physiology Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...

, acute local blood flow regulation refers to an intrinsic regulation, or control, of the

vascular tone Vascular resistance is the resistance that must be overcome for blood to flow through the circulatory system. The resistance offered by the systemic circulation is known as the systemic vascular resistance or may sometimes be called by another ter ...

arteries An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in ...

at a local level, meaning within a certain tissue type,

organ Organ and organs may refer to: Biology * Organ (biology), a group of tissues organized to serve a common function * Organ system, a collection of organs that function together to carry out specific functions within the body. Musical instruments ...

, or

organ system An organ system is a biological system consisting of a group of organ (biology), organs that work together to perform one or more bodily functions. Each organ has a specialized role in an organism body, and is made up of distinct Tissue (biolog ...

. This intrinsic type of control means that the

blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...

s can automatically adjust their own vascular tone, by dilating (widening) or constricting (narrowing), in response to some change in the environment. This change occurs in order to match up the tissue's oxygen demand with the actual oxygen supply available in the blood as closely as possible. For example, if a

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

is being utilized actively, it will require more oxygen than it was at rest, so the blood vessels supplying that muscle will vasodilate, or widen in size, to increase the amount of blood, and therefore oxygen, being delivered to that muscle. There are several mechanisms by which vascular tone, and therefore blood flow, is controlled. The

sympathetic nervous system The sympathetic nervous system (SNS or SANS, sympathetic autonomic nervous system, to differentiate it from the somatic nervous system) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous sy ...

and a variety of

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

s, for instance, both exert some degree of control over vascular tone. However, the local intrinsic regulatory system described here is completely independent of these other mechanisms. Many organs or organ systems have their own unique mechanism of local blood flow regulation, as explained below.

Individual Mechanisms

There are two major means of local regulation of blood flow, which are described below. # Metabolic control, which consists of metabolites and paracrine agents released from surrounding tissue that act on the blood vessel(s). For example, as tissue metabolism increases, driving up oxygen demand, the amount of available oxygen decreases, driving down the pH and triggering a release in

adenosine Adenosine (symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9- glycosidic bond. Adenosine is one of the four nucleoside build ...

, which triggers the blood vessel to vasodilate. #

Myogenic The myogenic mechanism is how arteries and arterioles react to an increase or decrease of blood pressure to keep the blood flow constant within the blood vessel. Myogenic response refers to a contraction initiated by the myocyte itself instead of ...

control, which originates from the wall of the blood vessel itself and consists of both muscle reflexes and products released from

endothelial cells The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the res ...

that line the vessel. These endothelial products include

nitric oxide Nitric oxide (nitrogen oxide, nitrogen monooxide, or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes den ...

and

endothelin-1 Endothelin 1 (ET-1), also known as preproendothelin-1 (PPET1), is a potent vasoconstrictor peptide produced by vascular endothelial cells, as well as by cells in the heart (affecting contractility) and kidney (affecting sodium handling). The prote ...

that are released in response to either chemical stimuli, like

histamine Histamine is an organic nitrogenous compound involved in local immune responses communication, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Discovered in 19 ...

, or increased

shear stress Shear stress (often denoted by , Greek alphabet, Greek: tau) is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross secti ...

on the blood vessel (meaning the amount of stress exerted by blood on the blood vessel walls). While nitric oxide causes vasodilation, endothelin-1 causes vasoconstriction.

Examples of local blood flow regulation

Below are several examples of differing types of local blood flow regulation by specific organ type or organ system. In each case, there is a specific type of intrinsic regulation occurring in order to maintain or alter blood flow to that given organ alone, instead of creating a systemic change that would affect the entire body. * Cerebral (brain) circulation is highly sensitive to changes in pCO₂, meaning the amount of dissolved

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂) present in the blood vessel, as well as the hydrogen ion concentration. Both of these factors affect pH and, in turn, the balance between vasodilation versus vasoconstriction in the brain. So, the blood vessels found specifically in the brain respond changes in dissolved carbon dioxide levels. * Coronary (heart) circulation is controlled at the local level primarily by metabolic control mechanism. More specifically it is regulated by adenosine, a local vasodilator produced by neighboring cells. Therefore, the heart is influenced by a form of metabolic control through the effects of paracrine signaling. * Renal (kidney) circulation is primarily controlled by

Tubuloglomerular Feedback In the renal physiology, physiology of the kidney, tubuloglomerular feedback (TGF) is a feedback system inside the kidneys. Within each nephron, information from the nephron#Renal tubule, renal tubules (a downstream area of the tubular fluid) is si ...

, which is a system of organ-specific autoregulation that directly affects renal blood flow. * Pulmonary (lung) circulation undergoes hypoxic vasoconstriction, which is a unique mechanism of local regulation in that the blood vessels in this organ react to

hypoxemia Hypoxemia (also spelled hypoxaemia) is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia is usually caused by pulmonary disease. Sometimes the concentration of oxygen in the ...

, or low levels of dissolved oxygen in blood, in the opposite way as the rest of the body. While tissues and organs tend to increase blood flow by vasodilating in response to low oxygen supply, the blood vessels in the lungs actually vasoconstrict to decrease blood flow in response to low oxygen. *

Splanchnic Splanchnic is usually used to describe organs in the abdominal cavity. It is used when describing: * Splanchnic tissue * Splanchnic organs - including the stomach, small intestine, large intestine, pancreas, spleen, liver, and may also include the ...

circulation, which supplies blood to several gastrointestinal organs (liver, gallbladder, pancreas, intestines) and the spleen, is influenced by

gastrointestinal hormone The gastrointestinal hormones (or gut hormones) constitute a group of hormones secreted by enteroendocrine cells in the stomach, pancreas, and small intestine that control various functions of the digestive organs. Later studies showed that most ...

s and metabolites, such as vasodilatory

kinin A kinin is any of various structurally related peptides, such as bradykinin and kallidin. They are members of the autacoid family. Kinins are peptides that are cleaved from kininogens by the process of kallikreins. Kallikreins activate kinins ...

s, released from the cells lining the intestines,

bile acid Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver in peroxisomes. Bile acids are conjugated with taurine or glycine residues to give anions called bile ...

s from the gallbladder, and by products of digestion. This is an example of control at the organ system level as this group of organs all receive blood flow from one central source, the splanchnic artery. *

Skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

is influenced by multiple factors. First, metabolites that are produced by active muscle use can alter skeletal muscle tone. Second, skeletal muscle can undergo hyperemia, which is a mechanism of local blood flow regulation with two major subtypes. Regardless of the subtype, the result of hyperemia is an increase in blood flow to the affected skeletal muscle. ** Active hyperemia is one subtype, which occurs in response to increased metabolic demand, meaning high oxygen requirements within the tissue. It follows the principle of metabolic control, with the release of vasodilatory substances in response to increased oxygen demand. This is classically seen in skeletal muscle in activities like running where muscles are activity being utilized and thus have increased oxygen demand. ** Reactive hyperemia is the second subtype, which occurs after a short interruption, or arrest, in blood flow. In response to the blood flow interruption, a temporary compensatory vasodilation occurs as soon as blood flow has resumed, before returning to normal. This response occurs because vasodilatory substances, like adenosine, are released in response to the blood flow interruption, meaning that when blood flow resumes it occurs in a wider blood vessel and thus at an increased flow rate. This is classically seen in weight lifting, as skeletal muscle can become occluded momentarily during this activity, thus suspending blood flow.

References

{{reflist Angiology

Individual Mechanisms

Examples of local blood flow regulation

See also

References