What Determines The Direction Of Gas Movement

The circulatory system plays a crucial role in supplying oxygen and nutrients to tissues while removing metabolic waste products. Capillaries, which make up the majority of the vasculature, facilitate gas and nutrient exchange between the blood and surrounding tissues. However, the mechanisms that efficiently distribute red blood cells (RBCs) and plasma throughout capillary networks are not fully understood, particularly in the brain.

The Complexity of Brain Vasculature

The brain’s vasculature consists of interconnected surface vessels that give rise to arterioles, which penetrate into the brain and supply a vast network of capillaries. Arterioles are composed of endothelial cells (ECs) surrounded by smooth muscle cells and an internal elastic lamina. Capillaries, on the other hand, lack smooth muscle cells and have pericytes embedded in their basement membrane.

Brain Vasculature

Pericytes, found in all capillary microcirculations, are especially abundant in the retina and cerebral circulations. These cells have distinct morphological characteristics, including a prominent nucleus aligning with the vessel lumen, extensions along the long axis, and projections wrapping around the endothelial tube. Their role in regulating blood flow within capillary networks is paramount.

Precision and Efficiency: Neurovascular Coupling

The brain requires a significant amount of energy and is highly sensitive to disruptions in blood flow. To ensure optimal functioning, the brain has evolved mechanisms to allocate blood flow to regions of higher neuronal activity. This phenomenon, known as neurovascular coupling (NVC), relies on intricate signaling between neurons and the cerebral vasculature.

Recent research has revealed that capillary endothelial cells possess inwardly rectifying potassium (Kir) channels. When neurons are active, they produce potassium (K+) as a byproduct. The activation of these Kir channels by K+ triggers a propagating electrical signal that induces arteriolar dilation and increases blood flow into the capillary network. While this mechanism communicates the need for increased blood flow to active regions, it does not address how blood flow is regulated within the capillary network itself.

The Role of Pericytes in Blood Distribution

In capillary networks associated with a single parenchymal arteriole, the distribution of blood within a nonuniform neural activity volume could be stochastic without downstream control mechanisms. However, research suggests that junctional pericytes may serve as dynamic controllers of blood distribution within capillary networks. These pericytes are primarily located at capillary junctions and have the ability to structurally and dynamically determine the geometry of these junctions.

Pericytes Regulating Blood Flow

By relaxing proximal pericytes in response to hyperpolarizing signals triggered by K+, blood flow can be directed towards the source of the signal. This mechanism also contributes to NVC, ensuring that blood flow is directed towards regions with increased neuronal activity. These findings challenge the notion that blood flow within the capillary network is a passive process solely dictated by the static architecture of the vasculature. Instead, pericytes play a crucial role in dynamically controlling blood perfusion to fine-tune the delivery of oxygen and nutrients to the surrounding tissue.


Q: What is the role of capillaries in the circulatory system?
A: Capillaries serve as the sites where gas and nutrient exchange occurs between the blood and surrounding tissues. They play a crucial role in delivering oxygen and nutrients to cells while removing metabolic waste products.

Q: What are pericytes?
A: Pericytes are specialized cells found in the capillary microcirculation of all vascular beds. They are particularly abundant in the cerebral and retinal circulations. Pericytes play a vital role in regulating blood flow within the capillary network and fine-tuning the delivery of oxygen and nutrients to tissues.


Understanding the mechanisms that determine the direction of gas movement, particularly in the brain, is an ongoing area of research. The presence of pericytes in the capillary microcirculation highlights their significance in regulating blood flow and optimizing the delivery of oxygen and nutrients to meet the metabolic demands of tissues. By unraveling these complexities, researchers can further enhance our understanding of the circulatory system’s intricate workings.

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