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Control Systems Negative Feedback Systems • Most control systems of the body act by negative feedback
• A stimulus causes a reaction that opposes the acting stimulus • Increased CO2 causes increased pulmonary ventilation, which decreases CO2 • Decreased arterial pressure activates the baroreceptor system which acts
increase heart rate and arterial constriction, which increases arterial pressure
• The negative feedback system acts to maintain homeostasis
Control Systems Gain of Control Systems • The gain of a control system is a parameter which describes the degree of
effectiveness with which a control system can maintain constant conditions • Gain = Correction / Error
• Example • In a normal person with a functioning baroreceptor control system, a defined
stimulus causes arterial pressure to increase from 100 mmHg to 125 mmHg • Error is +25 mmHg - if the baroreceptor system provided perfect control, there
would be no change in arterial pressure • A person with a non-functioning baroreceptor control system, the same stimulus
causes arterial pressure to increase from 100 mmHg to 175 mmHg • Difference from the normal response is 50 mmHg • Thus, the baroreceptor system provides a correction of -50 mmHg
• Gain = -50 mmHg / +25 mmHg = -2 • The baroreceptor system reduces the effect of the stimulus by two thirds
Mass Transport Diffusion Through a Membrane • With different assumptions, Fick’s Second Law can
also be written as
• Consider a steady state system where a membrane separates two large reservoirs of a dilute chemical, with a concentration of CL on the left (x = 0) and CR on the right (x = L), the above equation reduces to
• Assuming a “perfect source” and a “perfect sink”, we can solve the above to find
Momentum Transport Fluid Flow Between Parallel Plates
• Consider a volume element in a fluid between two parallel plates separated by a height, H. The bottom plate is stationary, while the top plate is pulled to the right at a velocity of V0. The fluid between the plates will also begin to move to the right due to the momentum imparted by the upper plate. Here, we wish to describe the velocity of the fluid as a function of position (height) between the plates.
• Our two boundary conditions state that the fluid next to the stationary plate is a rest (Vx = 0 at y = 0) and the fluid next to the plate in motion moves at the same velocity as the plate (Vx=V0 at y = H).
• Using this approach, we find that the velocity profile between the two plates is linear with position
• Release is much faster than conformational change • Intracellular solute concentration is low • Equilibrium state is reached between S + P and C • All membrane bound protein is either free or in a complex
• Similar to facilitated transport, however energy is required • Primary Active Transport: Energy is used directly, as in the consumption of ATP
to ADP • Secondary Active Transport: Energy is derived from another source, such as a
concentration gradient established by primary active transport • Transport can occur against the concentration gradient across the plasma membrane • Transport typically utilizes membrane bound carrier proteins • Transport can be regulated by the relative number of ligands and / or receptors
Membrane Potentials Membrane Potentials Are Caused by Diffusion • K+ is high inside a nerve fiber membrane,
but K+ is low outside the membrane
• Ignoring other ions, the large K+ gradient causes K+ to diffuse outward through the membrane • K+ carry positive electrical charges to
the outside, thus creating electropositivity outside the membrane and electronegativity inside because of negative anions that remain behind
• The potential difference ultimately is balanced with the concentration difference and further K+ transport is stopped
• In the normal mammalian nerve fiber, the potential difference created by the K+ concentration difference is about 94 mV, with negativity inside the fiber membrane
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
• The membrane potential, or Nernst potential, for a single ion is given as
• Vs potential difference • R gas constant • T absolute temperature • F Faraday’s constant • z charge on the ion • Sin intracellular ion concentration • Sout extracellular ion concentration
• Under the influence of sodium, potassium, and chloride ions, the Nernst Equation expands to the Goldman-Hodgkin-Katz Equation
• Notes about the GHK Equation • K+, Na+, and Cl- are the most significant ions in nerve and muscle fibers • Permeability controls significance • Note sign convention • Cl- does not change greatly during transmission of a nerve impulse, so it plays a
• If K+ is the only ion to move through the membrane (channel), and since there is a high ratio of K+ inside to outside (35:1), the resting potential inside the fiber would be equal to -94 mV
• Sodium Diffusion Potential • There is a slight permeability of the nerve
membrane to Na+ due to the K+ Na+ leak channel
• In addition, the ratio of Na+ from inside to outside the membrane is 0.1, and this gives a calculated Nernst potential for the inside of the membrane of +61 mV
• Combining with the GHK equation, the resting potential is -86 mV
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Nerve Action Potential Voltage-Gated Na+ and K+ Channels
• Voltage-gated Na+ channel drives the action potential
• Voltage-gated K+ channel speeds the repolarization of the membrane
• Voltage-Gated Na+ Channel • Channel has two gates, one near the
outside of the channel called the activation gate and one near the inside called the inactivation gate
• At rest (-90 mV) the activation gate is closed
• When the membrane potential rises from -90 mV, at approximately -70 to -50 mV, the activation gate opens (activated state) increasing the Na+permeability as much as 500 to 5000 times
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Nerve Action Potential Propagation of the Action Potential • A local circuit of current flow from the
depolarized areas of the membrane to the adjacent resting membrane areas • Positive charges are carried by the
inward-diffusing Na+ through the depolarized membrane and then for several millimeters in both directions along the core of the axon
• Positive charges increase the voltage for a distance of 1 to 3 mm inside the large myelinated fiber to above the threshold voltage value for initiating an action potential
• The action potential travels in all directions
away from the stimulus until the entire membrane has become depolarized
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Rhythmicity of Some Excitable Tissues Rhythmicity • The membrane of the heart control
center does not depolarize immediately after it has become repolarized
• Toward the end of each action potential, and continuing for a short period thereafter, the membrane becomes excessively permeable to K+
• The excessive outflow of K+ carries tremendous numbers of positive charges to the outside of the membrane, leaving inside the fiber considerably more negativity than would otherwise occur
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Molecular Mechanism of Muscle Contraction Walk-Along Theory of Contraction
• After Ca++ activation, the heads of the cross-bridges from the myosin filaments become attracted to the active sites of the actin filament, and causes contraction to occur
• Contraction is hypothesized to occur due to a walk-along mechanism • When a cross-bridge head attaches to an active site,
the attachment causes the head to tilt toward the arm and drags actin filament - the power stroke
• After tilting, the head then automatically breaks away from the active site
• Next, the head returns to its extended direction • The head then combines with a new active site farther
down along the actin filament • The head tilts again to cause a new power stroke
• Each cross-bridges acts independently • Number of cross-bridges in contact with the actin
filament determines the force of contraction
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Myosin - Actin Overlap Determines Tension • At no overlap, developed tension is zero • As overlap increases, tension increases • At a sarcomere length of 2.2 µm, the actin filament
has overlapped all the cross-bridges of the myosin filament but has not yet reached the center of the myosin filament
• With further shortening, the sarcomere maintains full tension until 2.0 µm
• Below 2.0 µm, the ends of the two actin filaments begin to overlap each other in addition to overlapping the myosin filaments • Two Z discs of the sarcomere abut the ends of
the myosin filaments • Ends of the myosin filaments are crumpled • Strength of contraction decreases rapidly and
approaches zero
Molecular Mechanism of Muscle Contraction
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Excitation of Skeletal Muscle Neuromuscular Junction
• The nerve fiber forms a complex of branching nerve terminals that invaginate into the surface of the muscle fiber but lie outside the muscle fiber plasma membrane • Structure is called the motor end plate • Schwann cells insulate it from the
surrounding fluids • Invaginated membrane is the synaptic
gutter or synaptic trough • Space between the terminal and fiber is
the synaptic space or synaptic cleft • Space is 20 to 30 nm • Subneural clefts increase the
surface area for the synaptic transmitter
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Excitation of Skeletal Muscle Propagation of Muscle Action Potential • Skeletal muscle fibers are so large that
action potentials spreading along the surface membrane cause almost no current flow deep within the fiber
• Excitation-contraction coupling • Interior transmission of action
potentials occurs along transverse tubules (T tubules) that penetrate all the way through the muscle fiber from one side of the fiber to the other
• T tubule action potentials cause release of Ca++ inside the muscle fiber in the immediate vicinity of the myofibrils, and these Ca++ then cause contraction
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Contraction and Excitation of Smooth Muscle Contraction Mechanisms in Smooth Muscle
• Smooth muscle is irregular compared to skeletal muscle
• Large numbers of actin filaments are attached to dense bodies • Dense bodies are attached to the cell membrane and dispersed
inside the cell - they serve the same role as the Z discs in skeletal muscle
• Some membrane bound dense bodies of adjacent cells are bonded together by intercellular protein bridges - these bonds transmit force from cell to cell
• Myosin filaments are interspersed among actin filaments • Approximately 15x less myosin than actin • Myosin filaments have sidepolar cross-bridges, where the
bridges on one side hinge in one direction and those on the other side hinge in the opposite direction
• Sidepolar cross-bridges allow myosin to pull an actin filament in one direction on one side while simultaneously pulling another actin filament in the opposite direction on the other side
• This allows smooth muscle cells to contract as much as 80% of their length, compared to < 30% in skeletal muscle
• The cardiac cycle consists of a period of relaxation called diastole, during which the heart fills with blood, followed by a period of contraction called systole
Guyton & Hall. Textbook of Medical Physiology, 11th
Work Output of the Heart Graphical Analysis of Ventricular Pumping
• Consider the pumping mechanics of the left ventricle
• The diastolic pressure curve is determined by filling the heart with progressively greater volumes of blood and then measuring the diastolic pressure immediately before ventricular contraction occurs, which is the end-diastolic pressure of the ventricle
• The systolic pressure curve is determined by recording the maximum systolic pressure achieved during ventricular contraction at each volume of filling, and without any outflow of blood from the heart Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Regulation of Heart Pumping Frank-Starling Mechanism
• Fundamental Concept: The amount of blood pumped by the heart each minute (venous return) is determined by the rate of blood flow into the heart • Peripheral tissue controls blood flow
• The greater the heart is stretched during filling, the greater it contracts and the greater quantity of blood is pumped into the aorta
• The intrinsic ability of the heart to adapt to increasing volumes of inflowing blood is called the Frank-Starling Mechanism
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Specialized Excitatory and Conductive System Organization of the A-V Node
• The A-V node is located in the posterior wall of the right atrium immediately behind the tricuspid valve
• After traveling through the internodal pathways, an impulse reaches the A-V node about 0.03 sec after its origin in the sinus node
• There is a delay of another 0.09 sec before the impulse enters the penetrating portion of the A-V bundle, where it passes into the ventricles
• A final delay of another 0.04 sec occurs mainly in this penetrating A-V bundle, which is composed of multiple small fascicles passing through the fibrous tissue separating the atria from the ventricles • The total delay in the A-V nodal and A-V bundle
system is about 0.13 sec • A total delay of 0.16 sec occurs before the excitatory
signal finally reaches the contracting muscle of the ventricles
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Electrocardiographic Interpretation Principles of Vector Analysis in Electrocardiograms • Similarly, a vector representing the instantaneous mean direction of current flow may
be projected onto all three leads to determine their relative contributions
Electrocardiographic Interpretation Principles of Vector Analysis in Electrocardiograms • Finally, this vector analysis may be applied to ventricular vectors during
depolarization and compared to the ECG’s QRS wave
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Electrocardiographic Interpretation Vector Analysis During Atrial Depolarization and
Repolarization • Again, a similar analysis may be performed during
atrial depolarization and the P wave as well as repolarization and the atrial T wave
• Important concepts here include • Depolarization begins in the sinus node • The direction of the electrical potential vector
is slightly to the right of the atrial septum • This direction persists throughout
depolarization • This direction is almost the same as the
vector for ventricular depolarization • Thus, the P wave is positive
• Repolarization also begins in the sinus node • Opposite to ventricular repolarization • Thus vector is in the opposite direction • Thus, the atrial T wave is negative
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Vascular Distensibility Hydrostatic Pressure • A column of liquid exerts pressure due to the
weight of the liquid • Pressure rises 1 mmHg for each 13.6 mm
below the liquid surface
• Thus, a person standing still has • Right atrial pressure = 0 mmHg • Venous pressure in the feet = 90 mmHg • Venous pressure in the hands = 35 mmHg
• Also, a standing person also has collapsed neck veins (0 mmHg) due to atmospheric pressure • Veins in the skull do not collapse • Sagittal sinus = -10 mmHg
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Vascular Distensibility Venous Valves and Venous Pumps • Veins contain small “valves” that allow blood to
flow towards the right atrium and prevents it flow back into the arterial system
• A standing person would have a +90 mmHg pressure in the feet • However, skeletal muscle and other
movements compresses the veins, moves blood unidirectionally through the valve system - venous pump
• Pressure in walking feet is + 25 mmHg
• Standing without movement allows venous pressure to rise, movement of fluid into the interstitial space, loss of circulating blood volume - and eventually unconsciousness
• Varicose veins result from valve dysfunction and result in edema
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Microcirculation and the Lymphatic System Nutrient Exchange • Nutrient exchange from the capillary to the
interstitial fluid is dominated by passive diffusion induced by thermal fluctuations (Brownian Motion)
• Lipid soluble molecules (oxygen and carbon dioxide) can diffuse directly through endothelial cell membranes
• Water soluble molecules (water, Na+, Ca++, and glucose) must pass through capillary pores • Diffusion is incredibly fast • Water diffuses through the capillary
membrane 80x faster than the rate of plasma flow along the capillary!!!
• The transport of molecules through the capillary pore depends upon molecule size and molecular concentration gradient
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Local Control of Blood Flow Acute Blood Flow Control • Autoregulation of Blood Flow
• An increase in blood pressure, increases blood flow, but within a minute blood flow returns to normal - autoregulation
• Metabolic Theory • Arterial pressure increases, blood flow
increases, nutrition is too high, blood vessels contract, flow decreases
• Myogenic Theory • Sudden pressure increase causes a
stretch of small blood vessels and smooth muscles to contract, causing vasoconstriction, and reduced blood flow
• Such a theory implies a possible positive feedback loop and a potential viscous cycle leading to death
• Thus theory likely only applicable to a few tissues and in limited circumstances
Effect of increasing arterial pressure on blood flow through a muscle. The solid curve shows the effect when change occurs quickly (minutes), the dashed curve shows
the effect when change occurs slowly (weeks).
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Integrated System for Pressure Control Hypertension • Hypertension due to reduced
renal mass and increase in salt intake
• Reduction of kidney mass reduces the ability of the kidney to remove salt, thus increasing arterial pressure - shifting the renal function curve to the right
• The intake of both salt and water also causes increased arterial pressure - shifting the water and salt intake line up Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Integrated System for Pressure Control Renin-Angiotensin System • Renin is a small protein produced by the kidneys and
released in response to a fall in arterial pressure
• Once released, renin acts on angiotensinogen to form angiotensin I
• Angiotensin II, a fragment of angiotensin I, is then formed by cleavage
• Angiotensin II is a powerful vasoconstrictor (increasing resistance and therefore pressure) and inhibitor of renal excretion of salt and water (increasing volume and therefore pressure)
• System actively maintains arterial pressure in response to significant changes in salt intake
• Angiotensin II is inactivated by angiotensinase Guyton & Hall. Textbook of
Fluid Regulation in Abnormal States Example: Patient in Water Deficit
• What is the effect on extracellular and intracellular fluid volumes when 2 liters of a hypertonic 2.9% sodium chloride solution is administered to a 70 kg patient with an initial osmolarity of 280 mOsm/liter
• Using the basic assumptions • Extracellular osmolarity = 280 mOsm/l x 14 liters = 3,920 mOsm • Intracellular osmolarity = 280 mOsm/l x 28 liters = 7,840 mOsm • Total body fluid osmolarity = 280 mOsm/l x 42 liters = 11,760 mOsm
• Next, 2 liters of a 29 g/l sodium chloride (58 g/mol) solution provides 1 mole or 2 osmoles (2,000 mOsm) of sodium chloride
• The instantaneous change would be • Extracellular concentration = (3,920 + 2,000) mOsm / 16 liters = 370 mOsm/l • Intracellular concentration remains constant = 280 mOsm/l
• The long term change would be • Total body fluid concentration = 13,760 mOsm / 44 liters = 312.7 mOsm/l • Extracellular volume = 5,920 mOsm / 312.7 mOsm/l = 18.9 liters • Intracellular volume = 7,840 mOsm / 312.7 mOsm/l = 25.1 liters
Kidneys: Glomerular Filtration Physiological Anatomy of the Kidneys
• Blood flow to the kidneys is normally 21% of cardiac output (1200 ml/min)
• Blood enters the kidney through the renal artery, and then branches progressively to form interlobar arteries, arcuate arteries, interlobular arterioles, and afferent arterioles
• Afferent arterioles lead to the glomerular capillaries, which then coalesce to form efferent arterioles, and then the peritubular capillaries • Glomerular capillaries are the site of
fluid and solute filtration • Efferent arterioles regulate pressure
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Carbohydrate Metabolism ATP Formation From Glucose • During glycolysis, 4 molecules of ATP are formed, and 2 are expended to cause the
initial phosphorylation of glucose - giving a net gain of 2 molecules of ATP • During each citric acid cycle, 2 molecules of ATP are formed, 1 from each pyruvic
acid molecules formed from glucose • During glucose breakdown, a total of 24 hydrogen atoms are released during
glycolysis and the citric acid cycle • 20 H atoms are oxidized in oxidative phosphorylation, with the release of 3 ATP
molecules per 2 atoms of hydrogen metabolized, giving 30 ATP molecules • 4 H atoms hydrogen atoms are released by their dehydrogenase in oxidative
phosphorylation; 2 ATP molecules are usually released for every 2 H atoms oxidized, thus giving 4 ATP molecules
• Thus a maximum of 38 ATP molecules formed for each molecule of glucose degraded to carbon dioxide and water • Thus, 456,000 calories of energy can be stored as ATP, whereas 686,000
calories are released from glucose, giving an overall maximum efficiency of 66%
Lipid Metabolism Transport of Lipids in the Body Fluids • Almost all fats, broken down into monoglycerides and fatty acids, are absorbed from
the intestines and into the intestinal lymph
• In intestinal epithelial cells, triglycerides are reformed and dispersed as chylomicron droplets, with the protein apoprotein B decorating their surface • Chylomicrons contain 87% triglycerides, 9% phospholipids, 3% cholesterol, 1%
apoprotein B • After eating, chylomicrons constitute 1 - 2% of plasma volume
• The capillary endothelium of the liver and adipose tissue contain lipoprotein lipase, which hydrolyzes triglycerides within the chylomicrons and thus releasing fatty acids and glycerol • Constituents are absorbed by surrounding cells and resynthesized into
Protein Metabolism Transport and Storage • Upon food digestion, almost all proteins are digested and absorbed in the form of
amino acids - rarely are polypeptides or proteins absorbed into the blood
• The normal blood concentration of amino acids is between 35 and 65 mg/dl • Concentration rises only slightly after eating, due to
• Digestion and absorption occur over the course of hours • Amino acid uptake by tissues, especially the liver, is quick
• Amino acids are too large for diffusion through cell membranes, thus are taken up by facilitated transport or active diffusion
• In the kidneys, amino acids are actively reabsorbed by the proximal tubular epithelium
• Absorbed amino acids are quickly reused in protein synthesis, thus storage of amino acids is low • Cells do contain small quantities of amino acids, releasable during deficiency • The liver contains proteins that are easily broken down for use as a amino acids • Excess amino acids can be converted into energy, fat, or glycogen
Energetics Metabolic Rate • A 70 kg man in bed consumes 1650 kcal per day • Eating and digestion consumes an additional 200
kcal per day • Sitting without exercising consumes an additional
150 to 400 kcal per day • Thus, a sedentary man consumes about 2000
kcal per day
• The minimal amount of energy needed to exist is defined as the basal metabolic rate (BMR) and accounts for 50 - 70% of daily energy needs in a sedentary person • Normal BMR is 65 - 70 kcal per hour • Skeletal muscle accounts for 20 - 30% of
BMR • BMR falls dramatically with age, following the
loss in skeletal muscle
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
• There are about 16 generations of conducting airways • The trachea (Z = 0), which bifurcate into the two main
bronchi • The bronchi (Z = 1), which divide into the lobar,
segmental and 4 to 5 further divisions of cartilaginous bronchi
• The next eight generations (Z = 8 - 15) constitute progressively smaller, ciliated noncartilaginous bronchioles, the last of which is the terminal bronchiole which is about 0.5 mm in diameter
• Total number of airways is approximately 216, or 65,000 • An adult contains 300 million to 1 billion alveoli, with a
total alveolar surface area of 70 m2 • The tracheal cross sectional area is about 2.5 cm2 and this
cross sectional area is approximatley constant until the third generation • Cross sectional area then increases by approximately
7/5 per generation to 180 cm2 at the 16th generation
• The movement of substances through the lung capillary membranes is similar to that observed in the peripheral tissues however • Pulmonary capillary pressure is low (7
mmHg) • Interstitial fluid pressure is lower than in
the peripheral tissue • Pulmonary capillaries are leaky, so colloid
osmotic pressure is twice that observed in peripheral tissue, approaching 14 mmHg
• Alveolar walls are extremely thin
• When fluid forces are summed, a +1 mmHg filtration pressure is found, meaning that there is a slight movement of fluid from the capillaries into the lymphatic system
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Pulmonary Gas Exchange Diffusion Through the Respiratory Membrane
• Some respiratory units lack for adequate blood flow, while others lack for adequate air flow - even though total ventilation and total pulmonary blood flow is normal
• This imbalance may be described by the ventilation-perfusion ratio • Here, alveolar ventilation (VA) and blood
flow (Q) are compared • VA/Q = 0 means no alveolar ventilation • VA/Q = ∞ means no blood flow
• As VA/Q falls below normal, inadequate ventilation is observed: physiological shunt
• As VA/Q rises above normal, inadequate blood flow is observed: physiological dead space • This takes anatomical dead space into
account
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Transport of Gas in Blood and Tissues Transport of Oxygen from Lungs to Tissues
• Once in the systemic circulation and capillary bed, blood PO2 falls from 95 mmHg to 40 mmHg as the interstitial fluid takes up oxygen through pressure driven transport
• Transport is a function of blood flow rate, with increasing rates increasing oxygen transport into the interstitial fluid up to the 95 mmHg limit
• Transport is also a function of tissue consumption, with increasing consumption decreasing interstitial fluid PO2 or requiring a significant increase in flow to maintain interstitial fluid PO2
• Cellular PO2 is low due to the cell’s consumption of oxygen, but still in great excess to the cell’s minimum level of 1 to 3 mmHg Guyton & Hall. Textbook of Medical Physiology,
Transport of Gas in Blood and Tissues Transport of Oxygen from Lungs to Tissues
• Carbon dioxide moves in opposition to oxygen, but diffuses approximately 20 times as quickly • Thus small pressure differences in carbon
dioxide are observed between cellular and interstitial space, interstitial space and capillary blood, and arterial and venous blood
• Again, only one third of the capillary length is needed for “complete” carbon dioxide transport
• Again, a decrease in blood flow increases interstitial fluid carbon dioxide pressure - and an increase in blood flow decreases interstitial fluid carbon dioxide pressure
• Again, a increase in metabolism increases interstitial fluid carbon dioxide pressure and a decrease in metabolism decreases interstitial fluid carbon dioxide pressure
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Transport of Gas in Blood and Tissues Transport of Oxygen from Lungs to Tissues
• Normally, 97% of oxygen is carried by hemoglobin, while 3% is dissolved in the water solvent in blood
• As blood PO2 increases, hemoglobin saturation increases, typically resting at about 97% under normal conditions • Normally, 100 ml of blood carries 15 gm
hemoglobin which can bind oxygen at 1.34 ml / gm • Thus, 100 ml of blood carries 20 ml of oxygen
bound to hemoglobin • This is described as 20 volumes %
• Upon passing through the capillaries, blood “gives up” about 5 ml of oxygen to the tissues • 25% utilization
• During heavy exercise, up to 15 ml of oxygen is transported from the blood to the tissues • 75% utilization
Guyton & Hall. Textbook of Medical Physiology, 11th Edition
Transport of Gas in Blood and Tissues Transport of Carbon Dioxide
• PCO2 of venous blood is 45 mmHg and contains 2.7 ml CO2/dl blood, while PCO2 of arterial blood is 40 mmHg and contains 2.4 ml CO2/dl blood • Thus, for 100 ml of blood, only 0.3 ml of carbon
dioxide is transported in the dissolved form, or about 7% of total carbon dioxide
• RBC produce an enzyme, carbonic hydrolase, which catalyzes the reaction of carbon dioxide with water, forming carbonic acid • Carbonic acid dissociates to H+ and HCO3
-
• H+ bind with hemoglobin • HCO3
- diffuse into the plasma, exchanging with Cl-
• Carbonic acid mechanism accounts for 70% of normal carbon dioxide transport
• Carbon dioxide also binds with amine radicals of hemoglobin, forming carbaminohemoglobin • Carbamino transport accounts for about 20% of
normal carbon dioxide transport Guyton & Hall. Textbook of Medical Physiology, 11th Edition