question: The human circulatory system and why bodies bleed when we get small cuts even if it's not hitting one of the veins or arteries
comment: There are many capillaries in the skin that get broken once a cut is deeper than the epidermis. If there weren't, the skin would receive no blood supply and not be able to survive. If you hit a vein or artery, you'll bleed quite a bit.
passages: In cut carotid arteries with 100 mL of blood through the heart at each beat (at 65 beats a minute), a completely severed artery will spurt blood for about 30 seconds and the blood will not spurt much higher than the human head. If the artery is just nicked, on the other hand, the blood will spurt longer but will be coming out under pressure and spraying much further.
   Leg veins have valves which prevent backflow of the blood being pumped against gravity by the surrounding muscles.
  
  Section::::Structure.:Types.
  
  There are various kinds of blood vessels:
  
  BULLET::::- Arteries
  
  BULLET::::- Elastic arteries
  
  BULLET::::- Distributing arteries
  
  BULLET::::- Arterioles
  
  BULLET::::- Capillaries (smallest blood vessels)
  
  BULLET::::- Venules
  
  BULLET::::- Veins
  
  BULLET::::- Large collecting vessels, such as the subclavian vein, the jugular vein, the renal vein and the iliac vein.
  
  BULLET::::- Venae cavae (the two largest veins, carry blood into the heart).
  
  BULLET::::- Sinusoids
  
  BULLET::::- Extremely small vessels located within bone marrow, the spleen, and the liver.
   BULLET::::- "Contusion:" Simple bruising. In this type of injury, the capillaries in the epidermis and dermis are damaged, without breaking the skin. Blood oozes out of these vessels into the spaces between cells or interstitial space, causing swelling and discoloration. Blood loss is generally limited, and not of serious consequence. It may, however, act as a signpost, pointing to more serious injuries.
   Blood viscosity increases as blood is more hemoconcentrated, and decreases as blood is more dilute. The greater the viscosity of blood, the larger the resistance will be. In the body, blood viscosity increases as red blood cell concentration increases, thus more hemodilute blood will flow more readily, while more hemoconcentrated blood will flow more slowly.
   The vascular volume is divided into the venous volume and the arterial volume; and the arterial volume has a conceptually useful but unmeasurable subcompartment called the effective arterial blood volume.
  
  Section::::Compartments by location.
  
  BULLET::::- Intracellular fluid
  
  BULLET::::- Extracellular fluid
  
  BULLET::::- Intravascular fluid (blood plasma)
  
  BULLET::::- Interstitial fluid
  
  BULLET::::- Lymphatic fluid (sometimes included in interstitial fluid)
  
  BULLET::::- Transcellular fluid
  
  Section::::Health.
   The circulatory system consists of the heart and blood vessels (arteries, veins and capillaries). The heart propels the circulation of the blood, which serves as a "transportation system" to transfer oxygen, fuel, nutrients, waste products, immune cells and signalling molecules (i.e. hormones) from one part of the body to another. Paths of blood circulation within the human body can be divided into two circuits: the pulmonary circuit, which pumps blood to the lungs to receive oxygen and leave carbon dioxide, and the systemic circuit, which carries blood from the heart off to the rest of the body. The blood consists of fluid that carries cells in the circulation, including some that move from tissue to blood vessels and back, as well as the spleen and bone marrow.
   The essential components of the human cardiovascular system are the heart, blood and blood vessels. It includes the pulmonary circulation, a "loop" through the lungs where blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provide oxygenated blood. The systemic circulation can also be seen to function in two parts – a "macrocirculation" and a microcirculation. An average adult contains five to six quarts (roughly 4.7 to 5.7 liters) of blood, accounting for approximately 7% of their total body weight. Blood consists of plasma, red blood cells, white blood cells, and platelets. Also, the digestive system works with the circulatory system to provide the nutrients the system needs to keep the heart pumping.
   In a normal circulatory system, the volume of blood returning to the heart each minute is approximately equal to the volume that is pumped out each minute (the cardiac output). Because of this, the velocity of blood flow across each level of the circulatory system is primarily determined by the total cross-sectional area of that level. This is mathematically expressed by the following equation:
  
  where 
  
  BULLET::::- v = velocity (cm/s)
  
  BULLET::::- Q = blood flow (ml/s)
  
  BULLET::::- A = cross sectional area (cm)
  
  Section::::Blood flow.:Turbulence.
   BULLET::::- The outer layer is the "tunica adventitia" and the thickest layer in veins. It is entirely made of connective tissue. It also contains nerves that supply the vessel as well as nutrient capillaries (vasa vasorum) in the larger blood vessels.
  
  Capillaries consist of a single layer of endothelial cells with a supporting subendothelium consisting of a basement membrane and connective tissue.
  
  When blood vessels connect to form a region of diffuse vascular supply it is called an anastomosis. Anastomoses provide critical alternative routes for blood to flow in case of blockages.
   Pressure drops gradually as blood flows from the major arteries, through the arterioles, the capillaries until blood is pushed up back into the heart via the venules, the veins through the vena cava with the help of the muscles. At any given pressure drop, the flow rate is determined by the resistance to the blood flow. In the arteries, with the absence of diseases, there is very little or no resistance to blood. The vessel diameter is the most principal determinant to control resistance. Compared to other smaller vessels in the body, the artery has a much bigger diameter (4  mm), therefore the resistance is low.
   BULLET::::- The myogenic response of blood vessels. In 1902 Bayliss described that many blood vessels respond to an elevation of transmural pressure by constriction, finally reaching a diameter that is smaller than the initial diameter at the lower pressure. This observation, which had not received much attention at the time, was studied by Folkow as part of his thesis work and subsequently for more than a decade. He described this mechanism as an essential element in vascular control. This is now well-established knowledge in cardiovascular physiology.
   Regardless of site, blood pressure is related to the wall tension of the vessel according to the Young–Laplace equation (assuming that the thickness of the vessel wall is very small as compared to the diameter of the lumen):
  
  where
  
  BULLET::::- "P" is the blood pressure
  
  BULLET::::- "t" is the wall thickness
  
  BULLET::::- "r" is the inside radius of the cylinder.
  
  BULLET::::- formula_16 is the cylinder stress or "hoop stress".
  
  For the thin-walled assumption to be valid the vessel must have a wall thickness of no more than about one-tenth (often cited as one twentieth) of its radius.
   Assuming steady, laminar flow in the vessel, the blood vessels behavior is similar to that of a pipe. For instance if p1 and p2 are pressures are at the ends of the tube, the pressure drop/gradient is:
  
  The larger arteries, including all large enough to see without magnification, are conduits with low vascular resistance (assuming no advanced atherosclerotic changes) with high flow rates that generate only small drops in pressure. The smaller arteries and arterioles have higher resistance, and confer the main blood pressure drop across major arteries to capillaries in the circulatory system.
   Section::::Function.:Capillaries.
  
  The capillaries are the smallest of the blood vessels and are part of the microcirculation. The microvessels have a width of a single cell in diameter to aid in the fast and easy diffusion of gases, sugars and nutrients to surrounding tissues. Capillaries have no smooth muscle surrounding them and have a diameter less than that of red blood cells; a red blood cell is typically 7 micrometers outside diameter, capillaries typically 5 micrometers inside diameter. The red blood cells must distort in order to pass through the capillaries.
   Relatively common causes include sepsis, surgery, major trauma, cancer, and complications of pregnancy. Less common causes include snake bites, frostbite, and burns. There are two main types: acute (rapid onset) and chronic (slow onset). Diagnosis is typically based on blood tests. Findings may include low platelets, low fibrinogen, high INR, or high D-dimer.
   Blood vessels carry blood throughout the body, which moves because of the beating of the heart. Venules and veins collect blood low in oxygen from tissues throughout the body. These collect in progressively larger veins until they reach the body's two largest veins, the superior and inferior vena cava, which drain blood into the right side of the heart. From here, the blood is pumped into the lungs where it receives oxygen and drains back into the left side of the heart. From here, it is pumped into the body's largest artery, the aorta, and then progressively smaller arteries and arterioles until it reaches tissue. Here blood passes from small arteries into capillaries, then small veins and the process begins again. Blood carries oxygen, waste products, and hormones from one place in the body to another. Blood is filtered at the kidneys and liver.
   As blood flows from the arterioles into the capillaries a change in pressure occurs. In order to maintain pressure, the capillaries branch off to a web of vessels that carry blood into the venules. Through this process blood undergoes micro-circulation. In micro-circulation, the Fåhræus effect will take place, resulting in a large change in hematocrit. As blood flows through the arterioles, red cells will act a feed hematocrit (Hf), while in the capillaries a tube hematocrit (Ht) occurs. In tube hematocrit plasma fills most of the vessel while the red cells travel through in somewhat of a single file line. From this stage, blood will enter the venules increasing in hematocrit, in other words the discharge hematocrit (Hd).
   Veins are present throughout the body as tubes that carry blood back to the heart. Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small. 
  
  BULLET::::- Superficial veins are those closer to the surface of the body, and have no corresponding arteries.
  
  BULLET::::- Deep veins are deeper in the body and have corresponding arteries.
  
  BULLET::::- Perforator veins drain from the superficial to the deep veins. These are usually referred to in the lower limbs and feet.
  
  BULLET::::- Communicating veins are veins that directly connect superficial veins to deep veins.
   BULLET::::- "Arterial bleeding:" As the name suggests, blood flow originating in an artery. With this type of bleeding, the blood is typically bright red to yellowish in colour, due to the high degree of oxygenation. Blood typically exits the wound in spurts, rather than in a steady flow; the blood spurts out in time with the heartbeat. The amount of blood loss can be copious, and can occur very rapidly.
   BULLET::::- Resistance vessels – arterioles, pre- and postcapillaries.
  
  BULLET::::- Metabolic vessels – capillaries
  
  BULLET::::- Capacitance vessels – veins
  
  Particular feature of resistance vessels is ability to change lumen crossectional area and influence blood pressure. Human arteries or arterioles that are around 0.2 mm or smaller contribute to creation of the blood flow resistance and are called resistance arteries.
  
  Section::::Physiology.
   Section::::Structure.:Cardiovascular system.:Capillaries.
  
  Arteries branch into small passages called arterioles and then into the capillaries. The capillaries merge to bring blood into the venous system.
  
  Section::::Structure.:Cardiovascular system.:Veins.
  
  Capillaries merge into venules, which merge into veins. The venous system feeds into the two major veins: the superior vena cava – which mainly drains tissues above the heart – and the inferior vena cava – which mainly drains tissues below the heart. These two large veins empty into the right atrium of the heart.
  
  Section::::Structure.:Cardiovascular system.:Portal veins.
   There are two types of trauma: penetrating trauma and blunt trauma.
  
  BULLET::::- Penetrating trauma is the most common cause of vascular injury and can result in internal bleeding. It can occur after a ballistic injury or stab wound. If penetrating trauma occurs in blood vessels close to the heart, it can quickly lead to hemorrhagic or hypovolemic shock , exsanguination, and death.
  
  BULLET::::- Blunt trauma is another cause of vascular injury that can result in internal bleeding. It can occur after a high speed deceleration in an automobile accident.
  
  Section::::Causes.:Non-traumatic.
   Blood flows from the heart through arteries, which branch and narrow into arterioles, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to become venules, which in turn widen and converge to become veins, which then return blood back to the heart through the venae cavae.
   Section::::Symptoms.
  
  Section::::Symptoms.:Complications.
  
  Following are some complications of coagulopathies, some of them caused by their treatments:
  
  Section::::Causes.
  
  While there are several possible causes, they generally result in excessive bleeding and a lack of clotting.
  
  Section::::Causes.:Acquired.
  
  Acquired causes of coagulopathy include anticoagulation with warfarin, liver failure, vitamin K deficiency and disseminated intravascular coagulation. Additionally, the haemotoxic venom from certain species of snakes can cause this condition, for example Bothrops, rattlesnakes and other species of viper. Viral hemorrhagic fevers include dengue hemorrhagic fever and dengue shock syndrome.
   Section::::Blood pressure.
  
  The blood pressure in the circulation is principally due to the pumping action of the heart. The pumping action of the heart generates pulsatile blood flow, which is conducted into the arteries, across the micro-circulation and eventually, back via the venous system to the heart. During each heartbeat, systemic arterial blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure. In physiology, these are often simplified into one value, the mean arterial pressure (MAP), which is calculated as follows:
  
  where:
  
  BULLET::::- MAP = Mean Arterial Pressure
  
  BULLET::::- BP = Diastolic blood pressure
  
presuppositions: Bleeding only comes from veins or arteries.
corrections: Capillaries in the skin contain blood and bleed when cut, as well as veins or arteries.
raw_presuppositions: Bleeding only comes from veins or arteries. Bleeding only comes from veins or arteries.
raw_corrections: Capillaries in the skin contain blood and bleed when cut, as well as veins or arteries. Bleeding does not only come from veins or arteries.