24.2 Peripheral Vascular System

Arteries

Tubelike vessels called arteries made of smooth muscles are responsible for transporting blood away from the heart via the aorta. This blood is referred to as arterial blood because it is fully oxygenated, giving it its bright red pigmented appearance. Arterial blood first and foremost delivers oxygen to the tissues in the body. The cells ultimately use the oxygen to drive cellular respiration in order to create adenosine triphosphate (ATP). ATP is a nucleotide that collects chemical energy produced by the breakdown of food molecules to provide the energy necessary for all cellular activities (e.g., muscle contraction, transmitting nerve signals [nerve impulse propagation], breaking down the liquid-like substances within cells [condensate dissolution], making new molecules [chemical synthesis]). It is also involved in signal transduction pathways, DNA synthesis, and cell communication.

Arteries consist of multiple layers that are strong, elastic, and capable of dilating and recoiling as they respond to cardiac systole and diastole. As the heart contracts and pumps blood out into the vessels (systole), arteries dilate to accommodate the high pressure. Recoil then occurs as the blood is pushed through the arteries, thus creating a wave, recognized as a pulse. This relaxed phase of the cardiac cycle when the chambers of the heart are refilling with blood is called diastole.

The primary functions of the arteries are not only transportation of oxygen but also transporting nutrients and hormones throughout the body and aiding in thermoregulation. Arteries also provide oxygen and nutrients to the uterus during pregnancy, allowing for proper fetal growth. These highly adaptable vessels respond to signals and stimuli received from the central nervous system and the environment, such as temperature, chemicals, air pollution, and atmospheric pressure. A hormone neurotransmitter called catecholamines is released into the blood, triggering the nerves to signal the arteries to constrict or dilate based on the stimuli, thus causing a change in blood pressure. This reaction is important in maintaining homeostasis as well as optimal perfusion to tissue.

While arteries primarily carry oxygenated blood away from the heart to the body, the pulmonary artery moves deoxygenated blood from the heart to the lungs to engage in gas exchange in the alveoli. After oxygenation occurs, the arteries and veins of the pulmonary system switch roles. The pulmonary vein transports oxygenated blood back to the heart to be pushed out through the aorta to the body.

Branching from the arteries are arterioles, where blood is directed into the capillaries (Figure 24.12). These vessels have sympathetic nerve fibers that receive signals from the sympathetic nervous system, which helps to regulate the amount of blood flow to tissues. This regulation of the sympathetic nervous system triggers the arterioles to constrict, thus increasing the resistance of blood flow. When the system triggers the arterioles to dilate, the resistance is decreased. Hormones, including angiotensin II, travel through the bloodstream to act on tissues throughout the body. A local signaling molecule, such as prostaglandin, is released and act on a specific part of the body. Both hormones and local signaling molecules can also trigger an arteriole diameter response.

Figure 24.12 Arterioles are the vessels that connect the arteries with the capillaries. As blood reaches the tissue, the arteries become smaller (arterioles) so that blood can completely saturate the tissues through capillary exchange. (credit: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Veins

Thin-walled, low-pressured vessels, known as veins, are less elastic-like than arteries. This allows for high capacitance, which is defined as a greater volume of blood at a much lower pressure. Venous blood is returned to the heart from the periphery by veins through skeletal muscle contraction. Unlike arteries that rely on cardiac systole to assist in the forward movement of blood, the veins are farther away from the heart and require the muscles surrounding them to contract and squeeze blood forward. Intraluminal valves located within the veins prevent blood from flowing backward, maintaining a forward flow.

Blood flow through the lower extremities relies heavily on muscle contraction to return blood to the heart. The forward movement of blood from these extremities also relies on changes in respiration that affect the pressure gradients in both the abdominal and thoracic cavities. Deep inspiration brings a higher pressure than what is observed throughout the entire respiratory cycle. This is another example of how the cardiovascular and respiratory systems work interdependently to maintain homeostasis.

When deoxygenated blood enters the capillaries, it moves through the venules into the veins. Like arterioles, venules are tiny vessels that transition from the capillaries into the larger return vessels that carry blood back to the heart. These vessels are small, yet highly porous, and play an integral role in gas exchange in the tissue. Working in conjunction with capillaries, the postcapillary venule regulates solute exchange. This is the segment of microvasculature most reactive to inflammation. Postcapillary venules contain intercellular endothelial junctions that allow plasma proteins and circulating cells (leukocytes) to exit from the bloodstream in response to foreign agents (e.g., infection, inflammation).

Capillaries and Fluid Exchange

Capillaries are the smallest vessels where fluid is exchanged throughout the body at the cellular tissue level. It is at this point where oxygen, nutrients, hormones, waste, and other molecules are exchanged. Smaller molecules (e.g., gases, lipids) can diffuse directly through endothelial cells of the capillary wall. Larger molecules (e.g., glucose, sodium, potassium, calcium) use transporters to move through the membrane by facilitated diffusion. Water moves across the membrane through osmosis. The thickness of the artery and arteriole walls limits premature exchange of oxygen-rich blood and nutrients, allowing for optimal perfusion at the tissue level. Capillaries are classified by their function as well as structure and arrangement of endothelial cells and basement membrane. This allows for the exchange of molecules needed for specific tissue function (Figure 24.13).

Figure 24.13 Loading and unloading of nutrients occur within the capillaries that are attached directly to tissues. Exchange of oxygenated and deoxygenated blood occurs at this level to maintain homeostasis. (credit: modification of work from Biology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Common in muscle, connective, and nervous tissues are continuous capillaries and they have the lowest permeability capabilities of all capillaries. Molecules needed for function (e.g., water, glucose, hormones, gases) can pass through these capillaries, but large molecules (e.g., plasma proteins, platelets) are blocked. Continuous capillaries create the blood-brain barrier that prevents toxins from entering into the tissues of the brain. The brain, however, does need rapid absorption and filtration of vital nutrients, water, and glucose, which is provided through continuous capillaries that have small pores in them. These are called fenestrated capillaries, and they are located in areas that require rapid absorption or filtration (e.g., kidneys, small intestines, brain). These capillaries have many fenestrae, which are like windows or pores, and provide no resistance to fluid flow across the membranes. Found in areas where white blood cells are formed are sinusoid capillaries and they have large pores that are required to allow for movement of these cells in and out of the bloodstream. These capillaries can be found in areas such as red bone marrow and the liver. Red bone marrow produces red and white blood cells and platelets. On the other hand, yellow bone marrow produces fat, cartilage, and bone.

When material is transferred between capillary blood and body tissues, this is capillary exchange, which is essential for delivery of nutrients and removal of waste products. Fluid exchange in the capillaries occurs through opposing forces called hydrostatic and osmotic pressures. The pressure of fluids against the walls of the capillaries that forces molecules, typically water, through the capillary wall is called hydrostatic pressure. While osmotic pressure is the minimal pressure of a solution needed to stop the movement of a solute across a semipermeable membrane. Osmotic pressure works to prevent the movement of water across a semipermeable membrane to maintain homeostasis between intracellular and extracellular fluid. It is important to note that in this process, solutes cannot pass through the capillaries, only the solvent (e.g., water) (Figure 24.14).

Figure 24.14 (a) Osmosis occurs when the solution becomes heavy with solvent or solute, causing an imbalance in the tonicity of the solution, which causes a buildup of pressure and forces the solvent to move through the semipermeable membrane. (b) The tonicity of a solution determines whether the solvent moves in or out of the cell. Isotonic solutions have equal amounts of solutes and solvent so no movement occurs; hypertonic solutions have more solutes outside the cell, causing water to move out of the cell to balance the solution and hypotonic solutions have lower solutes in the cell, causing water to move inside the cell to maintain balance. (credit a and b: modification of work from Chemistry. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Capillary exchange occurs through one of three mechanisms: diffusion, transcytosis, and bulk flow. Diffusion depends on a difference in the gradient between the blood and interstitial tissue to allow the flow of small molecules across the capillary wall. Molecules, such as glucose and oxygen, are moved from the bloodstream into the tissue, and waste products, such as carbon dioxide, move from the tissue into the bloodstream. This is the most widely used mechanism within the body that allows movement of molecules from areas of high concentration to areas of low concentration.

Transcytosis allows for larger, lipid-insoluble substances to cross the capillary membrane through processes called endocytosis and exocytosis. Endocytosis brings molecules into the cell, allowing for nutrient absorption and hormone regulation as well as maintaining fluid balance. Exocytosis is the process of moving molecules out of the cell to be eliminated or moved to another area of the body.

Net filtration pressure to modulate capillary dynamics using small, lipid-insoluble solutes in water to cross the capillary wall is called bulk flow. Movement across the wall is bidirectional based on hydrostatic or osmotic pressure (Figure 24.15). Osmotic pressure (also called oncotic or colloid osmotic pressure) is exerted by proteins, and hydrostatic pressure is generated by the pressure of fluid within or outside the capillary on the capillary wall. Net filtration pressure, the sum of forces, determines the fluid flow in or out of the capillary. Larger plasma proteins (e.g., albumin) cannot cross easily through the capillary walls, resulting in fluid leakage from the capillaries. However, when there is decreased volume of plasma proteins, or blood pressure is increased significantly, there is a change in net filtration, causing excess fluid buildup in the tissues (edema).

Figure 24.15 Capillary microcirculation involves hydrostatic and osmotic pressures that move molecules across the capillary wall. (credit: modification of work “Capillary microcirculation.svg” by “Kes47”/Wikimedia Commons, Public Domain)

Characteristics of Arterial and Venous Insufficiency

Patients experience an insufficient supply of blood throughout the body when narrowing of the arteries and veins occurs, which is caused by systemic atherosclerosis. The most affected areas are the lower extremities because of their distance from the heart. A primary symptom of peripheral vascular disease is intermittent claudication, a cramp-like muscle pain, burning sensation, or extreme fatigue in the calf, thigh, or buttocks that is induced by exercise and relieved by rest. Acute or critical limb ischemia may occur if left untreated.

Vascular insufficiency in the extremities manifests as delayed venous filling time, cool skin, and abnormal skin color. An abnormal pedal pulse may be palpated and femoral artery bruit may be heard on auscultation. Findings may be subtle or there may be no symptoms, making diagnosis difficult. Diagnostic testing is critical in determining proper treatment. The ankle-brachial index (ABI) is the most common test used for patients in the outpatient setting. Blood pressure from the brachial artery in the arm is taken and compared with blood pressure from the posterior tibial and dorsalis pedis arteries in the ankle (Figure 24.17). Accurate results of the ABI depend on the provider’s ability to perform the test correctly.

Figure 24.17 The ankle-brachial index (ABI) test is used to determine blood flow to the heart. (credit: modification of work “Pad abi.jpg” by National Heart Lung and Blood by (NIH)/Wikimedia Commons; Public Domain)

Both venous and arterial insufficiencies are the result of systemic atherosclerosis, but in different types of vessels. Blockage in the veins between the extremities and heart is venous insufficiency and can result in thick, tough skin that is brownish in color Figure 24.18).

Figure 24.18 Venous insufficiency is a decrease in blood flow back to the heart from the legs. The skin of the lower leg will be thick and brownish in color. (credit: modification of work “Venoplasty and Venous Stenting in Patients with Chronic Venous Insufficiency in the Lower Extremities” by National Library of Medicine, CC BY 3.0)

In contrast, arterial insufficiency is blockage in the arteries, limiting blood flow to the extremities and resulting in thin, shiny, dry skin that is cool to the touch.

Edema

Edema is the collection of excess tissue fluid in the interstitial space outside of the cell that results in swelling. This occurs most often in the lower extremities throughout the day caused by gravitational pull and is seen in people who stand or sit for long periods of time (e.g., long-distance airplane flights and car trips). The problem is immobility secondary to the lack of muscle contractions. However, clinical edema is the accumulation of fluids that is in excess of normal daily occurrences and requires medical intervention. Table 24.3 describes the various types of edema.

Type of Edema	Description
Peripheral edema	Swelling of the feet, ankles, legs, hands, and arms
Pulmonary edema	Collection of excess fluid in the lungs, making breathing difficult
Cerebral edema	Excess accumulation of water in the intracellular and/or extracellular spaces of the brain
Macular edema	Swelling of the macula (part of the retina) caused by fluid leakage and accumulation; a serious complication of diabetic retinopathy
Periorbital edema	Swelling around the eyes, most often temporary

Table 24.3 Types of Edema

Venous blood is forced toward the heart by the skeletal muscle pump or muscle contractions, which include one-way valves to prevent blood from backing up in the vessels. A lack of use of these muscles and pumps, or dysfunction in the cardiovascular system, contributes to lower extremity edema. Often, this is a result of patients being non ambulatory or relying primarily on a wheelchair. Blood flow to the legs becomes congested because of increased hydrostatic pressure caused by decreased function.

An early indicator of venous dysfunction is dependent edema, which is measured by placing slight pressure with a finger in the area of swelling to determine the degree of pitting. A pitting edema is rated based on the depth of indentation and the time it takes to recover. This rating is on a scale of 0 (absent, no clinical findings) to 4+ (very deep pit that lasts up to two to five minutes) (Figure 24.19) (Swenty & Hall, 2020).

Figure 24.19 Edema is measured by placing slight pressure on the area of swelling, releasing, and then determining the lapse in recovery. (credit: modification of work “Grading of Edema” by Chippewa Valley Technical College; CC BY 4.0)

Heart failure, liver disease, pregnancy, dietary intake, kidney disease, and some medications all contribute to the onset of edema as well as the severity of the condition. It is important to understand that edema is not a discrete disorder but a sign of an underlying disorder that must be addressed. Although edema can occur in any area of the body, the lower extremities are most often affected in peripheral vascular disease. Heart failure, hypertension, and deep vein thrombosis (DVT) are all disorders that must be accurately diagnosed to properly treat and alleviate edema.

Hypoalbuminemia contributes to the reduced oncotic pressure that manifests as hair loss in some chronic illnesses such as nephrotic syndrome, lupus nephropathy, and chronic glomerulonephritis. Cirrhosis and chronic liver disease (e.g., hepatitis C, alcohol abuse) also contribute to inadequate albumin absorption leading to edema. When swelling is unilateral, or asymmetrical, the cause is most often due to venous thrombosis. Accurate assessment and testing is needed to determine the exact location of edema and create a comprehensive nursing care plan. Cardiomyopathies that produce equal involvement of both left and right ventricles will manifest as both pulmonary and peripheral edema. However, right-sided heart failure (also called cor pulmonale) will manifest with edema in the extremities (Lent-Schochet & Jialal, 2023).

Signs of edema are determined by the underlying contributing factor but most often begin with swelling of the ankles. Swelling that begins to rise into the legs should warrant a thorough assessment. Signs and symptoms that may accompany swelling include shortness of breath and pitting edema.

Real RN Stories

Frequent Hospital Visitors

Nurse: Natasha, BSN
Clinical setting: Preoperative area of a busy operating room
Years in practice: 7
Facility location: A suburb of a large metropolitan area in Colorado

We serve a diverse, lower-to-middle class, large retirement-age population. I was working the 7 a.m. to 3:30 p.m. shift. One of my patients arrived with her husband and adult daughter around 9:15 a.m. in preparation for corneal transplant surgery early that afternoon.

After having the patient change into a hospital gown and slippers, I took vital signs including weight (171 lbs. [78 kg]), height (5′ 2″ [157 cm]), and pain level (3/10). I completed the ophthalmic portion of the assessment, and then began to perform the rest of the admission assessment. I noted that the patient had lower extremity edema and swelling that encompassed both feet and rose to the midcalf. I noted that it was pitting edema and I rated it as moderate, with pitting approximately 5 mm in depth that took almost thirty seconds to resolve.

I realized that edema is not a discrete disorder—it’s a sign of an underlying condition. So, I needed to get more information from the patient to determine the cause. First, I asked the patient about the edema—how long ago it started, does anything help it (such as elevation), does anything make it worse (such as sitting with feet dependent on the floor), whether she had any unexplained abdominal fullness or sudden weight gain, or any shortness of breath.

I also looked at the patient’s medical record and noted that she was a smoker, had high blood pressure, and that her mother had died of congestive heart failure. So, based on her history and the assessment, I was concerned that the edema was a sign of heart failure. I documented my findings in her chart and notified the surgeon.