Systemic arteries transport oxygenated blood from the left ventricle to the body tissues. Blood is pumped from the ventricles into large elastic arteries that branch repeatedly into smaller and smaller arteries until the branching results in microscopic arteries called arterioles. The arterioles play a key role in regulating blood flow into the tissue capillaries. About 10 percent of the total blood volume is in the systemic arterial system at any given time.
The wall of an artery consists of three layers. The innermost layer, the tunica intima also called tunica interna , is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers. The middle layer, the tunica media , is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure.
The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel.
Capillaries, the smallest and most numerous of the blood vessels, form the connection between the vessels that carry blood away from the heart arteries and the vessels that return blood to the heart veins. The primary function of capillaries is the exchange of materials between the blood and tissue cells. Diffusion, the most widely-used mechanism, allows the flow of small molecules across capillaries such as glucose and oxygen from the blood into the tissues and carbon dioxide from the tissue into the blood.
The process depends on the difference of gradients between the interstitium and blood, with molecules moving to low-concentrated spaces from high-concentrated ones. Transcytosis is the mechanism whereby large, lipid-insoluble substances cross the capillary membranes.
The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side. Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall. The movement of materials across the wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces that modulate capillary dynamics.
The net filtration pressure derived from the sum of the four forces described above determines the fluid flow into or out of the capillary. Movement from the bloodstream into the interstitium is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure. Alternatively, movement from the interstitium into the bloodstream is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.
Capillary Dynamics : Oncotic pressure exerted by proteins in blood plasma tends to pull water into the circulatory system. Transcytosis, or vesicle transport, is one of three mechanisms that facilitate capillary exchange, along with diffusion and bulk flow. Substances are transported through the endothelial cells themselves within vesicles. This mechanism is mainly used by large molecules, typically lipid-insoluble preventing the use of other transport mechanisms. Vesicles are capable of merging, allowing for their contents to mix, and can be transported directly to specific organs or tissues.
Due to the function of transcytosis, it can be a convenient mechanism by which pathogens can invade a tissue. Transcytosis has been shown to be critical to the entry of Cronobacter sakazakii across the intestinal epithelium and the blood-brain barrier. Listeria monocytogenes has been shown to enter the intestinal lumen via transcytosis across goblet cells. Shiga toxin secreted by entero-hemorrhagic E. These examples illustrate that transcytosis is vital to the process of pathogenesis for a variety of infectious agents.
Pharmaceutical companies are currently exploring the use of transcytosis as a mechanism for transporting therapeutic drugs across the human blood-brain barrier. Capillary fluid movement occurs as a result of diffusion colloid osmotic pressure , transcytosis, and filtration. Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.
Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall and is dependent on the physical characteristics of the capillary. This is why tissues that are very active, such as your muscles, liver , and kidneys , have an abundance of capillaries.
Capillaries connect the arterial system — which includes the blood vessels that carry blood away from your heart — to your venous system. Your venous system includes the blood vessels that carry blood back to your heart.
The exchange of oxygen, nutrients, and waste between your blood and tissues also happens in your capillaries. This happens through two processes:. Additionally, white blood cells from your immune system can use capillaries to reach sites of infection or other inflammatory damage. There are three types of capillaries.
Each has a slightly different structure that allows to function in a unique way. These are the most common types of capillaries. They contain small gaps in between their endothelial cells that allow for things like gases, water, sugar glucose , and some hormones to pass through. The continuous capillaries in the brain are an exception, however.
These capillaries are part of the blood-brain barrier, which helps to protect your brain by only allowing the most essential nutrients to cross.
They contain small pores, in addition to small gaps between cells, in their walls that allow for the exchange of larger molecules. This type of capillary is found in areas that require a lot of exchange between your blood and tissues. Examples of these areas include:. Sinusoid capillaries allow for the exchange of large molecules, even cells. The surrounding basement membrane is also incomplete with openings in many places. These types of capillaries are found in certain tissues, including those of your liver , spleen, and bone marrow.
For example, in your bone marrow, these capillaries allow newly produced blood cells to enter into the bloodstream and begin circulation. While capillaries are very small, anything unusual in their functioning can cause visible symptoms or even potentially serious medical conditions.
Port wine stains are a type of birthmark caused by the widening of capillaries located in your skin. This widening causes the skin to appear pink or dark red in color, giving the condition its name. Over time, they can darken in color and thicken.
Petechiae are small, round spots that appear on the skin. They happen when capillaries leak blood into the skin. But experts think it may be related to a substance in the blood that damages capillary walls. People with SCLS have recurring attacks during which their blood pressure drops very quickly.
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