tissue perfusion

Examples of tissue perfusion in the following topics:

• Physical Characteristics and Volume

  • Blood is a fluid that is technically considered a connective tissue.
  • In order to maintain homeostasis, blood volume and blood pressure must be high enough that blood can reach all of the body's tissues, a process called tissue perfusion.
  • Most tissues can survive without perfusion for a short amount of time, but the brain needs a continuous supply of oxygen and glucose to stay alive.
  • Many mechanisms exist to regulate blood volume and tissue perfusion, including renal water excretion in the kidney, the pumping activity of the heart, and the abilities of the arteries to constrict or dilate.
  • When blood volume becomes too low, such as from an injury, dehydration, or internal bleeding, the body will enter into a state of hypovolemic shock, in which tissue perfusion decreases too much.

• Internal Respiration

  • The first is the exchange of gasses between the bloodstream and the tissues.
  • Oxygen diffuses into the cells of the tissues, while carbon dioxide diffuses out of the cells of the tissues and into the bloodstream.
  • The factors that influence tissue gas exchange are similar to the factors of alveolar gas exchange, and include partial pressure gradients between the blood and the tissues, the blood perfusion of those tissues, and the surface areas of those tissues.
  • Each of those factors generally increase gas exchange as those factors are increased (i.e., more oxygen diffusion in tissues with more blood perfusion).
  • Human cells may use lactic acid fermentation in muscle tissue during strenuous exercise when there isn't enough oxygen to power the tissues.

• Functions of Blood

  • The main function of blood is to supply oxygen to tissues and remove carbon dioxide.
  • Tissues cannot survive very long without these two molecules.
  • These terms are used to describe oxygen or blood deficiency to tissues in the body:
  • Hypoxia- a state in which the tissues do not receive adequate oxygen supply, generally due to decreased tissue perfusion or decreased oxygen intake.
  • Most tissues can survive in a hypoxic or ischemic state for a few hours before infarction sets in.

• Compartment Syndrome

  • Compartment syndrome is caused by the compression of tissues within a limb from swelling following an injury, resulting in tissue necrosis.
  • Injuries are the most common cause of compartment syndrome, as they cause swelling of the affected tissue.
  • The swelling of the tissue forces pressure upon the muscle compartments, which has a limited volume.
  • Elevation of the affected limb in patients with compartment syndrome is not recommended, as this leads to decreased vascular perfusion of the affected region.
  • Failure to relieve the pressure can result in necrosis of tissue in that compartment, since capillary perfusion will fall leading to increasing hypoxia of those tissues.

• External Respiration

  • Respiration is the transport of oxygen to the cells within tissues and the transport of carbon dioxide in the opposite direction.
  • While a severe ventilation–perfusion mismatch indicates severe lung disease, minor imbalances can be corrected by maintaining air flow that is proportional to capillary blood flow, which maintains the balance of ventilation and perfusion.
  • Perfusion in the capillaries adjusts to changes in PAO2.
  • When airflow becomes higher relative to perfusion, PACO2 decreases, so the bronchioles will constrict in order to maintain to the balance between airflow (ventilation) and perfusion.
  • External respiration is a result of partial pressure gradients, alveolar surface area, and ventilation and perfusion matching.

• Lungs

  • The outer layer of the lungs are the pleura, a type of mesothelium (membrane tissue) that surrounds the lung and attaches it to the thoracic cavity.
  • Only a relatively small proportion of alveoli in the lungs are perfused with blood and actually take part in gas exchange.
  • The ratio of ventilation in the lungs and perfusion of the lungs (the air and blood supply of the alveoli respectively) is called the V/Q ratio (with Q being perfusion); it is an important indicator of efficiency in the lungs.
  • Too low perfusion (and a higher ratio) indicates alveolar dead space, while too low ventilation (and a lower ratio) indicates a shunt, which is a lack of air supply relative to perfusion.

• Congestive Heart Failure

  • Initially, this helps compensate for heart failure by maintaining blood pressure and perfusion, but it places further strain on the myocardium, increasing coronary perfusion requirements, which can lead to worsening of ischemic heart disease.
  • Reduced perfusion (blood flow) to the kidneys stimulates the release of renin – an enzyme that catalyzes the production of the potent vasopressor angiotensin .
  • This shifts the balance of forces in favor of interstitial fluid formation as the increased pressure forces additional fluid out of the blood, into the tissue.
  • This results in edema (fluid build-up) in the tissues.
  • Reduced perfusion (blood flow) to the kidneys stimulates the release of renin, an enzyme that catalyzes the production of the potent vasopressor angiotensin.

• Homeostatic Responses to Shock

  • In the early stages this is generally an inadequate tissue level of oxygen .
  • As anaerobic metabolism continues, increasing the body's metabolic acidosis, the arteriolar smooth muscle and precapillary sphincters relax such that blood remains in the capillaries and will lead to leakage of fluid and protein into the surrounding tissues.
  • The prolonged vasoconstriction will also cause the vital organs to be compromised due to reduced perfusion.
  • Adenosine easily perfuses out of cellular membranes into extracellular fluid, furthering capillary vasodilation, and then is transformed into uric acid.
  • Adenosine easily perfuses out of cellular membranes into extracellular fluid, furthering capillary vasodilation, and then is transformed into uric acid.

• Internal Anatomy of the Kidneys

  • The kidneys are surrounded by a renal cortex, a layer of tissue that is also covered by renal fascia (connective tissue) and the renal capsule.
  • The renal cortex is granular tissue due to the presence of nephrons—the functional unit of the kidney, that are located deeper within the kidney, within the renal pyramids of the medulla.
  • The cortex provides a space for arterioles, and venules from the renal artery and vein, as well as the glomerular capillaries to perfuse the nephrons of the kidney.
  • The medulla consists of multiple pyramidal tissue masses, called the renal pyramids, which are triangle structures that contain a dense network of nephrons.

• Blood Supply to the Lungs

  • Systemic circulation refers to blood supply to the rest of the body, for the purpose of supplying oxygen to the tissues.
  • Bronchial circulation (by the bronchial arteries) supplies blood to the tissues of the bronchi and the pleura, and is considered part of systemic circulation.
  • Capillaries are the thinnest and smallest type of blood vessel, and they supply oxygen to individual tissues everywhere in the human body.
  • The most serious issue in pulmonary circulation is a pulmonary embolism, which is where a blood clot travels to the lung and causes an infarction of the lung (tissue death from lack of oxygen).
  • This results in an increase in aveolar dead space and decreased perfusion, (leading to shortness of breath and chest pain) and can be fatal if not treated in time by fibrinolytics (medications that dissolve the clot).