Biology 322 – Human Anatomy Blood vessels

Blood Vessels Arteries carry blood FROM the heart Decrease in size with distance from heart Very small arteries = ARTERIOLES Veins carry blood TO the heart Increase in size closer to heart Very small veins = VENULES Capillaries microscopically thin vessels where nutrients, gases, wastes are exchanged with tissues Essentially link arterial system with venous system

All arteries and veins consist of three layers or TUNICS 1.Tunica intima – endothelial cells and basement membrane 2.Tunica media – thickest layer consisting of collagen, smooth muscle This layer is thicker in arteries and often contains elastic tissue - higher fluid pressure in arteries than veins Smooth muscle allows vessels to alter diameter – important for regulating blood flow and pressure 3.Tunica externa – loose connective tissue that helps anchor vessels to tissues, other vessels Tissues of small vessels exchange nutrients, wastes, gases by diffusion from bloodstream Larger vessels possess VASA VASORUM – tiny vessels located in tunica externa that supply outer layers

Arteries Known as RESISTANCE VESSELS because they resist high pressure of blood leaving the heart Also due to the fact that constriction of arteries increases resistance in the circulatory system → increases blood pressure Arteries are broken down into 3 categories loosely based on size 1.Conducting arteries – Largest arteries (aorta, common iliac, subclavian) Contain a great deal of elastic tissue in tunica media Expand during systole and contract during diastole Expansion prevents high pressure from reaching smaller arteries Contraction ensures blood pressure remains high enough in between heartbeats (WINDKESSEL EFFECT) 2.Distributing arteries – Smaller arteries that branch off of conducting arteries More smooth muscle than elastic tissue in tunica media Control blood flow to specific organs Femoral, brachial, renal arteries are distributing arteries 3.Resistance arteries - Smallest arteries that branch off distributing arteries Majority of tunica media is smooth muscle Usually not named, smallest resistance arteries are ARTERIOLES

Arterial Sense Organs Blood is very closely monitored for pressure and chemistry to maintain homeostasis BARORECEPTORS – monitor blood pressure CHEMORECEPTORS – monitor blood pH, O2, CO2, glucose levels Send information to brainstem that regulates heart rate, vascoconstriction/dilation, and respiratory rate 1.Carotid sinuses – located in the internal carotid artery. Detects changes in BP 2.Carotid bodies – located in external carotid artery. Detect changes in O2, CO2, and pH and alter resp. rate 3.Aortic bodies – located in aortic arch. Chemoreceptors similar to carotid bodies.

Capillary beds Complex network of capillaries that surround tissues of the body Exchange of gasses, nutrients, hormones, waste products occurs here Every cell in the body is very close to a capillary Essentially link between arterial and venous systems Capillaries Microscopic blood vessels consisting of only an endothelium and basement membrane All are relatively porous to allow exchange of products between blood and tissues 3 main types: 1.Continuous capillaries – most common type of capillary. Contain small clefts between endothelial cells – allow glucose, gasses, small molecules to move into tissue 2.Fenestrated capillaries – found in organs that need to filter or exchange large quantities of materials (kidney, endocrine glands, intestines). Endothelial cells contain many FILTRATION PORES 3.Sinusoids – found in organs such as liver, spleen, bone marrow where entire cells and large quantities of proteins are able to pass between endothelial cells. Actually have more open space than endothelial cells.

Veins Also known as CAPACITANCE VESSELS since they can accommodate large volumes of blood by expanding (most blood is found in venous system) Relatively thin walled, with little smooth muscle Don’t need to withstand high pressures like arteries Also broken down roughly on the basis of size 1.Venules – smallest veins that drain blood from the capillary bed. Somewhat porous like capillaries and allow leukocytes to exit into tissues 2.Medium veins – mid-sized veins such as ulnar or tibial veins. Contain valves that prevent low pressure venous blood from flowing backwards 3.Venous sinuses – very thin walls with no smooth muscle (i.e., Coronary sinus ) 4. Large veins – Relatively small amount of smooth muscle, but large diameters Sup. and inf. vena cava, common iliac veins, common jugular are examples

Circulatory Routes ANASTAMOSES – location where several arteries or veins converge/diverge before or after a capillary bed Allows blood to enter or exit a capillary bed by more than one route All blood leaving heart eventually needs to return This is accomplished in several different ways Most common way is from L.V. → artery → capillary bed → vein → R.A. In a PORTAL SYSTEM, blood flows through TWO capillary beds prior to returning to the heart Usually these two capillary beds are in different organs In an ARTERIOVENOUS SHUNT, blood is able to be “re-routed” and bypasses the capillary bed Blood flows directly from arterial system to venous system Helps regulated flow to specific organs/tissues

Blood flow and pressure In order to sustain life, the circulatory system needs to exchange O 2, nutrients, and waste at a pace based on the that tissue’s metabolic rate Blood supply to a tissue can be expressed as FLOW or PERFUSION Flow is defined as the volume of blood flowing into a tissue per minute (mL / min) Perfusion relates flow of blood to the mass of a tissue or organ (mL / min/ g) Example : 1.Two tissues each weigh 50 g and receive 20mL of blood per minute (FLOW and PERFUSION are the same between the two tissues) 2.Tissue A weighs 50 grams and Tissue B weighs 200 grams and both receive 50 mL of blood per minute FLOW is the same but PERFUSION is not! A = 1mL/min/g B=0.25mL/min/g 3.Tissue A weighs 50 grams and receives 50mL of blood per minute. Tissue weighs 25 g and receives 25mL/min Perfusion is same in both (1mL/min/g), but Flow is not!

Blood Pressure Defined as the force that blood exerts on the wall of a blood vessel Blood pressure (BP) is most commonly measured within the brachial artery with a sphygmomanometer and is referred to ARTERIAL BP A “healthy” BP in an adult is about 120/80 mm Hg SYSTOLIC pressure = pressure in artery when the heart contracts DIASTOLIC PRESSURE = pressure in artery during relaxation PULSE PRESSURE is the difference between systolic and diastolic pressures (40 mmHg for “normal” BP) MEAN ARTERIAL PRESSURE measures the average pressure within an artery over a given period of time (if you measured BP once every 1/10 th of a second for 1 second) Not a simple average (i.e., ( ) / 2 = 100 mm Hg Diastole lasts longer than Systole!! So, a good estimate = Diastolic pressure + 1/3 of pulse pressure 80 + (40/3) = 93.3 mm Hg

Blood pressure varies with distance from heart and between arteries and veins Blood in larger arteries (closer to heart) is under very high pressure and is PULSATILE – pressure varies with phase of cardiac cycle However, arteries expand during systole and absorb some pressure thereby reducing pressure as you get farther from the heart As a result, pressure in your brachial artery will be greater than in an artery in your foot As blood flows through smaller and smaller arteries, more pressure is absorbed so that blood returning to the heart is under very LOW pressure Venous blood flow is generally not pulsatile

Hypertension = elevated resting BP Commonly defined as BP >140/90 Hypotension = low resting BP Blood Pressure Blood pressure is regulated primarily by three factors: 1.Cardiac output – increased CO leads to increased BP 2.Blood volume – increased blood volume increases BP Largely controlled by kidneys (hydration and Na+ plays a big role) Dehydration reduces amount of water in plasma → less blood volume Na+ causes you to retain water → more water in plasma → more blood volume 3.Resistance in blood vessels – caused by friction between blood and vessel walls

Blood Vessel Resistance Resistance to flow creates pressure within vessels Resistance INCREASES blood pressure Resistance is influenced by: 1.Blood viscosity – thicker blood flows less easily and is under greater pressure Largely affected by RBC# and protein content 2.Vessel length – blood encounters more friction the farther it travels in a vessel causing a decrease in blood flow → decrease in pressure 3.Vessel radius – probably the most important and variable factor affecting resistance in a vessel. Blood flowing through a vessel demonstrates LAMINAR FLOW – essentially flowing in “sheets” Blood near the walls flows slower due to friction Blood in center flows faster As a vessel constricts, more blood flows closer to the walls → more friction → more resistance → greater pressure

Regulation of BP and Blood Flow Vasodilation/vasoconstriction have a huge impact on resistance and flow Resistance and flow is proportional to the 4 th power of the radius (r 4 ) of a vessel So very small changes in diameter lead to big changes in resistance and flow For example, a doubling in vessel diameter results in a 16-fold increase in flow (i.e., from 1mm to 2mm or 2mm to 4mm) Derivations of Poisseiulle’s equations Ultimately, blood pressure is dependent on resistance (increase in resistance causes increase in BP) But, blood FLOW is dependent on both pressure and resistance At a given pressure, flow can be altered by changing resistance (and vice versa)

Regulation of BP and Blood Flow Vasoconstriction/dilation can be controlled in 3 ways 1.Local control – a form of autoregulation where the blood vessel tissues control diameter Accumulation of waste products, release of NO (nitric oxide), and prostacyclin from endothelial cells can trigger relaxation of smooth muscle As these products get washed away, the vessel constricts again 2.Neural control – control of vessel diameter by ANS Cardiovascular centers in medulla oblongata process information received by baroreceptors and chemoreceptors Neural control of vessel diameter is primarily controlled by the SYMPATHETIC division of ANS More stimulation of smooth muscle by sympathetic fibers causes constriction, less stimulation causes relaxation

3.Hormonal control - a variety of hormones produced by the brain, liver, adrenal glands that alter blood vessel radius and/or fluid balance in the blood Angiotensin II – released into bloodstream by liver as angiotensinogen. Gets converted to Ang II by ANGIOTENSIN CONVERTING ENZYME (ACE) Ang II is a potent vasoconstrictor ACE inhibitors are potent drugs to treat hypertension Atrial naturetic peptide – promotes excretion of Na+ → H2O loss→ decrease in blood volume→ decrease in BP Also acts as a mild vasodilator Antidiuretic hormone (ADH) – promotes Na+ retention and also is a vasoconstrictor Epinephrine/norepinephrine – released into bloodstream by adrenal glands and also is the neurotransmitter released by sympathetic nerve fibers Causes smooth muscle contraction → vasoconstriction → increase in BP Aldosterone – hormone released by adrenal gland that promotes Na+ retention like ADH Receptor for aldosterone is called the MINERALOCORTICOID RECEPTOR The stress hormone CORTISOL can also bind that same receptor – explains increase in BP when you are under stress!!!

Regulation of blood pressure depends on a constant series of NEGATIVE FEEDBACK LOOPS Involves sensors and receptors in body, cardiovascular control centers in brainstem, as well as many EFFECTORS: heart, blood vessels, kidneys, etc… all working together to maintain homeostasis Keep in mind that this loop can go in BOTH directions depending if BP is too high or too low!!!