaphylactic shock, which results when a sensitized individual is exposed to an antigen source such as bee venom or horse serum.

Unlike most other arteries, cerebral arteries [45] dilate 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, fainting, or [46] syncope , is likely to occur. The death of brain tissue due to interruption of blood flow is called a stroke, or [47] cerebral vascular accident (CVA) . 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] transient ischemic attack (TIA) , in which brief cerebral ischemia produces dizziness, weakness, or other sensations.

In the muscular system, muscle contraction obstructs blood flow. Consequently, [49] isometric 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] reabsorption , thus preventing pulmonary congestion with fluid. Unlike arteries elsewhere, pulmonary arteries [51] constrict in response to hypoxia; this minimizes wasteful blood flow to areas of the lung that are poorly [52] ventilated .

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

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

The [63] common carotid 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] vertebral 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] circle of Willis , around the pituitary gland. Anterior, middle, and posterior [66] cerebral arteries distribute blood from here throughout the cerebrum.

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

In the thoracic cavity, the aorta gives off [73] bronchial 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] intercostal arteries between the ribs and superior [75] phrenic arteries to the diaphragm.

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

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

Blood that has circulated through the brain collects in large spaces called the superior and inferior sagittal [87] sinuses , among others. From here, blood flows down the neck in the internal and external [88] jugular veins. They empty into the [89] subclavian vein, which continues into the [90] brachiocephalic vein and finally the [91] superior vena cava , 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] radial vein on the lateral side and [93] ulnar vein on the medial side. They unite at the elbow to form the [94] brachial vein of the arm proper. This continues as the [95] axillary vein in the armpit and the [96] subclavian vein inferior to the clavicle. The major superficial veins of the upper extremity, visible through the skin, are the [97] cephalic vein on the lateral side and the [98] basilic 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] azygos on the right and the smaller vein with a longer name, the [100] hemiazygos , 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] common iliac veins from the legs. In its ascent through the abdominal cavity, the IVC receives the [102] renal veins from the kidneys, [103] hepatic veins from the liver, and [104] phrenic veins from the diaphragm. A special venous pathway, the [105] hepatic portal system , carries blood from the intestines via the inferior and superior [106] mesenteric veins to the liver. The vein that enters the inferior surface of the liver is the [107] hepatic portal vein .

Anterior and posterior tibial veins drain the foot and leg, then unite at the back of the knee to form the [108] popliteal vein. This continues into the thigh as the [109] femoral vein. The two major superficial veins of the lower extremity are the great and small [110] saphenous 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] common iliac vein .