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Unformatted text preview: Lecture 25 Lecture Circulation 10 December 2010 Cardiac muscle sustains its contraction due to the plateau phase of the cardiac action potential and thus prolonged depolarization.
Inward Ca++ current. Inward Na+ current. Outward K+ current. Repolarization is due to K+ efflux. Electrical activity initiates the cardiac cycle.
Mammalian cardiac cycle for the left side of the heart. depolarization contraction ↑ blood pressure blood flow open valves ↓ volume The heart is the ultimate source of pressure that drives blood through the vascular system. During the cardiac cycle, blood flow from the heart is intermittent, … but capillary perfusion (forced blood flow) is continuous. Aorta has elastic elements in its wall which stretch and store pressure during systole. Stored energy is released back into the blood during diastole, thus maintaining blood flow. The aorta and other arteries dampen pressure oscillations and serve as pressure reservoir. Cardiac Output Cardiac
Cardiac Output is the volume of blood pumped by each ventricle per minute. – Cardiac Output = C.O. = SV x HR
– Regulation of C.O. by control of SV and HR
– Extrinsic regulation via nerves and hormones – Intrinsic regulation via autoregulation Regulation of Heart Rate: Regulation Pacemaker Potentials
Extrinsic Control: Sympathetic stimulation increases HR Parasympathetic stimulation decreases HR Stroke Volume (SV) – EDV = volume of blood in ventricle at end of diastole – ESV = volume of blood in ventricle at end of systole – SV = volume of blood ejected from heart each cycle – SV = EDV - ESV EJECTION FRACTION: Fraction of end-diastolic volume ejected during a single heartbeat Ejection fraction = Ejection SV / EDV SV = 70 mL / 130 mL = 0.54 70 mL 130 mL Regulation of Stroke Volume: Regulation Family of Starling Curves
Intrinsic Control: Frank-Starling Extrinsic Control: Sympathetic stimulation increases ventricular contractility Relatively Low Pressure Closed Circulatory System Systemic (left) to Pulmonary (right) in Series
Relatively High Pressure Circulation (in Mammals and Birds) Circulation Relatively Relatively Low Pressure Pressure Relatively Relatively High Pressure Pressure Circulation (in Mammals and Birds) Circulation First things first: First
The primary role of the heart is to generate pressure to push blood through the vasculature. The magnitude of blood flow is directly proportional to the driving pressure and inversely proportional to the resistance in the vasculature: Flow = Pressure Gradient / Resistance = ΔP / R Mean Blood Pressure Profiles Mean
Note the relatively low pressures in the pulmonary circuit. From Flow=ΔP/R coupled with the fact that pulmonary blood flow = systemic blood flow, it follows that R is lower in the pulmonary than in the systemic circuit. Mean Blood Pressure Profiles Mean
Note the relatively low pressures in the pulmonary circuit. From Flow=ΔP/R coupled with the fact that pulmonary blood flow = systemic blood flow, it follows that R is lower in the pulmonary than in the systemic circuit. Flow is a constant from one place to another in the circulatory system. Thus, the large drop in pressure indicates high resistance in the arterioles. Regulation of Resistance: Regulation Poiseuille’s Law
R= 8ηL r4
R = resistance, η= fluid viscosity, L= vessel length, r = vessel radius Note: resistance is inversely proportional to the 4th power of radius Flow = ΔP/R = Note: flow in ΔP r4 8ηL directly proportional to the 4th power of radius ΔRadius → Mean Blood Pressure Profiles Figure 14.11b ΔResistance → Arterioles are the “resistance vessels” with highly muscular Organ Blood Flow = walls.Regulate Contraction/relaxation of vascular smooth muscle causes relatively small changes in /arteriolar diameter and MAP Organ Resistance distribution of CO relatively large need according to changes in vascular resistance. Recall that systemic circulatory paths are arranged in parallel and that this arrangement allows independent regulation of flow. Changes in arteriolar resistance determine local flow and thus the distribution of cardiac output. Flow varies due to differences in resistance. ΔRadius → ΔResistance → Regulate distribution of CO according to need Organ Blood Flow = MAP / Organ Resistance Flow varies due to differences in resistance. Rest → Exercise: Rest ↑C.O. 5x ΔDistribution of C.O. To meet demands of metabolically active tissues To dissipate heat (i. e., skin) Microcirculation Microcirculation A capillary consists capillary only of a single layer of endothelial cells bounded by a basement membrane. basement Photomicrograph: Photomicrograph: Endothelial Cells Vascular Fluid Dynamics Vascular Vascular Fluid Dynamics Vascular Vertebrate Phylogeny and Circulatory Plans Vertebrate Teleost Heart Teleost Gill-Breathing Fish As evidence of their critical physiological importance, mammalian kidneys comprise <1% of body mass but receive ~20% of C.O. and require ~16% of total ATP at rest. Mammals: Kidneys in Humans Mammals: Primary urine is formed in the renal corpuscle (glomerulus surrounded by Bowman’s capsule) by ultrafiltration.
Primary urine is modified in the tubules of the nephron by active reabsorption and secretion and by osmosis. Definitive urine is conducted away from the kidneys through the ureters into the urinary bladder and is excreted through the urethra. Insects: Malpighian Tubules in Ants
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookEXCRET.html formed in blindended tubules connected to the posterior midgut. Primary urine is modified in the hindgut prior to excretion as definitive urine. Primary urine is General Functions of Excretory Organs
Excretory organs do a lot of things. They regulate fundamental aspects of the milieu intérieur – the internal environment. In general, the functions of excretory organs include: 1. 2. 3. 4. 5. Volume regulation Osmoregulation Ion regulation Acid-base balance Nitrogen excretion Vertebrate Kidneys
Amphibian nephron exemplifies basic renal structures and functions. Vertebrate Kidneys
Ultrafiltration is driven by blood hydrostatic pressure to push ultrafiltrate from the glomerular plasma into Bowman’s capsule. Vertebrate Kidneys
Primary urine is formed by ultrafiltration and is subsequently modified by reabsorption and secretion. The final product of definitive urine is excreted. In humans, the glomerular filtration rate (GFR) is about 120 ml/min -- enough to filter all of body water every ½ hour! Mammalian juxtamedullary nephrons are distinguished by having long loops of Henle. These loops of Henle enable the kidney to form urine that is hyperosmotic to blood plasma (U/P osmotic ratio > 1). Urine Formation in Mammals Medullary thickness reflects the length of the renal papillae, which contain loops of Henle and collecting ducts. Vertebrate Kidneys In conjunction with the vasa recta, loops of Henle create a standing corticomedullary osmotic concentration gradient. The longer the renal papillae, the higher the deep medullary osmotic concentration. Vertebrate Kidneys The loop of Henle is a countercurrent multiplier. Vertebrate Kidneys Standing corticomedullary osmotic concentration gradient. Vertebrate Kidneys Difference in osmotic pressure between blood plasma and interstitium Water is absorbed by osmosis, and the osmotic concentration can equilibrate across the collecting duct. Vertebrate Kidneys With ADH Without ADH Principle of Principle Countercurrent Exchange in the Vasa Recta H2O H2O H2O Solutes Solutes ...
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