The flow of blood in the capillaries is controlled by precapillary sphincters. A precapillary sphincter is the muscular fibers that control the movement of blood between the arterioles and capillaries. Regulation of fluid movement between the capillaries and the surrounding interstitial tissues is determined by the balance of two forces: the hydrostatic pressure and osmotic pressure.
On the arterial side of the capillary, the hydrostatic pressure the pressure that comes from the heart pumping blood and the elasticity of the arteries is high. Since capillaries are "leaky" this pressure forces fluid and nutrients against the walls of the capillary and out into the interstitial space and tissues. On the vein side of the capillary, the hydrostatic pressure has dropped significantly.
At this point, it is the osmotic pressure of the fluid within the capillary due to the presence of salts and proteins in the blood that draws fluids back into the capillary. Osmotic pressure is also referred to as oncotic pressure and is what pulls fluids and waste products out of the tissues and into the capillary to be returned to the bloodstream and then delivered to the kidneys among other sites.
Capillaries are important medically in many ways, and there are ways that you can actually indirectly observe these tiny blood vessels. If you've ever wondered why your skin turns white when you put pressure on it the answer is the capillaries.
Pressure on the skin presses blood out of the capillaries resulting in the blanching or pale appearance when the pressure is removed. If you develop a rash, a physician may push on your skin to see if the spots turn white. When capillaries are broken, the blood leaks into the skin and the red spots will remain even with pressure. These are called petechiae and are associated with different conditions than rashes that do blanch with pressure. Doctors often check for "capillary refill.
An example of this use would be in people with burns. A second-degree burn may reveal capillary refill to be somewhat delayed, but in a third-degree burn, there would be no capillary refill at all.
Emergency responders often check capillary refill by pushing on a fingernail or toenail, then releasing pressure and waiting to see how long it takes for the nailbed to appear pink again. If color returns within two seconds the amount of time it takes to say capillary refill , circulation to the arm or leg is probably OK. If capillary refill takes more than two seconds, the circulation of the limb is probably compromised and considered an emergency.
There are other settings in which capillary refill is delayed as well, such as in dehydration. You may hear doctors talk about a phenomenon known as "third spacing. Capillary permeability can be increased by cytokines leukotrienes, histamines, and prostaglandins released by cells of the immune system.
The increased fluid third spacing locally can result in hives. When someone is very ill, this third spacing due to leaky capillaries may be widespread, giving their body a swollen appearance.
Most of the time when you have your blood drawn, a technician will take blood from a vein in your arm. Capillary blood may also be used to do some blood tests, such as for those who monitor their blood sugar. A lancet is used to cut the finger cut capillaries and can be used for testing blood sugar and blood pH.
There are several common and uncommon conditions that involve the capillaries. A small percentage of children are born with "birthmarks" consisting of an area of red or purple skin related to dilated capillaries. Most port-wine stains are a cosmetic problem rather than a medical concern, but they may bleed easily when irritated. Capillary malformation arteriovenous malformation syndrome may occur as part of an inherited syndrome present in roughly 1 in , people of European ancestry.
In this syndrome, there is more blood flow than normal through the capillaries near the skin, which results in pink and red dots on the skin. The may occur alone, or people may have other complications of this syndrome such as arteriovenous malformations abnormal connections between arteries and veins which, when in the brain, can cause headaches and seizures.
A rare disorder known as capillary leak syndrome involves leaky capillaries which result in constant nasal congestion and episodes of fainting due to rapid drops in blood pressure. Macular degeneration , now the leading cause of blindness in the United States, occurs secondary to damage in the capillaries of the retina. Though the tiniest of blood vessels, capillaries play the biggest role in being the location where oxygen and carbon dioxide are exchanged in all tissues, and where nutrients are delivered and waste is removed from cells.
Capillaries also very important in medical diagnosis and give sometimes critical information on a person's medical condition. While once thought to be responsible for primarily cosmetic conditions, that's changed as their role in macular degeneration was found.
The function of capillaries is to carry blood, nutrients, and oxygen to internal organs and tissues across the body. It does this by acting as the middle connection between veins and arteries. An aneurysm may have no symptoms until it is either very large or it ruptures. Aortic stenosis may be congenital present from before birth , but is often diagnosed during teenage years.
Bleeding may be minor or it may be a life-threatening medical emergency. Content on this website is provided for information purposes only. Information about a therapy, service, product or treatment does not in any way endorse or support such therapy, service, product or treatment and is not intended to replace advice from your doctor or other registered health professional.
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Summary Read the full fact sheet. Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. The presence of excess tissue fluid around the cells leads to a condition called edema. Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet like most health professionals.
However, clinical edema goes beyond normal swelling and requires medical treatment. Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated. Figure 7. Varicose veins are commonly found in the lower limbs. Edema may be accompanied by varicose veins, especially in the superficial veins of the legs.
This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy. More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing.
Without treatment, they tend to grow worse over time. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. Severe cases may require conventional surgery to remove the damaged vessels. As there are typically redundant circulation patterns, that is, anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation.
There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot. In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time. Their ability to hold this much blood is due to their high capacitance , that is, their capacity to distend expand readily to store a high volume of blood, even at a low pressure.
The large lumens and relatively thin walls of veins make them far more distensible than arteries; thus, they are said to be capacitance vessels. When blood flow needs to be redistributed to other portions of the body, the vasomotor center located in the medulla oblongata sends sympathetic stimulation to the smooth muscles in the walls of the veins, causing constriction—or in this case, venoconstriction. This increases pressure on the blood within the veins, speeding its return to the heart.
As you will note in the image above, approximately 21 percent of the venous blood is located in venous networks within the liver, bone marrow, and integument. This volume of blood is referred to as venous reserve. Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system.
This includes repair and replacement of diseased or damaged vessels, removal of plaque from vessels, minimally invasive procedures including the insertion of venous catheters, and traditional surgery.
Following completion of medical school, the physician generally completes a 5-year surgical residency followed by an additional 1 to 2 years of vascular specialty training. Vascular technicians are specialists in imaging technologies that provide information on the health of the vascular system. They may also assist physicians in treating disorders involving the arteries and veins.
This profession often overlaps with cardiovascular technology, which would also include treatments involving the heart. Although recognized by the American Medical Association, there are currently no licensing requirements for vascular technicians, and licensing is voluntary. The United States Bureau of Labor projects this profession to grow by 29 percent from to Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart.
Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart. The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross section.
The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa.
Capillaries have only a tunica intima layer. The tunica intima is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue. The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. It is the thickest layer in all but the largest arteries. The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle.
Blood flow through vessels can be dramatically influenced by vasoconstriction and vasodilation in their walls. Answer the question s below to see how well you understand the topics covered in the previous section.
Skip to main content. Search for:. Structure and Function of Blood Vessels Learning Objectives By the end of this section, you will be able to: Compare and contrast the three tunics that make up the walls of most blood vessels Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function Describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains Explain the structure and function of venous valves in the large veins of the extremities.
Disorders of the Cardiovascular System: Edema and Varicose Veins Despite the presence of valves and the contributions of other anatomical and physiological adaptations we will cover shortly, over the course of a day, some blood will inevitably pool, especially in the lower limbs, due to the pull of gravity.
Careers in Action: Vascular Surgeons and Technicians Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system. Visit this site to learn more about vascular surgery. Visit this site to learn more about vascular technicians.
Critical Thinking Questions Arterioles are often referred to as resistance vessels. Cocaine use causes vasoconstriction. Is this likely to increase or decrease blood pressure, and why? A blood vessel with a few smooth muscle fibers and connective tissue, and only a very thin tunica externa conducts blood toward the heart. 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.
These attacks can be severe and require emergency medical attention. People with arteriovenous malformation syndrome AVM have an abnormal tangle of arteries and veins that are connected to each other without capillaries in between. These tangles can occur anywhere in the body, but are most often found in the brain and spinal cord.
This can cause lesions that interfere with blood flow and oxygen delivery. These lesions may also cause bleeding into the surrounding tissue. However, in some cases, it can cause:. Treatment usually involves surgically removing or closing the AVM lesion.
Medication can also help to manage symptoms, such as pain or headaches. Microcephaly-capillary malformation syndrome is a rare genetic condition that starts before birth. People with this condition have smaller heads and brains.
They also have widened capillaries that increase the flow of blood near the skin surface, which can cause pinkish red spots on the skin. Mutations to this gene can result in cells dying during development, affecting the entire development process. Treatment for this condition can involve stimulation — particularly through sound and touch — bracing to maintain posture, and anticonvulsant medication therapy for management of seizures.
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