Flashcards in Medical Physiology Block 8 Week 1 Deck (62):
species that do regulate internal body temperature
Are relatively stable over a broad range of ambient conditions
Describe the components regulating human internal body temperature.
Components of the system: thermal sensors, afferent pathways, and integration system in the CNS, efferent pathways, and target organs that control heat generation and transfer (skeletal muscle shivering, dissipation of heat through skin, and dissipation of heat through sweat glands)
When is body temperature lowest? highest? why?
3-6 AM; 3-6 PM
Entrained by light-dark cues to a 24-hour cycle
How is body temperature changes in a postovulatory women?
raised by half a degree Celsius
Why do infants have difficulty controlling body temperature?
cannot shiver or sweat
Why do the elderly have difficulty controlling body temperature?
weakened sensation, decreased metabolism, and reduced ability to dissipate heat
Which hormones increase metabolic rate?
thyroxine and epinephrine
Why is the skin an ideal location for heat transfer to the environment?
large surface area
Describe heat transfer by radiation.
occurs between the skin and solid bodies in the environment
Describe heat transfer by conduction. Is this a major factor in humans?
occurs when the body touches a solid material of different temperature
Describe heat transfer by evaporation.
is proportional to the water vapor pressure gradient between skin and environment
Is the rate of heat exchange greater or smaller in water compared to air?
What happens to the temperature gradient and water vapor gradient in windy conditions (increased air velocity)
With forced air movement (wind), the cooler "ambient" air replaces the air overlying the skin more rapidly (increased heat exchange)
Increased air velocity over the skin increases the effective vapor pressure gradient between skin and the overlying air because of the faster movement of water vapor away from the skin
Describe the body's sensors for temperature.
Free nerve ending distributed over the entire skin surface are specialized sensory neurons that provide the CNS with information about the body's thermal condition
Body core sensors are at high densities in the preoptic area and anterior hypothalamus (important for sensing increases in internal temperature during exercise)
Warm receptor fibers are very sensitive to an acute rise in skin temperature but deactivate at higher temperatures
How does the body increase heat dissipation?
With a moderate heat load, the autonomic response primarily increases the heat transfer rate from core to skin by increasing cutaneous blood flow
However, when the heat load is sufficiently great, the autonomic nervous system also activates the eccrine sweat glands, which secrete sweat on the skin surface, thus elevating the partial pressure of water vapor there and promoting increased evaporation
How does the body preserve or increase heat production?
When a cold stress is sufficiently great, the physiological response includes increasing heat production by involuntary, clonic, rhythmic contraction and relaxation of skeletal muscle (shivering); in addition to vasoconstriction of cutaneous vessels
What innervates the sweat gland?
sympathetic nervous system (Ach)
Describe internal body temperature during exercise.
Steady-state core temperature is slightly elevated during exercise (persists as long as exercise continues)
The slight initial drop in core temperature at the onset of exercise is caused by flushing out of blood from the cooler peripheral circulation when the muscle and skin beds vasodilate
Mean skin temperature decreases during exercise because of the increased evaporative cooling of the skin caused by sweating
Why does the steady-core body temperature rise during exercise?
Consequence of the initial imbalance between rates of heat production and dissipation.
What is the defense mechanism for preventing hypothermia when immersed in cold water?
A thick layer of insulating fat retards heat loss to the water and postpones or even prevents hypothermia during prolonged exposures
How does heat transfer change when ambient temperature is greater than 30 degrees Celsius?
When ambient temperature reaches the mid-30s (ºC), evaporation becomes the only effective avenue for heat dissipation.
How does heat transfer change when exposed to high humidity?
high ambient humidity reduces the skin-to-environment gradient for water vapor pressure and thus reduces evaporation
Fever is caused by the action of circulating cytokines called pyrogens, where are low-molecular weight polypeptides produced by cells of the immune system
Fever begins when heat production temporarily exceeds heat dissipation
Modulated by interleukins and production of prostaglandins (E2)
What is the bodies response to a fever when a patient is in a warm environment? cold?
If fever strikes when the patient is in a warm environment in which the cutaneous vessels are dilated, the response will be to vasoconstrict to decrease heat loss
If the patient is in a cold environment the cutaneous vessels are already constricted, so the response will be to shiver
What is the difference between warm-blood and cold-blooded animals?
Warm-blooded creatures, like mammals and birds, try to keep the inside of their bodies at a constant temperature. They do this by generating their own heat when they are in a cooler environment, and by cooling themselves when they are in a hotter environment.
Cold-blooded creatures take on the temperature of their surroundings. They are hot when their environment is hot and cold when their environment is cold.
What is the difference between endothermy and ectothermy?
endothermy: a warm
-blooded animal that controls its body
temperature by producing its own heat
ectothermy: an animal that relies on external environment for temperature control instead of generating
its own body heat (cold-blooded)
What is the heat balance equation?
Heat production (metabolism - work done) - heat losses (radiation, convection, evaporation)
What is the Van't Hoff rule?
For every 10 degree Celsius increase/decrease in temperature, the reaction rate is doubled/halved.
What is the energy source for muscle contraction for a 1.5 minute run?
Nonoxidative breakdown of carbohydrates from glycogen breakdown and oxidative breakdown of fat and glucose
What is VO2 max?
Maximal aerobic power/maximal oxygen consumption
What is the fuel source for low intensity exercise (less than 30% VO2 max)? high intensity (greater than 70% VO2 max)?
What is fatigue?
decrease in the number of active cross bridges formed and the force produced per cross bridge (decreased force of contraction)
What is central fatigue?
reduced excitatory input to motor control centers in the brain and spinal cord; reduced output to alpha and gamma motor neurons
What is peripheral fatigue?
impairments in initiation and propagation of muscle action potentials; depleted substrates for energy metabolism and accumulation of lactate
Describe peripheral fatigue caused by high frequency stimulation.
caused by continuous firing of action potentials during intense exercise
sodium entry and potassium exit exceed ability of sodium/potassium pump (inactivation of voltage gated sodium channels)
recovery is rapid
Describe peripheral fatigue caused by low frequency stimulation.
caused by prolonged, moderate intensity exercise
calcium release from SR falls, which decreases intracellular calcium
recovery requires several hours
During the first 2-3 minutes of an aerobic activity, prior to the ability to consume enough oxygen to totally meet the energy needs, the difference in the energy requirement and the energy supplied by the aerobic systems is called?
What is oxygen debt?
the extra volume needed to restore the energetic system
How are athletes and sedentary people different in terms of oxygen uptake (VO2 max)?
athletes have higher oxygen consumption prior to beginning exercise
a sedentary person will reach VO2 max quicker than an athlete during exercise
An elite/trained athlete will have a higher resting O2 consumption, a higher VO2 max and is able to reach this at a higher intensity than an untrained person
Describe blood flow during exercise
decrease in blood flow to kidney, liver, and GI tract
increased blood flow to working skeletal muscle
oxygen uptake by the muscle depends on blood flow and oxygen contraction
redistribution of blood depends on metabolic rate and exercise intensity
convective delivery rate = cardiac output x arterial oxygen content
How does cardiac function change with exercise?
heart rate increases with exercise intensity and reaches a new steady state (changes dependent on intensity)
stroke volume increases but plateaus because: increased heart rate decrease stroke volume but increased contractility, preload, and decreased afterload all increase stroke volume
mean arterial pressure increases because: cardiac output increases and total peripheral resistance slightly decreases as a consequence of muscle vasodilation
What is rete mirabile?
There is a network of blood capillaries between red swimming muscles (in a mackerel shark with a length of 1m, it has a surface of 4m²!), which acts as a heat exchanger. The heat created through muscle activity is transported by the blood in the vessels that takes oxygen-depleted blood to the gills. Blood vessels with thin walls and cold, oxygen-rich blood from the gills are running in the opposite direction along the heated vessels and are warmed this way.
Thus, heat is transferred to the blood flowing into the body, while the blood flowing towards the gills cools off already and cannot lose as much heat to the outer medium anymore. This principle of opposite streams is so efficient that hardly any body heat is lost through the gills
How does exercise affect the lungs?
What converts chemical energy to mechanical work?
cross-bridge cycling and SERCA pump
What is the size principle?
small motor units are recruited before larger motor units
Small motor neuron depolarizes rapidly and has high membrane resistance (however conduction velocity is low)
associated with type 1 fibers
What is rate coding?
determinant of force is the frequency with which the muscle fibers are stimulated by their innervating axon.
T/F: When a motor nerve reaches threshold to fire an action potential, all the muscle fibers it innervates contract
What are muscle fibers classified by?
myosin heavy chain
T/F: all of the muscle fibers a motor neuron innervates are the same fiber type?
Which fiber type would be most beneficial to a marathon runner?
The ability to generate force is proportional to what?
number and diameter of the fibers
Describe anaerobic training paradigm.
Neural adaptations (4-6 weeks of program): increased motor unit activation, firing frequency, and firing synchronization; decrease in Golgi tendon organ inhibition; selective recruitment of type 2 fibers
Muscular adaptations (second month of training): hypertrophy, facilitation of fiber type transitions; increased enzyme and substrate concentration; tissue repair (satellite cells and down-regulation of myostatin)
Describe muscle learning.
When learning a new skill, motions are uncoordinated (recruitment of antagonistic muscles)
Fatigue of the muscle will lead to recruitment of larger motor units to maintain energy resulting in loss of fine control (costly due to greater energy expenditure)
T/F: For athletes with high intensity, short duration exercise, oxidative and glycolytic pathways are modes for replenishing phosphocreatine lost during the exercise period (improve lactate threshold)
Describe parallel muscle fibers.
a muscle with a few long fibers in parallel with the axis of shortening produces rapid shortening but less tension
Describe oblique muscle fibers.
a muscle with many short fibers at an angle to the axis of shortening produces slow shortening but more tension
Which sweat glands contribute to temperature regulation?
Describe eccrine glands.
Eccrine sweat glands are tubules comprising a secretory coiled gland and a reabsorptive duct
The release of acetylcholine stimulates the muscarinic receptors on the acinar cells and causes them to secrete into the lumen a protein-free plasma-like fluid (PLC; a Na/K/Cl cotransporter mediates the uptake of chloride across the basolateral membrane, and the chloride exits across the apical membrane through a chloride channel; lumen-negative voltage drives sodium secretion through the paracellular pathway; osmotic gradient for water)
Duct cells reabsorb salt and water until the fluid reaches the skin surface through an opening (sweat pore)
water reabsorption is limited
Describe acclimatization to heat.
Aldosterone mediates the ability to reabsorb sodium chloride
Why are type IIb fibers ideal for sprinters?
fast contraction speed, large motor neuron size, and high anaerobic enzyme content
What is an adaptation to aerobic exercise?
increase maximum heart rate, increase mitochondrial content, increase GLUT4 content, angiogenesis, and increase in acetylcholine release