Hyperaemia
Encyclopedia
Hyperaemia or hyperemia describes the increase of blood flow to different tissues in the body. It can have medical implications, but is also a regulatory response, allowing change in blood supply to different tissues through vasodilation
. Clinically, hyperaemia in tissues manifest as erythema
, because of the engorgement of vessels with oxygenated blood. Hyperaemia can also occur due to a fall in atmospheric pressure outside the body.
is unclear, but it may have something to do with the opening of precapillary sphincter
s.
flow that occurs when tissue
is active.
When cells
within the body are active in one way or another, they use more oxygen
and fuel, such as glucose
or fatty acid
s, than when they are not. Increased metabolic processes create more metabolic waste. The byproducts of metabolism are vasodilators. (Vasodilating metabolites: CO2, H+, K+, lactate, adenosine) Local arterioles respond to metabolism
by dilatating
, allowing more blood to reach the tissue. This prevents deprivation of the tissue.
Conversely, when a tissue is less metabolically active, it produces fewer metabolites which are simply washed away in blood flow.
This system is called autoregulation.
Since most of the common nutrients in the body are converted to carbon dioxide
when they are metabolized, smooth muscle around blood vessels relax in response to increased concentrations of carbon dioxide within the blood and surrounding interstitial fluid
. The relaxation of this smooth muscle results in vascular dilation and increased blood flow.
Some tissues require oxygen and fuel more quickly or in greater quantities. Examples of tissues and organ
s that are known to have specialized mechanisms for functional hyperaemia include:
. Following ischaemia there will be a shortage of oxygen
and a build-up of metabolic waste
.
This is commonly tested in the legs using Buerger's test
.
Reactive hyperaemia often occurs as a consequence of Raynaud's phenomenon
, where the vasospasm
in the vasculature leads to ischaemia and necrosis
of tissue and thus a subsequent increase in blood flow to remove the waste products and clear up cell debris.
Vasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...
. Clinically, hyperaemia in tissues manifest as erythema
Erythema
Erythema is redness of the skin, caused by hyperemia of the capillaries in the lower layers of the skin. It occurs with any skin injury, infection, or inflammation...
, because of the engorgement of vessels with oxygenated blood. Hyperaemia can also occur due to a fall in atmospheric pressure outside the body.
Hyperaemia and the regulation of blood flow
Functional hyperaemia is an increase in blood flow to a tissue due to the presence of metabolites and a change in general conditions. When a tissue increases activity there is a well-characterized fall in the partial pressure of oxygen and pH, an increase in partial pressure of carbon dioxide, and a rise in temperature and the concentration of potassium ions. The mechanism for vasodilationVasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...
is unclear, but it may have something to do with the opening of precapillary sphincter
Precapillary sphincter
The precapillary sphincter is a band of smooth muscle that adjusts the blood flow into each capillary. At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary. This is called the precapillary sphincter. This sphincter can open and...
s.
Functional hyperaemia
Functional hyperaemia, or active hyperaemia, is the increased bloodBlood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
flow that occurs when tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
is active.
When cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
within the body are active in one way or another, they use more oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and fuel, such as glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
or fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s, than when they are not. Increased metabolic processes create more metabolic waste. The byproducts of metabolism are vasodilators. (Vasodilating metabolites: CO2, H+, K+, lactate, adenosine) Local arterioles respond to metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
by dilatating
Vasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...
, allowing more blood to reach the tissue. This prevents deprivation of the tissue.
Conversely, when a tissue is less metabolically active, it produces fewer metabolites which are simply washed away in blood flow.
This system is called autoregulation.
Since most of the common nutrients in the body are converted to carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
when they are metabolized, smooth muscle around blood vessels relax in response to increased concentrations of carbon dioxide within the blood and surrounding interstitial fluid
Interstitial fluid
Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid...
. The relaxation of this smooth muscle results in vascular dilation and increased blood flow.
Some tissues require oxygen and fuel more quickly or in greater quantities. Examples of tissues and organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s that are known to have specialized mechanisms for functional hyperaemia include:
- The brainBrainThe brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
through the neuronNeuronA neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
-dependent haemodynamic responseHaemodynamic responseHaemodynamics is a medical term for the dynamic regulation of the blood flow in the brain. It is the principle on which functional magnetic resonance imaging is based....
. - PenilePenisThe penis is a biological feature of male animals including both vertebrates and invertebrates...
erectionErectionPenile erection is a physiological phenomenon where the penis becomes enlarged and firm. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal...
tissue by release of nitric oxideNitric oxideNitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
.
Reactive hyperaemia
Reactive hyperaemia is the transient increase in organ blood flow that occurs following a brief period of ischaemiaIschemia
In medicine, ischemia is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue. It may also be spelled ischaemia or ischæmia...
. Following ischaemia there will be a shortage of oxygen
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
and a build-up of metabolic waste
Metabolic waste
Metabolic wastes or excretes are substances left over from excretory processes, which cannot be used by the organism , and must therefore be excreted. This includes nitrogen compounds, water, CO2, phosphates, sulfates, insoles, medicals, food additives etc. Animals treat these compounds as excretes...
.
This is commonly tested in the legs using Buerger's test
Buerger's test
The vascular angle, which is also called buerger's angle, is the angle to which the leg has to be raised before it becomes pale. In a limb with a normal circulation the toes stay pink, even when the limb is raised by 90 degrees. In an ischaemic leg, elevation to 15 degrees or 30 degrees for 30 to...
.
Reactive hyperaemia often occurs as a consequence of Raynaud's phenomenon
Raynaud's phenomenon
In medicine, Raynaud's phenomenon is a vasospastic disorder causing discoloration of the fingers, toes, and occasionally other areas. This condition can also cause nails to become brittle with longitudinal ridges. Named for French physician Maurice Raynaud , the phenomenon is believed to be the...
, where the vasospasm
Vasospasm
Vasospasm refers to a condition in which blood vessels spasm, leading to vasoconstriction. This can lead to tissue ischemia and death . Cerebral vasospasm may arise in the context of subarachnoid hemorrhage. Symptomatic vasospasm or delayed cerebral ischemia is a major contributor to...
in the vasculature leads to ischaemia and necrosis
Necrosis
Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...
of tissue and thus a subsequent increase in blood flow to remove the waste products and clear up cell debris.