question: Why is it that when you cut yourself, even if you don't touch a blood vessel, you bleed?
comment: There are millions and millions of blood vessels all throughout your body. Some are so tiny that blood cells have to travel through them single file. Every cell in your body needs blood flow to live, except for dead cells like your hair and the very top layer of your skin. Even if you can't see a vein, there's still a blood vessel there. It's just microscopic.
passages: Vessel length is the total length of the vessel measured as the distance away from the heart. As the total length of the vessel increases, the total resistance as a result of friction will increase.
  
  Vessel radius also affects the total resistance as a result of contact with the vessel wall. As the radius of the wall gets smaller, the proportion of the blood making contact with the wall will increase. The greater amount of contact with the wall will increase the total resistance against the blood flow.
  
  Section::::Disease.
   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.
   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.
   Coagulation or blood clotting relies on, in addition to the production of fibrin, interactions between platelets. When the endothelium or the lining of a blood vessel is damaged, connective tissue including collagen fibers is locally exposed. Initially, platelets stick to the exposed connective tissue through specific cell-surface receptors. This is followed by platelet activation and aggregation in which platelets become firmly attached and release chemicals that recruit neighboring platelets to the site of vascular injury. A meshwork of fibrin then forms around this aggregation of platelets to increase the strength of the clot.
  
  Section::::Transient interactions.:Cell interactions between bacteria.
   Vascular permeability
  
  Vascular permeability, often in the form of capillary permeability or microvascular permeability, characterizes the capacity of a blood vessel wall to allow for the flow of small molecules (drugs, nutrients, water, ions) or even whole cells (lymphocytes on their way to the site of inflammation) in and out of the vessel. Blood vessel walls are lined by a single layer of endothelial cells. The gaps between endothelial cells (cell junctions) are strictly regulated depending on the type and physiological state of the tissue.
   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.
   There are five types of blood vessels: the arteries, which carry the blood away from the heart; the arterioles; the capillaries, where the exchange of water and chemicals between the blood and the tissues occurs; the venules; and the veins, which carry blood from the capillaries back towards the heart.
  
  The word "vascular", meaning relating to the blood vessels, is derived from the Latin "vas", meaning vessel. Some structures -- such as cartilage, the epithelium, and the lens and cornea of the eye -- do not contain blood vessels and are labeled "avascular".
  
  Section::::Structure.
   The phenomenon of ischemic preconditioning (IPC) was discovered in 1986 by C. E. Murry and colleagues, who observed that repeated, temporary cross-clamping of the left anterior descending artery (LAD) in dogs protected the LAD territory of the heart against a subsequent prolonged ischemic event, reducing infarct size by 75%. This was thought to be a local effect and was termed local ischemic preconditioning. The phenomenon was confirmed by other researchers in dogs, pigs, mice, and rats.
   Campbell was the first to discover that cells of bone marrow origin contribute to intimal (the innermost coat of blood vessels) thickening in arteries subjected to injury, rather than solely from cells of the artery wall. This showed that current strategies to prevent restenosis after angioplasty of blocked arteries may have been targeting the incorrect cell type.
   Blood, like electric current, flows along the path of least resistance. Resistance is affected by the length and width of a vessel (i.e. a long, narrow vessel has the greatest resistance and a short, wide one the least), but crucially in the human body width is generally more limiting than length because of Poiseuille's Law. Thus, if blood is presented with two paths, a short one that is narrow (with a high overall resistance) and a long one that is wide (with a low overall resistance), it will take the long and wide path (the one with the lower resistance).
   BULLET::::- the occipital artery which runs posteriorly to supply much of the posterior aspect of the scalp
  
  BULLET::::- the posterior auricular artery, a branch of the external carotid artery, ascends behind the auricle to supply the scalp above and behind the auricle.
  
  Because the walls of the blood vessels are firmly attached to the fibrous tissue of the superficial fascial layer, cut ends of vessels here do not readily retract; even a small scalp wound may bleed profusely.
  
  Venous drainage
   BULLET::::4. Vascular sprouting: With the breakdown of endothelial basement membrane, detached endothelial cells from pre-existing capillaries and post-capillary venules can divide and migrate chemotactically towards the wound, laying down new vessels in the process. Vascular sprouting can be aided by ambient hypoxia and acidosis in the wound environment, as hypoxia stimulates the endothelial transcription factor, hypoxia inducible factor (HIF) to transactivate angiogenic genes such as VEGF and GLUT1. Sprouted vessels can self-organise into luminal morphologies, and fusion of blind channels give rise to new capillary networks.
   Considering that the maximal expression of T-cadherin has been observed in nervous and cardiovascular systems, it is likely that T-cadherin is involved in guiding the growing vessel as well. The mechanism of T-cadherin mediated negative guidance in nervous system involves homophilic interaction and contact inhibition; in vascular system it is supposed that T-cadherin expressing blood vessels would avoid T-cadherin expressing tissues.
   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.
   VOLT capillaries do not have a blood-brain barrier, and so neurons in this region can respond to circulating factors present in the systemic circulation.
   Under normal conditions, to avoid atherogenesis, thrombosis, smooth muscle proliferation and endothelial apoptosis, shear stress maintains its magnitude and direction within an acceptable range. In some cases occurring due to blood hammer, shear stress reaches larger values. While the direction of the stress may also change by the reverse flow, depending on the hemodynamic conditions. Therefore, this situation can lead to atherosclerosis disease.
  
  Section::::Blood vessels.:Capacitance.
  
  Veins are described as the "capacitance vessels" of the body because over 70% of the blood volume resides in the venous system. Veins are more compliant than arteries and expand to accommodate changing volume.
   BULLET::::2. When the confluency of the cells are ideal, use a pipette tip to scratch a wound through the entire center of the well. As mentioned previously, this is where the potential inconsistency comes into play with this assay. If the scratch is made manually, it is important to make sure that the wound is visible on both side of the field of view and should be around 0.5 mm wide.
  
  BULLET::::3. The cells can then be placed on a microscope with a relative objective of 20x.
   The human skin has a somewhat elastic property as a self-defense; when the human body is stabbed by a thin object such as a small Kitchen knife, the skin often closes tightly around the object and closes again if the object is removed, which can trap some blood within the body. It has thus been speculated that the fuller, an elongated concave depression in a metal blade, functions to let blood out of the body in order to cause more damage. This misconception has led to fullers becoming widely known as "blood grooves". The fuller is actually a structural reinforcement of the blade similar in design to a metal I-beam used in construction. However, internal bleeding is just as dangerous as external bleeding; if enough blood vessels are severed to cause serious injury, the skin's elasticity will do nothing to prevent blood from exiting the circulatory system and accumulating uselessly in other parts of the body.
   In EVH, clinical experience, comprehensive training and a careful approach are needed when cutting and removing side branches from the main vessel being harvested. As branches are cut, they must be sealed, or cauterized, to prevent bleeding after disconnection from the main vessel. In surgical practice it is common to use a form of heat or electrical energy to cauterize, or seal tissues in order to stop bleeding. If not properly controlled, the application of heat or energy to seal off the branch vessels can cause unintended thermal injury to the surrounding cells including the endothelium of the main vessel or conduit.
   In large vessels with low hematocrit, viscosity dramatically drops and red cells take in a lot of energy. While in smaller vessels at the micro-circulation scale, viscosity is very high. With the increase in shear stress at the wall, a lot of energy is used to move cells.
  
  Section::::Shear rate relations.
   There are several techniques to measure vascular permeability to certain molecules. For instance, the cannulation of a single microvessel with a micropipette, the microvessel is perfused with a certain pressure, occluded downstream and then the velocity of some cells will be related to the permeability. Another technique uses multiphoton fluorescence intravital microscopy through which the flow is related to fluorescence intensity and the permeability is estimated from the Patlak transformation of the intensity data 
   Section::::Background.
  
  If the blood supply to an organ or a tissue is impaired for a short time (usually less than five minutes) then restored so that blood flow is resumed, and the process repeated two or more times, the cells downstream of the tissue or organ are robustly protected from a final ischemic insult when the blood supply is cut off entirely and permanently.
   During times of blockage of one of the arteries that supply the watershed area, such as in atherosclerosis, these regions are spared from ischemia by virtue of their dual supply. However, during times of systemic hypoperfusion, such as in disseminated intravascular coagulation or heart failure, these regions are particularly vulnerable to ischemia because they are supplied by the most distal branches of their arteries, and thus the least likely to receive sufficient blood.
   Section::::Family history.
   Studies have shown that when vascular trauma occurs, EPCs and circulating endothelial progenitors (CEPs) are attracted to the site due to the release of specific chemokines. CEPs are derived from EPCs within the bone marrow, and the bone marrow is a reservoir of stem and progenitor cells. These cell types accelerate the healing process and prevent further complications such as hypoxia by gathering the cellular materials to reconstruct the endothelium.
  
presuppositions: You can cut yourself without touching a blood vessel. You can cut yourself without touching a blood vessel
corrections: Every live cell has blood flowing in and out of it, so if a live cell is cut, blood vessels are cut. There are millions and millions of blood vessels all through your body. Even if you can't see it, there's still a blood vessel there. 
raw_presuppositions: You can cut yourself without touching a blood vessel. You can cut yourself without touching a blood vessel
raw_corrections: Every live cell has blood flowing in and out of it, so if a live cell is cut, blood vessels are cut. There are millions and millions of blood vessels all through your body. Even if you can't see it, there's still a blood vessel there. 