Biology 2672a: Comparative Animal Physiology Circulation II: Regulation of Circulation
What happens in the ‘systemic circuit’?
The ‘Systemic circuit’ Blood is delivering oxygen and nutrients and picking up waste products (and delivering them to the kidneys) Organs & muscles Linear flow rate is reduced Cross-sectional area increases
Major Arteries Muscular, elastic thick walls (smooth muscle and elastin) Pressure of kPa Elastic Dampen pressure differences Store some elastic energy e.g. Aorta, carotid artery, femoral artery 16/10 kPa
Terminal arteries 12/8 kPa
Arteriole: 8/3.5 kPa Capillaries: c kPa Venules: c kPa Fig Right ventricle then increases pressure to c. 1.9 kPa for passage through lungs
Precapillary sphincter Can close off flow to capillary bed Arteriolar-venular anasomosis Allows blood to bypass the capillary bed Fig
Veins Low pressure Have a system of one-way valves Much thinner-walled than arteries
Pressure drop across vascular system Fig b
What about birds? Broadly similar Heart pumps more and faster to meet greater oxygen demands Higher pressures
What about birds? Jugular Anastamosis
Gravity also affects pressure ΔP = ρgΔh Fluid density (mercury>seawater>water>oil) Acceleration due to gravity Height difference across the system See Fig 24.7
The problem of being a giraffe The brain of a standing giraffe is 2m above its heart To maintain a pressure of c. 13 kPa in brain arteries, needs an aortic pressure of c. 29 kPa (!)
Tight skin on legs Muscular arteries High interstitial fluid pressure, efficient return of venous blood
Giraffes have a drinking problem Very high pressure blood into brain Blood can pool in brain ΔP = ρgΔh
Solving the giraffe drinking problem Vasodilation in lower body reduces blood pressure Elastic arteries near brain absorb some increased pressure One-way valves in jugular vein prevent backflow of blood into head
Brain Heart Kidneys – Require a regulated blood pressure to function Need blood flow to be maintained
Animals with a closed circulatory system are able to regulate Where blood goes How much of it goes there Need to respond to central requirements e.g. fight-or-flight Also need to respond to local conditions O 2 demand, localised damage
How to regulate blood flow? Change Energy input Q = ΔPπr 4 8Lη8Lη Change tube diameter
Energy input: Cardiac Output stroke volume = heart rate × Cardiac Output Can be modulated by both endocrine and nervous systems Modulated by nervous activity (via norepinephrine) and circulating epinephrine (=adrenaline) Equation 24.1
Myogenic autoregulation (stretch response) Increased blood flow Increased pressure on arteriole wall Smooth muscle stretched Smooth muscle contracts Increased resistance Decreased blood flow
Neural control of vasoconstriction Sympathetic nervous system can be activated to induce vasoconstriction Thermoregulation Fight or flight Mediated by Norepinephrine released by sympathetic neurons
Neural Regulation of vasomotor tone sympathetic nerves noradrenaline : smooth muscle receptors : constriction : dilation Relative receptor population density
Smooth Muscle Cell NO Produced Viagra inhibits cGMP breakdown
Endocrine control of vasomotor tone Epinephrine (Adrenaline) from Adrenal medulla induces vasodilation Fight or flight Vasopressin (ADH) & Angiotensin II – vasoconstriction Activate Adrenergic receptors on smooth muscle Not necessarily mediated by nerves
Paracrine control of vasomotor tone NO Produced by endothelial cells to maintain vasomotor tone in response to hormonal cues Same mechanism as for parasympathetic activation Nitroglycerine Also responses to local factors indicating hypoxia and damage
Hormonal Adrenaline (Epinephrine) receptors Local Control low O 2, pH, ATP high CO 2, K + dilates vessels locally override neural & hormonal control Fig Local regulation
Reading for Tuesday Intro to Gas Exchange + breathing in water pp