THE CARDIOVASCULAR SYSTEM: CIRCULATION
The Arterial System
Carrying blood to the tissues
o Arterial pressure = CO · R
o Distensibility/elastic properties
o Systolic pressure
o Diastolic pressure
o Pulse pressure
pulse pressure =systolic pressure - diastolic pressure
o Measurement of arterial pressure
o Mean arterial pressure (MAP)
MAP=1/3 pulse pressure + diastolic pressure
o Arteriole size: 30 µm or less internal diameter
o Resistance vessels: primary regulatory site in determining local blood flow
o Relationship between resistance and the diameter of vessels
§ R µ 1/radius4
§ The diameter of a blood vessel is the most important contributing factor for the resistance of flow
o concept of a resting vascular tone
contributed by spontaneous depolarization of SM cell membrane
o Autoregulation
§ myogenic activity
§ locally released metabolites
o CO2 , adenosine, H+ , K+ , heat, depletion of O2, etc.
o example: occlusion of blood flow to a tissueà reactive hyperemia (reactive vasodilation)
o Regulation of local blood flow by nerves and circulating hormones
§ role of sympathetic nerves
o most arterioles are richly innervated by sympathetic nerves (exceptions: coronary vessels and cerebral vessels)
o continuous neural activity contributes to the resting vascular tone
o binding of NE a receptors mediates vasoconstriction
§ role of parasympathetic nerves
not generally involved in neural regulation of blood flow
o role of some circulating hormones
§ epinephrine
§ binding of EP to b receptors produces vasodilation
§ minimum effect of EP on a receptors
§ angiotensin II - very potent vasoconstrictor
§ atrial natriuretic factor (ANF)
§ potent vsodilator
§ secreted from the atria of the heart and a variety of other tissues
§ increase excretion of salt and water by the kidneys
The Capillary System
Providing the site for exchange of molecules
o Microcirculation
§ metarteriole - terminal arteriole
§ thoroughfare channel
§ true capillaries
§ small venule
§ precapillary sphincter
o Structure of the capillary wall
§ a unicellular layer of endothelium surrounded by a basement membrane
§ 0.5 µm thick
§ "pores" in the capillary membrane (6 -7 nm in size)
o By transcytosis and endocytosis
large molecules
o Diffusion
§ through endothelial membrane - lipid soluble molecules: O2, CO2
§ through the water-filled pores - lipid insoluble molecules: glucose, Na+, K+, Cl-
§ factors that facilitate diffusional exchange
o the distance that the molecules must move is short - >20µm between the capillary and most cells
o extensive surface area of the capillary wall - 7000 sq. ft
o slow rate of blood flow in the capillr elationship between velocity of blood flow and the cross-sectional area
o Bulk flow
§ water and dissolved molecules are driven through the pores by bulk flow
§ driving pressure: net pressure gradient of hydrostatic pressure and oncotic pressure
§ filtration and absorption: Starling’s Hypothesis
The Venous System
o Returning blood to the heart
o large diameter of veins, thinner wall than corresponding arteries
o Capacitance vessels: function as low pressure reservoir for blood
o One-way venous valves, skeletal muscle contraction, and respiration facilitate the return of blood to the heart
o Varicose veins
The Lymphatic System
o A second system of vessels
o The function of lymphatic vessels is to return extra plasma proteins and fluid in the interstitial space back to the circulation
o Three kinds of lymphatic vessels: lymphatic capillaries, collecting vessels, and lymphatic ducts
Regulation of Systemic Arterial Pressure
Major factors that influence BP are cardiac output, blood volume and peripheral resistance
o Neural controls: ANS (SNS/PNS)
§ vasomotor center (cardiovascular control center) in medulla
o transmitting parasympathetic impulses through the vagus nerve to the heart
o transmitting sympathetic impulses through the spinal cord to blood vessels of the body
§ sympathetic nervous system (SNS) innervates: the blood vessels and the heart
§ parasympathetic nervous system (PNS) influences the heart (HR)/little influence in blood vessels
§ Baroreceptor-initiated reflexes (a negative feedback system)
o pressure sensors: activated by stretching
o located in carotid artery (carotid sinus), aortic arch
o afferents to cardiovascular control center
o efferents to heart, arteries, and veins
§ Chemoreceptor-initiated reflexes
o chemoreceptors are sensitive to lack of O2, increases in pH and CO of blood
o located in carotid arteries (two carotid bodies) and aortic arch (several aortic bodies)
o afferents to vasomotor center
o effernts (SNS) to vessels - vasoactivity (usually constriction)
§ humoral controls
o adrenal medulla
o atrial natriuretic factor
o angiotensin II
o antidiuretic hormone
o endothelins/nitric oxide (NO)
o prostaglandins
o Hemorrhage: loss of blood
o Exercise and cardiovascular homeostasis