Microsoft PowerPoint - specific circulation.ppt []
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1. Define the special features of the circulation in the brain,
coronary vessels, skin, and fetus, and how these are
regulated.
2. Delineate how the oxygen needs of the contracting myocardium are
met by the coronary.
3. Understand how the fetus is supplied with oxygen and nutrients
in utero, and the circulatory events required for a transition to
independent life after birth.
4. List the vascular reactions of the skin.
Circulation
Cerebral circulation--- vessels
Blood is supplied to the brain, face, and scalp via two major sets
of vessels: the right and left common carotid arteries and the
right and left vertebral arteries.
1. The total cerebral blood flow: 55 ml/min/100g brain. 2. Brain is
the least tolerant of ischemia (> 5 seconds ---- loss of
consciousness)
Circle of Willis
1. Cerebrospinal Fluid (CSF) fills the ventricles and subarachnoid
space.
2. 2/3 of the CSF is formed in the choroid plexuses and the
remainder is formed around blood vessels and along ventricular
walls. The CSF in the ventricles flows through the foramens of
magendie and luschka to the subarachnoid space and is absorbed
throuh the arachnoid villi into veins.
3. The most critical role for CSF is to protect the brain.
4. The composition of CSF is essentially the same as that of brain
extracellular fluid.
The BBB maintains the constancy and protects the brain from
endogenous and exogenous toxins in the blood.
Blood-Brain Barrier
• Postganglionic Cholinergic neurons---acetylcholine, vasoactive
intestinal peptide (VIP), and peptide histidyl methionine
(PHM-27)
• Sensory nerves---substance P, neurokininA, and calcitonin
gene-related peptide (CGRP)
• Vasodilation substance P, CGRP, VIP, and PHM-27 • Vasoconstrictor
neruopeptide Y
Local factors • Total cerebral blood flow is constant. However,
regional cortical blood
flow is associated with regional neural activity. (nitric oxide and
adenosine may be involved).
Different stimuli elicit specific regional blood flow in human
cerebral cortex
•
The cerebral vessels are very sensitive to blood CO2
tension
(PaCO2).Increased PaCO2 elicit marked cerebral vasodilation;
reduced PaCO2 causes hyperventilation, decreased cerebral blood
flow.
•
This change may be probably due to alteration of intracellular pH,
vessels diameter (blood flow) and pH are inversely related
• PaCO2
pH leads to vessels vasodilation, increases blood
flow.
• O2
Regulation of cerebral circulation
The cerebral circulation shows reactive hyperemia and excellent
autoregulation between pressure of about 60 and 160 mm Hg; pressure
> 160 mm Hg may increase the permeability of the blood brain
barrier and cause cerebral edema. The noradrenergic discharge (NE)
occurs when the blood pressure is markedly elevated.
produced by sympathetic stimulation
Brain metabolism
• O2
consumption by human brain about 3.5 ml/100g brain/min
approximately 20%
of the total body resting O2
consumption.
• Brain is extremely sensitive to hypoxia.
• Glucose
is the major ultimate source of energy for the brain.
•
In general, glucose utilization at rest parallels blood flow and O2
consumption. •
The brain’s uptake of glutamate is approximately balanced by its
output of glutamine. •
Ammonia is very toxic to nerve cells.
•
In brain glutamate + ammonia and release glutamine as a
detoxifying mechanism
Coronary circulation
Coronary arteries
Heart chambers
Arteriouluminal vessels
Thebesian veins
Extracoronary arteries
Pressure (mm Hg) in
Aorta Left Vent
80 80 800 0
Coronary blood flow
•
Changes of aortic pressure generally shift
coronary blood flow •
Heart influences the coronary blood flow by squeezing
effect
of the contracting myocardium on the blood vessels
•
The force is so great during early ventricular systole that blood
flow in left coronary artery is briefly reversed.
•
Left coronary inflow is maximal in early diastole, when the
ventricles have relaxed and extravascular compression of the
coronary vessels is absent.
•
Right coronary artery shows a similar pattern, but because of
the lower pressure developed by the thin right ventricle
during systole, blood flow does not reverse in early systole.
•
During systole there is no blood flow in the
subendocardial portion of the left ventricle, this region is
prone to ischemic damage and is the most common site
of myocardial infarction.
• Patients with stenotic aortic valves
develop symptoms of myocardial ischemia.
•
Coronary flow is also decreased when the aortic diastolic
pressure is low
in congestive heart failure
Regulation of Coronary blood flow ---Chemical factors
• A decrease in the ratio of O2 supply / O2 demand release a
vasodilators from myocardium into the interstitial fluid, leading
to relax the coronary resistance vessels.
• Numerous metabolites such as CO2, K+, H+, lactate,
prostaglandins, adenine nucleotides and adenosine cause coronary
vasodilation.
• Accumulation of vasoactive metabolites (adenosine) may also be
responsible for reactive hyperemia in the heart.
• Increased metabolic activity of the heart decreases coronary
resistance, whereas a reduction in cardiac metabolism increases
coronary resistance.
• Under normal conditions, blood pressure is kept within relatively
narrow limits by the baroreceptor reflex.
• Therefore, changes in coronary blood flow are caused primarily by
caliber changes of the coronary resistance vessels in response to
the metabolic demands of the heart.
Neural factors • The coronary arterioles contain • -adrenergic
receptors ----- vasoconstriction, • -adrenergic receptors -----
vasodilation
• Activity in noradrenergic nerves to the heart and injections of
norepinephrine cause coronary vasodilation.
• However, norepinephrine increases the heart rate and the force of
cardiac contraction, and the vasodilation is due to production of
vasodilator metabolites in the myocadium secondary to the increase
in its activity.
• Stimulation of vagal fibers to the heart dilate the
coronaries.
Cutaneous circulation
• Core temperature and skin temperature.
• The primary function of the cutaneous circulation is maintenance
of a constant body temperature.
• Blood flow to the skin widely depends on the need for loss or
conservation of body heat. ----- mainly by changes in ambient and
internal body temperatures.
AV anastomoses shunt
V
• AV anastomoses
thick muscle walls and richly supplied
with nerve fibers.
•
AV anastomoses are highly sensitive to sympathetic nerves
vasoconstrictor agents such as epinephrine and
norepinephrine causing vasoconstriction.
•
In skin, neural control is more important than local factors
•
Thus, the regulation of blood flow through these
anastomotic channels is governed mainly by temperature
receptors or by higher centers of the central nervous system
(CNS).
• Parasympathetic vasodilator nerve fibers not supply the cutaneous
blood vessels.
• Sweat glands innervated by cholinergic fibers of sympathetic
nervous system vessels dilation
• Sweat contains an enzyme to release of bradykinin, act locally to
dilate the arterioles and increase blood flow to the skin
CNS control: particularly in the head, neck and upper chest •
Blushing: inhibition of sympathetic nerve fibers to the face •
Blanching : stimulation of the sympathetic nerve fibers to
the face.
Under normal conditions, ambient temperature is main factor in
the regulation of skin blood pressure
• Respond to cold causing direct vasoconstriction may be mediated
by the nervous system
• Cooled blood returning to the general circulation and stimulating
the temperature-regulating center in the anterior hypothalamus
----- reflex vasoconstriction
• Prolonged exposure of the hand to severe cold ( near 0 oC) has a
secondary vasodilator effect ----- reddening, alleviation of
pain.
• Heat causes local vasodilation of cutaneous vessels is regulated
by temperature-regulating center in the anterior
hypothalamus.
• The rosy face in the cold is a example of cold vasodilation.
However, blood flow through the skin of the face may be very low
despite the flushed appearance.
• The reddness is largely the result of the reduced O2 uptake by
the cold skin and the change in the affinity of hemoglobin for O2
as reflected by the cold-induced an increased O2 affinity for
hemoglobin.
• Amount of blood in the skin and degree of oxygenation of blood in
the subcutaneous vessels determine the color of face.
Skeletal muscle circulation
• Blood flow to skeletal muscle varies directly with the
contractile activity of the tissue and the type of muscle.
• Blood flow and capillary density in red muscle are greater in red
muscle than in white muscle.
• In resting muscle the arterioles exhibit asynchronous
intermittent contractions and relaxations.
• Total blood flow in quiescent muscle (1.4~ 4.5 ml/min/100g),
exercise may increase 15 to 20 times its resting level.
Skeletal muscle circulation
• At rest, neural and myogenic control predominates, during
exercise ---- metabolic control is much more important. Basal tone
and sympathetic nerve activity to muscle vessels regulate blood
flow in resting condition
• The tonic activity of the sympathetic nerves is greatly
influenced by the baroreceptor reflex.
• At rest, the effect of norepinephrine release from the
sympathetic nerves is more important than epinephrine released from
adrenal medulla.
• In skeletal muscle and skin do not receive parasympathetic
innervation.
Autoregulation of blood flow
Over the pressure range of 20 to 120 mm Hg, the steady-state flow
is relatively constant.
Myogenic mechanism
The vascular smooth muscle contracts in response to
increased stretch and relaxes with a reduction in
stretch.
An abrupt increase in perfusion pressure initially
distends the blood vessels then followed by
contraction of vessels and return of blood flow to the
previous control level.
Metabolic hypothesis • Blood flow is governed by the metabolic
activity of
the tissue. • When the metabolic rate of the tissue increases
or
the O2 delivery to the tissue decreases, more vasodilator substance
is formed and blood flow increases.
• Lactic acid, CO2, H+, Na+, inorganic phosphate ions, adenosine
and nitric oxide have been proposed as mediators of metabolic
vasodilation.
• Because blood pressure is kept fairly constant, tissue metabolic
activity and blood flow may vary together under physiological
conditions.
Metabolic regulation of tissue blood flow
The peak flow and duration of the reactive hyperemia (as a result
of the accumulation of these metabolites) are proportional to the
duration of the occlusion period.
Occlusion ---- decrease O2 delivery ----- metabolites accumulation
------ vasodilation --- -- increase blood flow
Splanchnic circulation
The blood of splanchnic capillary beds ultimately flows into the
portal vein, which normally provides most of the blood supply to
the liver. However, the hepatic artery also supplies blood to the
liver.
Intestinal circulation
Neural regulation • Neural control of the mesenteric circulation is
almost
exclusively sympathetic causing constriction mediated by a-
receptors.
• However, b-receptors are also present causing vasodilation.
Autoregulation
• The principal mechanism responsible for autoregulation is
metabolic, although a myogenic mechanism probably also
participates. Functional hyperemia
• Food ingestion and food absorption increase intestinal blood
flow. The principal mediator are glucose and fatty acids.
• Secretion of gastrin and cholecystokinin augments intestinal
blood flow
Hepatic circulation
Regulation of flow •
Blood flow in the portal venous and hepatic arterial
systems varies reciprocally. When blood flow is curtailed
in one system, the flow increases in the other.
•
The sympathetic nerves constrict the vessels in the
portal venous and hepatic arterial systems mediated by
receptors.
•
The liver contains about 15% of the total blood volume
of the body. If hemorrhage liver act as an important
blood reservoir in human.
Clinical case
•
Extensive fibrosis of the liver, such as hepatic
cirrhosis, increases hepatic vascular resistance,
which raises portal venous pressure
substantially.
•
The increased capillary pressure throughout
the splanchnic circulation leads to extensive
fluid transduction (ascites) into the peritoneal
cavity.
Placental and fetal circulation
Uterine circulation--- During pregnancy, blood flow increases
rapidly as the uterus increase in size.
Vasodilator metabolites Estrogens Corticotrophin-releasing
hormone
Placenta
• The placenta is the fetal lung and also the route providing
nutritive materials
• O2 is taken by the fetal blood and CO2 is discharged into the
maternal circulation. However the exchange is much less efficient
than in lung.
Fetal circulation
• The blood in the umbilical vein ----- ductus venosus -----
inferior vena cava, the remainder mixes with the portal blood of
the fetus -- --- to the left atrium via foramen ovale (FO) -----
left ventricle (better-oxygenated blood) ----- head of fetus
• Blood in superior vena cava to right ventricle and expelled into
the pulmonary artery.
• Since the resistance of the collapsed lung of fetus is high -----
most of the blood in the pulmonary artery passes through the ductus
arteriosus (DA) to the aorta ----- to the trunk and lower body (
lower O2 saturation).
• From the aorta, some of the blood is pumped into umbilical
arteries and back to the placenta.
Changes in fetal circulation at birth
• The fetal red cells contain fetal hemoglobin F with greater
affinity to O2. After 4 months 90% of the circulating hemoglobin is
hemoglobin A.
• After birth ----- placental circulation cut off ----- the
peripheral resistance suddenly rise.
• Lung expansion ----- the pulmonary vascular resistance falls
----- pulmonary blood flow increases markedly ----- returning to
left artium - ---- closing the foraman ovale ----- the ductus
arteriosus constricts within a few hours after birth producing
functional closure.
• The increase in arterial O2 tension plays an important role •
Relatively high concentrations of vasodilators
(prostaglandin)are
present in the ductus in utero.
Thermoregulation
• Homeostasis---
All homeostatic mechanisms use negative feedback to maintain a
constant value (called the set point). •
Thermoregulatory responses to cold
• Thermoregulatory responses to heat
Introduction •
Alterations of metabolic activity and temperature of
environment body temperature change
•
Different regions of the body have different
temperatures at rest.
•
The highest temperature (core temperature): brain,
thoracic and abdominal cavities
•
The lowest temperature (shell temperature): skin
•
When temperature is measured in the mouth: (95%)
36.337.1oC
The circadian rhythm of core body temperature in day.
Effect of ambient temperature on body temperature.
In women: an increase in body temperature of about 0.5oC following
ovulation, which persists until steroid levels fall.
> 42oC : proteins and enzymes denaturation ----- cell damage,
death
< 33oC : temperature regulation is impaired and consciousness is
lost
Heat exchange •
The more metabolically active, the more heat produced.
•
The most heat production in organs such as brain, skeletal
muscle, liver and kidney. •
Thermoneutral zone: the range of environmental temperature
2731oC
easy for the body to maintain its core temperature.
•
Within thermoneutral zone thermoregulation is achieved
solely by alterations in the blood flow to the skin.
•
Superficial venous plexus : accommodate a large volume of
blood •
In toes, ears, fingers and nose, arteriovenous anastomoses can
open and close according to thermoregulatory requirement
leading to loss or reduce of heat
Mechanisms of heat exchange
• The evaporation of sweat
Evaporation •
Approximately 580 Kcal of heat are lost for each
liter of water that evaporates from the body
surface.
•
The insensible water loss is about 600 ml/day =
390 Kcal of heat per day. (from lung, mucosa of
the mouth, skin in resting state.
•
During vigorous muscle activity increases more
sweat production 16 liters/h.
• Body temperature is chiefly regulated by neurons that lie within
the hypothalamus
• Hypothalamus receives afferent input from peripheral
thermoreceptors in skin and central thermoreceptors
• The skin has two kinds of thermoreceptor. Cold receptor :
δ-myelinated afferents (maximal rate of discharge at 25-30oC); warm
receptor: C-fibers (maximal rate of discharge at 40oC)
Regulation of body temperature – Role of the hypothalamus
• Body temperature regulation
set point = 37oC controlled by the
hypothalamus.
•
The hypothalamus exerts its thermoregulatory actions on the vasculature of
the skin, sweat glands, and adipose tissue via autonomic nervous system.
Thermoregulatory responses to cold
1. Cutaneous vasoconstriction •
Within the thermoneutral zone, blood flow to the
skin is around 200 ml/min
•
Increase in sympathetic outflow to the cutaneous
vessels initiated by the neurons in the posterior
hypothalamus (chiefly by the action of
norepinephrine at receptor)
reduce blood flow (20 ml/min)
reduce heat lost
During long periods of cold exposure, Hunting reaction is designed
to reduce the risk of ischemic tissue damage. The mechanism is
unclear but may result from a temporary loss of sensitivity to
norepinephrine.
Periodic vasodilation is thought to delay the onset of tissue
damage.
2. Increased heat production from shivering •
Metabolic heat production is increased by voluntary
muscle contraction or shivering in response to signals
from the somatic motor neurons arised in the
hypothalamus.
•
When muscle contracts, ATP is hydrolyzed and heat is
produced.
3. Nonshivering thermogenesis •
Heat stimulated by calorigenic hormones :
glucocorticoids, insulin and glucagon; or stimulation of
brown fat metabolism
Thermoregulatory responses to heat 1. Cutaneous
vasodilation •
The dilatation is mediated by the autonomic nervous system,
mainly through a reduction in vasomotor tone
increases heat loss
2. Enhanced sweating •
> 3032 oC increase sweating to 1.56 L/day from 500 ml/day in
cold condition •
The sweat glands are innervated by sympathetic fibers, most of
cholinergic via muscarinic receptors •
Sweat rates increase
in response to increased circulating
catecholamines form adrenal medulla •
It is important that fluid and salt must be replaced quickly.
•
The sodium chloride content of sweat may be lower, possibly as a
result of increased aldosterone secretion.
Disorders of thermoregulation
Hypothermia • It is inadvisable to warm the surface of a
hypothermic patient too rapidly, since the increased blood flow to
the periphery may compromise blood flow to the body’s vital organs
such as brain and lead to further problems.
Fever • Most often associated with infectious diseases,
dehydration, pyrogen produced by bacteria or by immune system
(monocytes , macrophages and astrocytes in brain) in response to
infection.
• The development of fever seems to involve a shift in the
set-point which the core temperature is regulated.
• The mechanism may involve an alteration in the firing rates of
preoptic neurons in the hypothalamus.
The time-course of typical febrile profile
Temperature regulation in newborn infant
•
The neonate has a high surface area to volume ratio
readily lost from the skin’s surface; its layer of
insulating fat is comparatively thin
causes the
core temperature of the body to drop to around 35
oC during the first hours of its life.
•
The thermoregulatory mechanisms are only partly
functional at birth
•
The extra heat is generated by nonshivering
thermogenesis via the metabolism of brown adipose
tissue or brown fat.
Brown fat in neonate
• Metabolism of brown fat is triggered by increased plasma levels
of circulating norepinephrine released by sympathetic nerve
endings.
• Cold stress increases sympathetic nerve activity and secretion of
epinephrine and norepinephrine by the adrenal medulla.
• The brown fat as a effective source of heat for the newborn
baby.
References
• Ganong’s Review of Medical Physiology 23rd
edition , Chapter 34.
• Vander’s Human physiology, Chapter14.
•
Guyton & Hall, Textbook of Medical Physiology.
Chapter 73
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