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 & musclesLinear flow rate is reduced

Cross-sectional area increases

Major Arteries Muscular, elastic

thick walls (smooth muscle and elastin)

Pressure of 10-20 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. 2.5-3 kPa

Venules: c. 1.3-2 kPa

Fig. 24.11

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. 23.11

VeinsLow pressureHave a system of one-way valvesMuch thinner-walled than

arteries

Pressure drop across vascular system

Fig. 24.12b

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 = ρgΔhFluid 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 goesHow much of it goes thereNeed to respond to central

requirements e.g. fight-or-flight

Also need to respond to local conditions O2 demand, localised damage

How to regulate blood flow?

Change Energy input

Q = ΔPπr4

8Lη

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 contractsIncreased

resistance

Decreased blood flow

Neural control of vasoconstrictionSympathetic 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 toneEpinephrine (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 toneNO

Produced by endothelial cells to maintain vasomotor tone in response to hormonal cues

Same mechanism as for parasympathetic activation

NitroglycerineAlso responses to local factors

indicating hypoxia and damage

Hormonal Adrenaline (Epinephrine) receptors

Local Control low O2, pH, ATP

high CO2, K+

dilates vessels locally override neural &

hormonal control

Fig. 23.11

Local regulation

Reading for Tuesday Intro to Gas Exchange +

breathing in waterpp 533-560