Cardiovascular System: Vessels and Circulation

Cardiovascular System: Vessels and Circulation

• Arteries carry blood away from the heart.
• Veins carry blood back to the heart.
• Capillaries facilitate gas and nutrient exchange.

Vessel Wall Composition

• Walls are composed of three layers called tunics.
• These are the tunica externa, tunica media, and tunica intima.
• Arteries and veins have all three tunics.
• Capillaries only have a tunica intima, which consists of an endothelial layer and a basement membrane.
• The space inside a vessel is called the lumen.

Tunica Intima

• The tunica intima is the innermost layer of a vessel wall.
• It is composed of endothelium, which is a simple squamous epithelium.
• The endothelium provides a smooth surface for blood to move through the lumen.
• It contains the subendothelial layer or areolar connective tissue.
• The tunica intima releases substances to regulate smooth muscle contraction like nitric oxide, which causes relaxation, and endotunicn, which causes vasoconstriction.

Tunica Media

• The tunica media is the middle layer of a vessel.
• It comprises circularly arranged layers of smooth muscle cells with elastic fibers.
• Contraction causes vasoconstriction and relaxation causes vasodilation.

Tunica Externa

• The tunica externa is the outermost layer of a vessel, consisting of areolar connective tissue with elastic and collagen fibers.
• This layer anchors the vessel to other structures.
• It contains vasa vasorum which are small arteries for supplying very large vessels.

Companion Vessels

• Arteries and veins serving the same body region lie next to each other.

Artery vs Vein Comparison

• Arteries generally have a narrower lumen diameter than veins and veins a wider lumen.
• Artery walls are thicker than veins and veins walls are narrower.
• Arteries have a more maintained cross-sectional shape, even without blood, whereas veins tend to flatten out (collapse) without blood.
• The thickest tunic in arteries is the tunica media, while in veins it is the tunica externa.
• Arteries contain more elastic and collagen fibers in their tunics while veins have less.
• Valves are are abundant in veins but arteries contain none.
• Arteries have a higher blood pressure, around 100 mm Hg in larger arteries to 40 mm Hg in smaller arterioles, while veins have lower pressure ranging from 20 mm Hg in venules to 0 mm Hg in the inferior vena cava. Blood Flow and Oxygen Levels:
• Systemic arteries transport blood high in O₂, and systemic veins transport blood low in O₂.
• Pulmonary arteries transport blood low in O₂, and pulmonary veins transport blood high in O₂.

Capillaries

• Capillaries only contain tunica intima without a sub-endothelial layer.
• They are composed of endothelium and a basement membrane, with a thin wall that allows for exchange.

Elastic (Conducting) Arteries

• These are the largest, thick-walled arteries near the heart with a diameter between 2.5 to 1 cm.
• Their large lumen allows low-resistance conduction of blood.
• They contain elastin, especially in the tunica media so the artery can stretch and accommodate blood during ventricular systole and diastole.

Muscular (Distributing) Arteries

• These arteries deliver blood to body organs and are distal to elastic arteries, ranging from 1 cm to 0.3 mm. They have a thick tunica media and are active in vasoconstriction and dilation. They have elastic tissue in two layers: internal and external elastic lamina.

Arterioles

• These are the smallest arteries with diameters of 0.3 mm to 10 UM.
• Larger arterioles have three tunics, while smaller ones have only thin endothelium and a single layer of smooth muscle.
• Smooth muscle is usually somewhat constricted, regulated by vasomotor tone in the brainstem.
• Arterioles regulate systemic blood pressure and flow.

Atherosclerosis

• This is a progressive disease of elastic and muscular arteries.
• It involves the presence of atheroma, or atheromatous plaque.
• It involves the thickening of the tunica intima and narrowing of the arterial lumen, which may be caused by an injury response to endothelium.
• The injury response hypothesis is considered to be caused by infection, trauma, and hypertension.
• Atherosclerosis results in an inflammation reaction and atheroma formation.
• Lumen becomes narrower with atheroma enlargement, and plaques are not noticed until they restrict blood flow to a specific region.

Angioplasty

• During angioplasty, an uninflated balloon and compressed stent are passed through a catheter to the area of the obstructed artery.
• The balloon inflates, expanding the stent and compressing the atheroma.
• The stent typically remains in the vessel as the balloon is deflated and the catheter withdrawn.

Capillaries

• These are the smallest blood vessels, 8-10 UM in diameter.
• The capillary walls consist of an endothelial layer on a basement membrane.
• Red blood cells travel through capillaries in a single file.
• Three Structural Types: continuous capillaries, fenestrated capillaries, and sinusoidal capillaries
• Permeability varies based on the type of capillary
• Gas and nutrient exchange occurs in the capillaries, which connect arterioles with venules.

Continuous Capillaries

• The most common type of capillaries and are found in skin, muscles, lungs, and the CNS.
• They have endothelial cells that provide an uninterrupted lining, with tight junctions connecting the cells.
• Intercellular clefts are the gaps that allow small particles such as glucose and gases to pass, while large particles, cells, or proteins can not pass.
• Continuous capillaries constitute the blood-brain barrier and have no intercellular clefts.

Fenestrated Capillaries

• These are found in kidneys, small intestines, endocrine organs, etc., where a lot of fluid transport occurs.
• Characterized by an endothelium riddled with pores or fenestrations.
• They exhibit a greater permeability than other capillaries and allow for smaller plasma protein movement.

Sinusoidal Capillaries

• Leaky, fenestrated capillaries with large lumens with incomplete lining of endothelial cells.
• They are found in the liver, bone marrow, lymphoid tissue, and some endocrine glands.
• Blood flow is sluggish so they Allow large molecules such as proteins and blood cells to pass between the blood and surrounding tissues.

Capillary Beds

• Groups of 10 to 100 capillaries functioning together. They are fed by metarterioles, a vessel branch of an arteriole.
• The proximal part of a metarteriole is encircled by scattered smooth muscle cells.
• The distal part, or thoroughfare channel, lacks smooth muscle cells and connects to a post capillary venule, serving as a draining bed..
• Vessels are branching from metarterioles.

Precapillary Sphincter

• A smooth muscle ring surrounds the origin of a true capillary.
• Relaxation permits blood flow and contraction causes blood to bypass the capillary bed.
• Vasomotion is the cycle of contracting and relaxing of precapillary sphincters and allows for only 1/4 of the body's capillary beds being open at any given time.
• Perfusion is the blood flow through body tissues.

Venules

• The smallest veins, with a 8-100 UM diameter.
• They have all three tunics.
• They serve as companion vessels with arterioles.
• They merge to form veins.

Veins

• Small and medium-sized veins are companion vessels with muscular arteries.
• Large veins travel with elastic arteries.
• They ensure blood flow toward the heart.
• Veins are made of tunica intima + elastic and collagen fibers with a similar valve structure to the heart's semilunar valves.
• The act as blood reservoirs.
• They contain valves to prevent blood pooling.

Blood Distribution

• At rest, 70% of blood is in systemic circulation so 55% of the total blood is in systemic veins, 10 percent in systemic arteries, and 5 percent in systemic capillaries. ,
• In pulmonary circulation, 18% of blood is circulating.
• 12% of the total volume of blood is in the heart.

Blood Vessel Pathways

• Blood vessels can be arranged in either simple pathways or in alternative pathways with anastomoses.
• Simple pathway: Artery -> capillary bed -> veins -> heart
• Anastomoses: Joining together of blood vessels, provides an alternative pathway for blood transportation

Capillary Exchange of Respiratory Gases and Nutrients

• O₂, CO₂, nutrients, and metabolic wastes diffuse or are transported between the blood and the interstitial fluid along concentration gradients.

Diffusion

• Substances moves in or out of blood dependent on its concentration gradient..
• Oxygen, hormones, nutrients, and small solutes move from blood to the interstitial fluid, .
• Carbon dioxide and wastes diffuse from tissue to blood.

Vesicular Transport

• Endothelial cells use endocytosis/pinocytosis to transport fluids across capillary walls.
• It moves substances across capillaries.

Bulk Flow

• Fluids move along a pressure gradient.
• Fluids and dissolved substances move in large amounts.
• Movement direction is determined by the net pressure of opposing forces, notably the balance between Hydrostatic and colloid pressure
• In filtration fluid moves out of the blood on arterial end.
• In reabsorption, fluid moves back into the blood at venuous end.

Hydrostatic Pressure

• Force exerted by a fluid

Colloid Osmotic Pressure

• The "pull" on water due to proteins

Net Filtration Pressure

• Equal to all the forces acting on a capillary bed.
• NFP is the difference between net hydrostatic pressure and net colloid osmotic pressure.
• At the arterial end of a bed, hydrostatic pressure dominates (fluids flow out).
• At the venous end of a bed, colloid osmotic pressure dominates leading to fluids flow in.
• Excess fluid is returned to the blood via the lymphatic system.

Degree of Vascularization

• The amount of blood vessels in a tissue.
• It depends on the tissue type.
• Metabolically active tissues have high vascularity.
• Tendons, ligaments, epithelia, and cornea, have little or no vascularity.
• The amount of vascularization may increase over time through angiogenesis and decrease through regression.

Angiogenesis

• Angiogenesis is the formation of new blood vessels.
• Two types are sprouting and intussusception and can occur over weeks to months to increase potential perfusion.
• It is the Skeletal muscle responds used to increase blood flow in response to aerobic training.
• It occurs in adipose tissue with weight gain and is the way coronary vessels respond to gradual blockage.

Steps of Angiogenesis

• Induction of VEGF signaling: Vascular endothelial growth factor
• Formation of tip cells.
• Stalk cell development.
• Vessel outgrowth.
• Anastomosis & perfusion.
• Maturation & stabilization.

Atherosclerosis

• Angiogenesis forms more blood vessels to feed tumors
• If tumor angiogenesis can be blocked. this deprives the tumor of nutrients.
• Many angiogenesis inhibitors are monoclonal antibodies that end in "-mab".

Regression

• Regression is the return to previous state of blood vessels.
• Examples include, skeletal muscle after individual becomes sedentary or in adipose tissue when weight is lost

Myogenic Response

• Contraction and relaxation of smooth muscle in BV walls to increase or decrease blood flow
• Constriction occurs if systemic blood pressure rises and more blood enters an arteriole.
• Dilation occurs if systemic blood pressure decreases and amount of blood decreases
• Myogenic response involves both contraction of smooth muscle in blood vessel walls in response to an increase in stretch and relaxation of smooth muscle in blood vessel walls in response to a decrease in stretch to maintain a constant blood flow..

Local, Short-Term Regulation

• Blood flow is regulated based on need Changes occur when metabolic activity changes or tissue is damaged.

Vasoactive Chemicals

• Vasoactive chemicals can alter blood flow.
• Vasodilators will dilate arterioles and relax precapillary sphincters.
• Vasoconstrictors will constrict arterioles and cause contraction of precapillary sphincters..