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