Chapter 19 - The Cardiovascular System: Blood Vessels
Introduction
- Blood is carried in a closed system of vesselsthat begins and ends at the heart
- The three major types of vessels are arteries, capillaries, and veins
- Arteries carry blood away from the heart, veins carry blood toward the heart
- Capillaries contact tissue cells and directly serve cellular needs
- Arteries and veins are composed of three tunics
- tunica interna (tunica intima)
- Endothelial layer that lines the lumen of all vessels
- In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present
- tunica media
- Smooth muscle and elastic fiber layer, regulated by _________ ____________ ___________
- Controls vasoconstriction/vasodilation of vessels
- Tunica externa (tunica adventitia)
- Collagen fibers that protect and reinforce vessels
- Larger vessels contain vasa vasorum
- Lumen - central blood-containing space surrounded by tunics
- Capillaries
are composed of endothelium with sparse basal lamina
Arteries
- Elastic Arteries - Thick-walled arteries near the heart; the aorta and its major branches
- Large lumen allow low-resistance conduction of blood
- Contain elastin in all three tunics
- Withstand and smooth out large blood pressure fluctuations
- Allow blood to flow fairly continuously through the body
- Muscular arteries - distal to elastic arteries; deliver blood to body organs
- Have thick tunica media with more smooth muscle and less elastic tissue
- Active in vasoconstriction
- Arterioles - smallest arteries; lead to capillary beds
- Control flow into capillary beds via vasodilation and constriction
Capillaries
- Walls consisting of a thin tunica interna, one cell thick
- Allow only a single RBC to pass at a time
- Pericytes on the outer surface stabilize their walls
- There are three structural types of capillaries
- Continuous -abundant in the skin and muscles, and have:
- Endothelial cells that provide an uninterrupted lining
- Continuous capillaries of the brain:
- Have tight junctions completely around the endothelium
- Constitute the blood-brain barrier
- Adjacent cells that are held together with tight junctions
- Intercellular clefts of unjoined membranes that allow the passage of fluids
- Fenestrated
- Found wherever active capillary absorption or filtrate formation occurs
- Found in the small intestines, endocrine glands, and kidneys
- Characterized by:
- An endothelium riddled with pores (fenestrations)
- Greater permeability to solutes and fluids than other capillaries
- Sinusoids
- Blood flows sluggishly, allowing for modification in various ways
- Found in the liver, bone marrow, lymphoid tissue, and in some
endocrine organs
- Highly modified, leaky, fenestrated capillaries with large lumens
- Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues
- Capillary Beds
- A microcirculation of interwoven networks of capillaries, consisting of:
- Vascular shunts - metarteriole-thoroughfare channel connecting an arteriole directly with a postcapillary venule
- True capillaries - 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed
- Precapillary sphincter
- Cuff of smooth muscle that surrounds each true capillary
- Regulates blood flow into the capillary
- Blood flow is regulated by vasomotor nerves and local chemical conditions, so it can either bypass or flood the capillary bed
Venous System
- Venules
- Are formed when capillary beds unite
- Allow fluids and WBCs to pass from the bloodstream to tissues
- Postcapillary venules - smallest venules, composed of endothelium and a few pericytes
- Large venules have one or two layers smooth muscle (tunica media)
- Veins
- Are formed when venules converge
- Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks
- Capacitance vessels (blood reservoirs) which contain 65% of the blood supply
- Veins have much lower blood pressure and thinner walls than arteries
- To return blood to the heart, veins have special adaptations
- Large-diameter lumens, which offer little resistance to flow
- Valves (resembling semilunar valves), which prevent backflow of blood (varicose veins)
- Venous sinuses - specialized, flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)
Vascular Anastomoses
- Merging blood vessels, more common in veins than arteries
- Arterial anastomoses provide alternate pathways (collateral channels) for blood to reach a given body region
- If one branch is blocked, the collateral channel can supply the area with adequate blood supply
- Thoroughfare channels are examples of arteriovenous anastomoses
Capillary Exchange of Respiratory Gases and Nutrients
- Oxygen, carbon dioxide, nutrients, and metabolic wastes diffuse between the blood and interstitial fluid along ______________ gradients
- Oxygen and nutrients pass from the _____ __ _______
- Carbon dioxide and metabolic wastes pass from ______ __ _____
- Water-soluble solutes pass through _____ ___ ____________
- ____ _______molecules diffuse directly through endothelial membranes
- Capillary Exchange: Fluid Movements
- Direction and amount of fluid flow depends upon the difference between:
- Capillary hydrostatic pressure (HPc) which is actually _______ ______
- Capillary colloid osmotic pressure (OPc)
- HPc - pressure of blood against the capillary walls:
- Tends to force fluids through the capillary walls
- Is greater at the _______ end of a bed than at the ______ end
- OPc - created by nondiffusible plasma proteins, which draw water toward themselves
- Net Filtration Pressure (NFP)
- NFP - considers all the _______ acting on a capillary bed
- ____ = (HPc - HPif) - (OPc - OPif)
- At the arterial end of a bed, __________ forces dominate (fluids flow ___)
- At the venous end of a bed, _______ forces dominate (fluids flow ___)
- More fluids enter the tissue beds than return blood, and the excess fluid is returned to the blood via the _________ _______.
- The pumping action of the heart generates blood flow through vessels along a pressure gradient, always moving from higher- to lower-pressure areas
- Systemic pressure:
- Is highest in the aorta
- Declines throughout the length of the pathway
- Is 0 mm Hg in the right atrium
- Venous BP alone is too low for adequate blood return and is aided by:
- Respiratory "pump" - pressure changes created during breathing suck blood toward the heart by squeezing local veins
- Muscular "pump" - contraction of skeletal muscles "milk" blood toward the heart
- Capillary BP - low capillary pressure is desirable because high BP would rupture fragile, thin-walled capillaries
- Blood Pressure (BP = CO x PR)
- Force exerted by blood on wall of blood vessel
- Pressure Points
- Measurements
- Systolic - ____ mm Hg (ventricles contract)
- Diastolic - ____ mm Hg (ventricles relax)
- Pulse Pressure = SP - DP = _____
- 5 Factors that influence arterial BP
- Mean Atrial Pressure (MAP) =(2(diastolic pressure) + Systolic Pressure)/3
- MAP =_________________________ (about 60 necessary for functioning properly)
- Cardiac output = stroke volume x heart rate
- ____________ = 70ml x 72 beats/min
- BP = Cardiac Output x Peripheral Resistance
- Peripheral Resistance
- Resistance - opposition to flow
- Measure of the amount of friction blood encounters as it passes through vessels
- Generally encountered in the systemic circulation
- Referred to as peripheral resistance (PR)
- The three important sources of resistance
- Resistance factors that remain relatively constant are:
- Blood viscosity - thickness or "stickiness" of the blood
- Blood vessel length - the longer the vessel, the greater the resistance encountered
- Peripheral resistance - changes in vessel diameter are frequent and significantly alter
- Resistance varies inversely with the fourth power of vessel radius (one-half the diameter)
- For example, if the radius is doubled, the resistance is 1/16 as much
- Small-diameter arterioles are the major determinants of peripheral resistance
- Control of Cardiac Output
- Cardiac output is determined by
- Neural - Cardiac center receives impulses from baroreceptors
- Mechanical (Starlings Law)- venous return
- Chemical - Epinephrine
- Resting heart rate is controlled by the cardioinhibitory center via the vagus nerves
- Under stress, the cardioacceleratory center increases heart rate and stroke volume - the (ESV) decreases and MAP increases
- Controls of Blood Pressure
- Short Term Mechanisms
- Vasomotor Center
- A cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter
- Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles
- Cardiovascular center - vasomotor center plus the cardiac centers that integrate blood pressure control by altering cardiac output and blood vessel diameter
- Vasomotor Activity
- Sympathetic activity causes:
- Vasoconstriction and a rise in blood pressure if increased
- Blood pressure to decline to basal levels if decreased
- Vasomotor activity is modified by:
- Baroreceptors (pressure-sensitive), chemoreceptors (O2, CO2, and H+ sensitive), higher brain centers, bloodborne chemicals, and hormones
- Baroreceptor-Initiated Reflexes
- Increased blood pressure stimulates the cardioinhibitory center to:
- Increase vessel diameter
- Decrease heart rate, cardiac output, peripheral resistance, and blood pressure
- Declining blood pressure stimulates the cardioacceleratory center to:
- Increase cardiac output and peripheral resistance
- Low blood pressure also stimulates the vasomotor center to constrict blood vessels
- Chemical Controls
- Blood pressure is regulated by chemoreceptor reflexes sensitive to oxygen and carbon dioxide
- Prominent chemoreceptors are the carotid and aortic bodies
- Reflexes that regulate blood pressure are integrated in the medulla
- Higher brain centers (cortex and hypothalamus) can modify BP via relays to medullary centers
- Chemicals that increase BP
- Adrenal medulla hormones - norepinephrine and epinephrine increase blood pressure
- Antidiuretic hormone (ADH) - causes intense vasoconstriction in cases of extremely low BP
- Angiotensin II - kidney release of renin generates angiotensin II, which causes intense vasoconstriction
- Endothelium-derived factors - endothelin and prostaglandin-derived growth factor (PDGF) are both vasoconstrictors
- Chemicals that decrease BP
- Atrial natriuretic peptide (ANP) - causes blood volume and pressure to decline
- Nitric oxide (NO) - has brief but potent vasodilator effects
- Inflammatory chemicals - histamine, prostacyclin, and kinins are potent vasodilators
- Alcohol - causes BP to drop by inhibiting ADH
- Long-Term Mechanisms: Renal Regulation
- Kidneys act directly and indirectly to maintain long-term blood pressure
- Direct renal mechanism alters blood volume
- Indirect renal mechanism involves the renin-angiotensin mechanism
- Declining BP causes the release of renin, which triggers the release of angiotensin II
- Angiotensin II is a potent vasoconstrictor that stimulates aldosterone secretion
- Aldosterone enhances renal reabsorption and stimulates ADH release
- Alterations in Blood Pressure
- Hypotension- low BP in which systolic pressure is below 100 mm Hg
- Orthostatic hypotension - temporary low BP and dizziness when suddenly rising from a sitting or reclining position
- Chronic hypotension - hint of poor nutrition and warning sign for Addison's disease
- Acute hypotension - important sign of circulatory shock
- Threat to patients undergoing surgery and those in intensive care units
- Hypertension - condition of sustained elevated arterial pressure of 140/90 or higher
- Transient elevations are normal and can be caused by fever, physical exertion, and emotional upset
- Chronic elevation is a major cause of heart failure, vascular disease, renal failure, and stroke
- Primary or essential hypertension - risk factors in primary hypertension include diet, obesity, age, race, heredity, stress, and smoking
- Secondary hypertension - due to identifiable disorders, including excessive renin secretion, arteriosclerosis, and endocrine disorders
Autoregulation: Local Regulation of Blood Flow
- Automatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in time
- Blood flow through an individual organ is intrinsically controlled by modifying the diameter of local arterioles feeding its capillaries
- Metabolic Controls
- Declining tissue nutrient and O2 levels are stimuli for autoregulation
- Hemoglobin delivers nitric oxide (NO) as well as oxygen to tissues
- NO induces vasodilation at the capillaries to help get O2 to tissue cells
- Other autoregulatory substances include: potassium and hydrogen ions, adenosine, lactic acid, histamines, kinins, and prostaglandins
- Myogenic Controls
- Inadequate blood perfusion or excessively high arterial pressure:
- Are autoregulatory
- Provoke myogenic responses - stimulation of vascular smooth muscle
- Vascular muscle responds directly to:
- Increased vascular pressure with increased tone, causes vasoconstriction
- Reduced stretch with vasodilation, which promotes increased blood flow to the tissue
Circulatory shock
- Any condition in which blood vessels are inadequately filled and blood cannot circulate normally
- Results in inadequate blood flow to meet tissue needs
- Three types include:
- Hypovolemic shock - results from large-scale blood loss
- Vascular shock - poor circulation resulting from extreme vasodilation
- Cardiogenic shock - the heart cannot sustain adequate circulation
Central Venous Pressure and Edema
- Pressure in the right atrium
- Factors that influence it alter flow of blood into the right atrium
- Weakly beating heart ________ central venous pressure which will _________ pressure within the peripheral veins
- If central venous pressure is above normal it will cause _________ _______
- Types of Edema
- Anasarca - throughout the body
- left or right ventricle failure
- damaged lungs
- Dependent Edema - one position to long
- Pulmonary Edema - Left or right ventricle failure?
- Dependent edemaPeriorbital edema
- Pitting edema - impression left in skin when touched
- Mechanical edema - tight clothing
- Lymphedema - lymph blockage
CIRCULATIONS
References
Renin Angiotensin Sytem Mechanisms in Cardiology
Renin Angiotensin Aldosterone Sytem Inhibitors Mechanisms in Cardiology
Primary and Secondary Prevention of CVD: Population Strategy and High-risk Strategy Mechanisms in Cardiology
Physical Activity and exercise in weight control Mechanisms in Cardiology
Transient Ischemic Attack and Stroke Medivisuals
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