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Animal Physiology
Summary

Physiology of Nerve and Muscle

Neurons:
Nerve cells and muscle cells are electrically excitable
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Forebrain – cerebrum (higher cognitive functions), thalamus, and hypothalamus
Midbrain
Hindbrain – pons, cerebellum, medulla
Spinal cord

Spinal Cord:
Grey matter = mainly cell bodies/dendrites
White matter = mainly axons
Dorsal = sensory input
Ventral = motor output

Resting Membrane Potential

Resting Membrane Potential:
The membrane potential (Vm) of a neuron is typically about -70 mV
...

Proteins pump Na+ out and K+ in, called Na+/K+ ATPase's
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2
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4
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Higher K+ concentration on left
Diffusion left to right
Negative charge causes increase in Vm
Vm causes movement right to left
Opposing forces reach equilibrium

Only a very small concentration gradient needed to
produce a voltage difference, so we assume the
concentration gradient doesn’t change
...

If [X]out higher than [X]in then ln[X] < 1, so EX will be negative
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Hypothesis was wrong as EK = -93 mV, but Vm = -65 mV
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As the graph deviates from the experimental values, the
membrane cannot be exclusively permeable to K+
...

Relative membrane permeability (p) to ions needs to be
taken into account
...
0) – membrane most permeable to K at rest
• Na
(pNa = 0
...
45)
Ion channels may be open at resting
membrane potential and make the membrane
permeable to ions
...

• Directly, because it is electrogenic
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The indirect contribution is more significant
and important than the direct pumping

Ionic Basis of Action Potentials

Properties of APs:
•
•
•
•
•
•
•

Triggered by membrane depolarisation
Triggered above a threshold
All-or-nothing response
Amplitude never varies much
Increasing stimulus intensity/size increases AP frequency not amplitude
Conduced along axons
Overshoot to beyond 0mV then hyperpolarisation (undershoot) and after-HP

Ionic Basis of APs:
When membrane permeability to an ion increases, the
change in Vm is towards the equilibrium potential of that ion
...

Closing of Na channels (voltage-gated inactivation) at +30mV
is the main cause of repolarisation
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Electrotonic Potentials

Properties of Electrotonic Potentials:
•
•
•
•

Local potential changes due to current flow (e
...
subthreshold depolarisation)
Vary in amplitude (are graded)
Spread passively, so loss of amplitude over distance
Cannot be used for signalling over long distances

Time constant (tau: τ)
• Time taken to reach maximum stimulus intensity
• Time taken for Vm to rise to 63% of maximum, or
decay to 37% of original value
• Longer time constant means an axon is slower to
respond to changes (B has longer time constant than A)

Space/Length constant (lambda: λ)
• Distance potential spreads passively from the site of origin
• Distance where potential decayed to 37% of original value
• This is approx
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• Relative refractory period: AP can be generated, but larger stimulus needed because
of AHP
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• Prevents APs “bouncing back” when they reach axon terminals
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A sodium channel cannot be used again until voltage-dependent inactivation is removed,
so a neuron must repolarise before it can generate another AP
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• Conduction velocity of AP propagation is proportional to square-root of the axon
diameter in unmyelinated axons
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Myelin is produced by Glial cells
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It is dangerous because:

• It changes the cell resting membrane potential from -70 mV to slightly more positive
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• It could disable the AP mechanism, or stop gates from resetting, as repolarisation
would only reach the new Vm, not -70 mV, so a significant number of gates could
therefore not have their inactivation removed
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Impairment of cardiac conduction can result in ventricular fibrillation (uncoordinated
contraction of the cardiac muscle) or asystole (flat line – death)
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Voltage-gated Ca2+ channels activated, influx of calcium
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Vesicles release NT into synaptic cleft by exocytosis
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Activation of postsynaptic receptors elicits response in postsynaptic cell
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Postsynaptic Response:
This depends on type of receptor present, which is specific for a particular NT
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Metabotropic receptors are slower, NT does not directly affect channel
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EPSP:
• Increase in permeability to K and Na
• Drives Vm towards 0/threshold (between
ENa and EK)
• Small depolarising events (~1 mV), so a
single EPSP doesn’t produce an AP

IPSP:
• Increase in permeability to Cl (or
sometimes selectively K)
• Drives Vm more –ve (towards ECl/EK)
• Often small hyperpolarising events
(~1 mV)

Neurotransmitters:
Main excitatory transmitter in the brain = glutamate
Main inhibitory in brain = GABA
in spinal cord = glycine

Function of Synaptic Transmission:
Many EPSPs at the same time can reach threshold and produce an action potential
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• Spatial summation – signals from different dendrites (different spaces) at same time
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To be effective
the gap between signals has to be less than the time constant
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EPSPs typically involve a small depolarisation of the postsynaptic neuron
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PSPs are passive (electrotonic) events dependent on ligand-gated ion channels,
whereas APs are active (regenerative) events dependent on voltage-gated channels
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• PSPs can summate
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Mammalian skeletal muscles are twitch muscles
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A single presynaptic AP results in a single postsynaptic AP
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Epp = end plate potential
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Neuromuscular transmission is mediated by
release of ACh from motor axons
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Activation of nicotinic receptors causes depolarisation of the muscle fibre membrane
(“end-plate potential”), which triggers a muscle AP
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Excitation-Contraction Coupling:
The AP has to spread all over the muscle
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1
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2
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Dihydropyridine receptors = voltage-gated calcium channels in T-tubule membrane
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APs spread down T-tubules, opening
dihydropyridine receptors, causing Ca2+ to flow
down a concentration gradient into the interior of
muscle cells (sarcoplasm)
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So calcium passively enters the muscle from both the T-tubules and the SR (separately)
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Indirect effect = opening of ryanodine receptors
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Mechanisms of Contraction in Skeletal Muscle

Sarcomeres:
Sarcomere = the unit of muscle contraction (from
one Z disk to another)
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I-band = only actin
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In contracted muscle the I-band gets smaller but
the A-band does not
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Contraction involves sliding of thick and thin filaments past each other
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More overlap = more cross bridges
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Thin Filaments:
Consist of two F-actin chains wound around each other in a double helix
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Thick Filaments:
Consists of 200-300 myosin molecules
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Sliding Filament Theory of Muscle Contraction:
Both ATP and Ca2+ have to be present for muscle contraction to take place
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This exposes myosin binding sites on actin filaments
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Actin bound to myosin via a crossbridge
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ATP binds to the myosin head, causing
it to disassociate from the actin
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ATP hydrolysed to ADP + Pi
4
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5
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Myosin
head drops back to the lower energy
state as the energy has been spent
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ADP dissociates from the myosin head
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This induces rigor mortis
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1 action potential = 1 twitch = several cycles
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Vertebrates and cephalopods have closed circulations
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• Blood carried to the depths of each tissue by discrete vessels
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• Blood returns to the heart from each tissue by way of discrete vessels
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• Blood flows through lacunae (small spaces between tissue cells) and sinuses (larger
spaces between cells)
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Open system:
Closed system:

↓ pressure, ↑ volume (blood continuous with extracellular fluid)
↑ pressure, ↓ volume (blood separate)

Closed circulation permits rapid adjustments of the circulation in response to tissue
demands, and can sustain high metabolic rates
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A neurogenic heart requires input from the nervous system (APs)to trigger contraction
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Invertebrate hearts tend to be neurogenic, whereas vertebrate hearts are myogenic
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Pacemaker cells = depolarise spontaneously
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Depolarisation begins in the SA (sinoatrial) node (contains pacemaker cells) and spreads
throughout the atrial muscle, causing them to contract
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AV bundle allows conduction down the heart via gap junctions
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Cardiac Output:
This is the volume of blood pumped by a heart per unit time
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Cardiac output = heart rate x stroke volume (vol
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Higher pressure in atria than ventricles =
blood forced out of atria
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Ventricular systole: ventricular blood
pressure increases dramatically
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Aortic valve opens when ventricular
pressure higher than aortic pressure
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Aortic valve shuts once ventricular
pressure decreases to lower than aortic
pressure
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More tunica media (muscle) in arteries than veins
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Tissue Perfusion:
Tissue perfusion maintains adequate blood flow through the capillaries
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(MABP  2/3 diastolic + 1/3 systolic)
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TPR depends on viscosity (haematocrit) and vascular resistance
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Significant changes in TPR can be achieved by small changes in vessel diameter
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MABP must not be allowed to vary too much, so TPR can compensate for changes in CO
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There are two major divisions of the autonomic nervous system
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Parasympathetic
• Decreases CO and TPR
• Acetylcholine is major NT
• Vagus nerve is major route

Cardiac Electrophysiology:
Cardiac APs are similar to neuronal APs
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• Dependent upon voltage-gated Na+ channels
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• Dependent upon voltage-gated Ca2+ and Na+ channels
– gives plateau phase
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Pacemaker cells and ventricular myocytes produce different APs
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Voltage-gated Na+ channels open
1
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Voltage-gated Ca2+ channels open
3
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This causes a slow
depolarisation between APs, with Vm tending towards ENa
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Parasympathetic Stimulation ↓ CO:
• Cholinergic fibres release ACh onto
receptors on pacemaker cells
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• Slower rise of pacemaker potential
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• Heart rate decreased
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• Increases pacemaker rate and heart rate
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• Hormonal control via epinephrine
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Pressor centre: Increases blood pressure via sympathetic outflow
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Neural control of vascular resistance is mainly sympathetic
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Capillary Exchange:
Hydrostatic pressure – causes filtration
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Colloid osmotic pressure – causes absorption
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Oedema is caused by tissue swelling due to accumulation of
fluid in tissues outside of the capillaries
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The Baroreceptor Reflex:
This is a negative feedback reflex
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Blood being forced out of the aorta causes a slight stretch
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Reflex works on a short-term basis
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Increased firing = ↓ sympathetic output =
↓ norepinephrine release
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• Decreased force of contraction  decreased
cardiac output
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All result in decreased MAP
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Local products of metabolism act as vasodilators:
• Increased CO2, H+, K+, adenosine  vasodilation  increased blood flow
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Endothelial Control:
The presence of endothelium is necessary for
many vasodilation responses
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• Phosphorylation of myosin decreases number
of cross-bridges  relaxation
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• Increased intracellular cGMP inhibits calcium
entry into the cell  decreased[Ca2+]in 
reduced contraction
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Respiratory pigments combine reversibly with O2
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Affinity is expressed as P50 – the partial pressure required
for 50% saturation
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Myoglobin has a lower P50, so a higher affinity for O2
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↓ P50 = ↑ affinity – facilitates movement of O2 into blood
from environment  favours loading
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Hb-O2 affinity reduced by:
• Increased temperature
• Binding of organic phosphate ligands such as 2,3-diphophoglycerate (DPG)
• Increased PCO2 (indirectly by decreased pH/increased H+ concentration)

Transport of CO2:
1
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The blood contains buffers (proteins) that ‘mop up’ the H+ ions produced, thereby
shifting the equilibrium to the right
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Formation of carbamino compounds
CO2 can bind to proteins such as haemoglobin, not through binding sites but through
free amino groups in the side chains
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Molecular CO2 – around 10%
CO2 can dissolve in aqueous solution
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Deoxygenated blood has a higher affinity for
CO2 than oxygenated blood, and so carries more
CO2
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This is the Haldane effect
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Inside red blood cell = fast bicarbonate ion production
(due to carbonic anhydrase enzyme)
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Higher HCO3– concentration inside the cell than outside
causes it to diffuse out down a concentration gradient,
shifting the equilibrium even more to the right
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Binding of H+ to Hb decreases its affinity for O2, as the pH
decreases, so promoting the unloading of O2
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CO2 Transport in the Lungs:
Opposite of tissues – bicarbonate ions converted to CO2,
causing CO2 to be released and Hb to bind to O2 due to
an increased affinity
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Diaphragm – sheet of striated muscle extending across the bottom of the rib cage
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The alveolar membrane is the gas exchange surface
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Intrapleural pressure helps maintain lung inflation
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• Elastic recoil from lung and chest wall (if chest
wall expands, lungs expand then recoil)
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Diaphragm: relaxed  end of expiration  curved upwards  smaller lung volume
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External Intercostal Muscles: Contract for any additional inspirational activity beyond
tidal breathing
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Internal Intercostal Muscles: Contract for any additional expirational activity beyond tidal
breathing
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Ventilation:
V̇T = ƒ VT
Tidal ventilation (L/min) = tidal volume (vol/breath) x respiratory rate (breaths /min)
V̇A = ƒ (VT - VD)
Alveolar ventilation = respiratory rate x alveolar volume
Alveolar volume – available for gas exchange
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Vital capacity – air available for breathing, not including residual volume
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If dead space is larger than alveolar volume then breathing occurs without gas exchange
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If an alveolus receives little fresh air  PO2 drops  vasoconstriction of local arterioles
 blood flow diverted away from non-functional alveoli
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Neural Control of Breathing:
Breathing is under automatic control by neurons in medulla/pons which fire rhythmically
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Increase ventilation by frequency, tidal volume, or both
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Increase ventilation
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• ↓ pH = ↑ rate and depth of ventilation = ↑ pH
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• Conscious control = ↑ rate and depth of ventilation
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Hypoxemia – below normal oxygen blood saturation
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Hypocapnia – below normal blood CO2 levels
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Short-term:
Peripheral chemoreceptors fire more 
increased ventilation  decreased arterial
PCO2  increase in pH (alkalosis) 
promotes loading (high Hb O2 affinity)
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• Increase 2,3-DPG levels  decreases Hb
O2 affinity  promotes unloading
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Some tissues can respire anaerobically (up to a point) and tolerate anoxia
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Marine mammals breathe air and cannot extract oxygen from water
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Oxygen Stores:
Lungs (air) – not ideal to increase lung volume as this increases buoyancy and makes
submerging more difficult
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Blood (bound to Hb) – Hb O2 affinity is already at maximum at sea level, cannot increase
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• Increasing haematocrit  make more red blood cells (only up to a point)
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Muscles (myoglobin) – myoglobin is a private oxygen store for muscles
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Maximising Oxygen Stores:
Marine mammals maximise their oxygen stores by:
• Increasing blood Hb content
• Increasing haematocrit
• Increasing blood volume
• Increasing myoglobin levels in muscle

Minimising Oxygen Demand:
Diving bradycardia – reduction in heart rate to ~20 bpm, triggered by sensory receptors
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Achieved through extensive selective peripheral
vasoconstriction (priority for brain and heart maintained)
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Decreased blood flow to muscles due to myoglobin unloading O2 only when limited
blood flow/low PO2
Behavioural adjustments – movements during a dive maximise efficiency and the use of
gliding
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Water – aquaporin proteins increase water
permeability 100 x
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Solutes – asymmetrical distribution of transporters in
basal and apical membranes
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Paracellular transport is between the cells (not possible
if tight junctions between cells)
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Ion Regulation in the Kidney

The Kidney:
Vertebrate kidneys have six roles in homeostasis:
• Ion balance
• Osmotic balance
• Blood pressure
• pH balance (depends heavily on removal of CO2)
• Excretion
• Hormone production
The nephron – functional unit of the kidney, composed of multiple
tubules
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Capillary networks –
• 1st capillary network: afferent arteriole feeds into glomerulus in
Bowman’s capsule
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Role is to exchange with tubule along its length
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Hydrostatic pressure encourages liquid components (plasma) to move out of blood into
Bowman’s capsule
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Mesangial cells control blood pressure and filtration within the glomerulus
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• Proximal tube: reabsorption begins
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• Distal tubule: reabsorption completed for most solutes
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Primary urine is initial filtrate filtered in Bowman’s capsule
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Secretion – requires transport proteins (antiport cotransporters) and energy
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Gradients maintained by vasa recta capillaries
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Osmotic concentration of final urine depends on permeability (aquaporins) of distal
tubule and collecting duct
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Excretion – urine in kidney  urinary bladder  urethra (voluntary control of sphincters)

Regulation of Urinary Function:
Endocrine hormones affect kidney function
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• Antidiuretics – reduce excretion of water (increase reabsorption of water)
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• Peptide hormones – rapid response
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Gives an indication of renal function
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Vasoconstriction of afferent arteriole  less filtration
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If too high – needed substances are lost in urine
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Regulation of GFR
Regulation of GFR occurs by intrinsic and extrinsic pathways
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• Tubuloglomerular feedback – senses changes in NaCl (increased GFR  increased
NaCl in ascending limb  chemical signalling  vasoconstriction  decreased GFR)
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Causes aldosterone release
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Increases water
reabsorption by collecting duct by increasing number of aquaporins  increased GFR
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Increases reabsorption of ions
and water, increasing blood volume and pressure  increased GFR
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This leads to increased CO  increased
resistance  increased MAP and GFR
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Low GFR stimulates the renin-angiotensin system
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