Arteries & Veins Allied Health II

Arteries  Strong, elastic vessels that are adapted for carrying blood away from the heart under high pressure.  Subdivide into progressively thinner tubes and eventually give rise to finer, branched arterioles

Arteries  The wall of an artery consists of 3 distinct layers  Tunica Interna – innermost layer  Simple Squamous epithelium = endothelium  Rests on connective tissue membrane that is rich in elastic and collagenous fibers

Endothelium  Helps prevent blood clotting  Smooth surface, allows blood cells & platelets to flow through without being damaged  Secretes biochemicals that inhibit platelet aggregation  Helps regulate blood flow  Secretes substances to help dilate or constrict blood vessels  Ex: endothelium releases nitric oxide which relaxes the smooth muscle of the vessel

Tunica Media  Middle layer  Makes of bulk of arterial wall  Smooth muscle fibers encircle the tube  Thick layer of connective tissue

Tunica Externa  Relatively thin  Mostly connective tissue  Irregularly organized elastic and collagen fibers  Attaches the artery to the surrounding tissues

Vasoconstriction/Vasodilation  Sympathetic branches of autonomic nervous system intertwine with smooth muscle in artery and arteriole walls  Impulses on vasomotor fibers stimulate smooth muscle to contract  reducing diameter of vessel = vasoconstriction  Vasomotor impulses inhibited, muscle fibers relax, diameter of blood vessels increases  Changes in diameters of arteries and arterioles directly affect blood flow and blood pressure

Arterioles  Walls of larger arterioles have 3 layers  As arterioles approach capillaries, these layer become thinner  Walls of very small arterioles consist of an endothelial lining and some smooth muscle fibers, surrounded by connective tissue

Capillaries  Smallest in diameter  Connect the smallest arterioles and venules  Extensions of arterioles = walls consist of endothelium  Form the semipermeable layer through which substances in blood are exchanged for substances in the tissue fluid surrounding body cells

Capillaries  Openings in walls are thin slits where endothelial cells overlap  Size/permeability depends on the tissue  Openings are smaller in capillaries of smooth, skeletal and cardiac muscle  Openings are larger in capillaries associated with endocrine glands, the kidneys and the lining of the small intestine

Capillaries  Capillary density depends on surrounding area’s rates of metabolism  Nerve/Muscle – use abundant O2 & nutrients = richly supplied with capillaries  Cartilage/Epidermis/Cornea – slow metabolic rates, little to no capillaries

Capillary Sphincters  Precapillary sphincters: smooth muscle that surrounds capillary entrances  Regulate blood distribution in capillary pathways  Can close or open a capillary by contracting or relaxing  Responds to the demands of the cells that the capillary supplies

Capillary Sphincters  When cells have low concentrations of oxygen & nutrients, sphincter relaxes to let oxygenated blood in  When cellular requirements are met, sphincter contracts  Blood flow can follow different pathways through tissues to meet the requirements of cells

Capillary Sphincters  During exercise, blood enters the capillary of skeletal muscles which have increased O2 & nutrient demand  While this is happening, blood can bypass some of the capillary networks in the digestive tract tissues since blood demand is less immediate there

Exchanges in Capillaries  Gasses, nutrients and metabolic by-products move through capillary walls by diffusion  Blood entering systemic capillaries carry high concentrations of O2 and nutrients, so they diffuse through capillary walls to tissues (high concentration  low concentration)  Concentrations of Co2 are greater in tissues, these wastes tend to diffuse into capillary blood

Exchanges in Capillaries  Plasma proteins – too large to pass through membrane pores. Stay in blood.  This creates an osmotic pressure that draws water into the capillaries (colloid osmotic pressure)  This causes more fluid to leave capillary than went into it  Lymphatic capillaries collect excess fluid and return in through lymphatic vessels to the venous circulation

Venules & Veins  Venules are microscopic vessels that continue from the capillaries to form veins  Veins follow pathways almost parallel to arteries  Carry blood back to atria

Venules & Veins  Walls of veins have 3 layers  Middle layer is poorly developed  Have thinner walls, contain less smooth muscle, less elastic connective tissue than arteries, but their lumens are wider

Venules & Veins  Many veins (mostly in upper & lower extremities) have flap – like valves that project inward  2 leaflets that close if blood begins to back up in a vein  Aid in returning blood back to the heart  Valves open if blood flow is towards the heart  Valves close if blood flow is away from the heart

Venules & Veins  Also function as blood reservoirs  Drop in arterial pressure (hemorrhage) sympathetic nerve impulses stimulate muscular walls of veins to constrict  Helps maintain blood pressure by returning more blood to the heart  Ensures a nearly normal blood flow even when as much as 25% of blood volume is lost

Characteristics of Blood Vessels VesselType of WallFunction ArteryThick, strong with 3 layersCarries blood from heart to arterioles under high pressure ArterioleThinner than artery, but with 3 layers Connects artery to capillary; helps control blood flow into capillary by vasoconstriction/ vasodilatation VenuleThinner wall, less smooth muscle & elastic tissue Connects capillary to a vein VeinThinner wall than artery but similar layers. Middle layer poorly developed. Some have flap like valves Carries blood under low pressure from venule to heart. Serves as blood reservoir. Valves prevent backflow of blood

Arterial Blood Pressure  Blood pressure is the force blood exerts against the inner walls of vessels  Arterial blood pressure rises and falls based on the stages of the cardiac cycle  Ventricular contraction = maximum strength = systolic pressure  Ventricles relax = diastolic pressure  Expanding & recoiling of arterial wall can be felt in the pulse

Factors that Influence Arterial Pressure  Heart Action  Blood volume  Peripheral resistance  Blood viscosity

Heart Action  Volume of blood discharged from left ventricle with each contraction  Stroke Volume  About 70 mL in an average weight male at rest  Cardiac output = volume discharged from left ventricle per minute  CO = Stroke Volume x HR  CO = 70mL x 72  CO = 5,040mL per minute  If stroke volume/HR rises, BP initially rises

Blood Volume  Blood volume = sum of formed elements and plasma volumes in vascular system  Usually about 5L for adults, or 8% of body weight in kg  Directly proportional to BP  For example:  Hemorrhage = decreased BP  Will rise with blood replacement  Dehydration = decreased BP  Will rise with fluid replacement

Peripheral Resistance  Friction between blood and walls of blood vessels  Hinders blood flow  Blood pressure must overcome this force  For example: contracting smooth muscles in an arterial wall increase peripheral resistance. Blood tends to back up in the arteries & arterioles  arterial pressure rises

Blood Viscosity  The ease with which a fluid’s molecules flow past each other  The greater the viscosity, the greater the resistance to flowing  More force is used to push blood that is of higher viscosity, causing BP to rise

Control of BP  Baroreceptors in walls of aorta and carotid arteries sense changes in BP  Arterial pressure increases  nerve impulses travel from baroreceptors to cardiac center of medulla oblongata  parasympathetic impulses to SA node  HR decreases  Cardio inhibitor reflex  CO falls, BP falls

Venous Blood Flow  Blood flow back to the heart relies less on arterial pressure and more on other factors like:  Skeletal muscle contraction – muscles press on nearby veins during contraction  pushes blood to next valve  Breathing movements – During inhalation, pressure from thoracic cavity squeezes abdominal veins  Venoconstriction

Arterial System  1 st portion of Aorta = ascending aorta  Located at its base are the 3 cusps of the aortic valve  The aortic sinus is located here, the right and left coronary arteries arise from here

Arterial System  3 major arteries originate from the aortic arch  Brachiocephalic artery  Left common carotid  Left subclavian artery  The upper part of the descending aorta is left of the midline  Gradually moves medially and finally lies directly in front of the thoracic wall and visceral organs = thoracic aorta  Below the diaphragm, it becomes the abdominal aorta, branches into abdominal wall and organs  Ends near brim of pelvis – common iliac arteries

Arterial System  Neck, Head & Brain  Branches of the subclavian and common carotid supplies blood to neck, head & brain

Arterial System  Shoulder & Upper Limb  Subclavian  Axillary  Brachial  Ulnar & Radial

Arterial System  Pelvis & Lower Limb  Abdominal aorta  common iliac arteries  internal/external iliac  femoral  popliteal artery  anterior tibial /posterior tibial

Venous System  Veins from the Brain, Head, and Neck  External Jugular – drains blood from face, scalp and superficial neck  Empty into right & left subclavian veins  Internal Jugular – Venous sinuses of brain, deep veins in face & neck  Join the subclavian veins  Brachiocephalic – Unions of internal jugular and subclavian veins  Merge & give rise to superior vena cava which enters right atrium

Venous System  Upper Limb & Shoulder  Deep & superficial veins  Basilic vein - ascends from forearm to the middle of arm where it joins the deep brachial vein and merges to form the axillary vein  Cephalic vein – travels up from hand to shoulder. In the shoulder, it drains into the axillary vein and then becomes the subclavian vein

Venous System  Abdominal & Thoracic Walls  Azygos vein – dorsal abdominal wall  inferior vena cava  Hepatic portal vein

Venous System  Lower Limb & Pelvis  Deep & Superficial Veins  Tibial veins  popliteal vein  femoral vein  external illiac  Superficial veins of foot, leg and thigh connect to form a complex network  Small & great saphenous veins run along the back of the calf