Transcript circulatory system - Zanichelli online per la scuola

David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Anatomia e fisiologia dei viventi S
Circulatory System and
Gas Exchange
How Is the Circulatory System Made?
A circulatory system (also known as the
cardiovascular system) consists of:
• Muscular pump—the heart
• Fluid—blood
• Series of conduits—blood vessels
It’s a closed systems
(completely contain the fluid in a
continuous system of vessels)
with four-chambered hearts and
completely separate pulmonary
and systemic circuits.
The Human Heart and Circulation
Valves prevent back-flow of blood:
• Atrioventricular (AV) valves
between atria and ventricles prevent
back-flow when ventricles contract.
• Pulmonary valve and aortic valve
between ventricles and major
arteries prevent back-flow when
ventricles relax.
How Does the Mammalian Heart Function?
The right atrium receives deoxygenated blood from
the body through two large veins:
• Superior vena cava—blood from upper body
• Inferior vena cava—blood from lower body
Blood passes from the right atrium through an AV
valve into the right ventricle.
The atrium contracts, then the ventricle contracts. The
AV valve closes and blood is pumped through the
pulmonary artery to the lungs.
How Does the Mammalian Heart Function?
Pulmonary veins return oxygenated blood to the left
atrium.
Oxygenated blood moves into the left ventricle through
an AV valve.
The left atrium contracts, then the left ventricle. The
aortic valve opens and blood flows into the aorta.
How Does the Mammalian Heart Function?
In the cardiac cycle both sides of the heart contract at
the same time—first the atria, then the ventricles.
Two phases:
• Systole—when ventricles contract
• Diastole—when ventricles relax
How Does the Mammalian Heart Function?
Cardiac cells are in electrical contact with each other
through gap junctions—action potentials can spread
rapidly.
Large groups of cardiac muscle contract in unison—
essential for pumping blood efficiently.
Pacemaker cells can initiate action potentials without
input from the nervous system.
The primary pacemaker is the sinoatrial node.
The Heartbeat (Part 1)
The action potential in the atria stimulates the
atrioventricular node.
The node consists of non-contracting cells that send
action potentials to the ventricles via the bundle of
His.
The Heartbeat (Part 2)
Fibers spread throughout
the ventricles and are
called Purkinje fibers.
What Are the Properties of Blood Vessels?
Arteries carry blood away from the heart and branch
into arterioles that feed into capillary beds.
Capillaries: tiny, thin-walled vessels; site of exchange
between blood and tissue fluid.
Venules drain the capillary beds and form veins,
which deliver blood back to the heart.
What Are the Properties of Blood Vessels?
The anatomy of different types of vessels reflects their
functions.
Walls of large arteries have extracellular collagen and
elastin fibers, which enable them to withstand high
blood pressures.
Smooth muscle cells in
artery and arteriole walls
constrict or dilate those
vessels.
What Are the Properties of Blood Vessels?
Arterioles are called resistance vessels because their
resistance can vary to control the blood flow to specific
tissues.
Beds of capillaries lie between arterioles and venules.
Capillary walls are thin and permeable to water and
many solutes.
Blood flows slowly through capillaries, allowing time for
exchange.
What Are the Properties of Blood Vessels?
Capillary walls are a single
layer of endothelial cells and
are permeable to water, ions,
and small molecules, but not
to large proteins.
Permeability varies widely in
different capillary beds, water
and small solutes pass
through spaces in the capillary
wall, in some cases through
holes called fenestrations.
What Are the Properties of Blood Vessels?
Precapillary sphincters on arterioles can shut off
blood supply.
Auto-regulation depends on smooth muscle being
sensitive to its chemical environment.
Hyperemia: Low
O2 and high CO2
levels cause
smooth muscle
to relax,
increasing blood
supply to
capillary bed.
What Are the Properties of Blood Vessels?
Blood flows through large arteries rapidly at high
pressures, but when it reaches the capillaries,
pressure and rate of flow decrease.
The sum total cross-sectional area of the capillaries is
much greater than that of any other class of vessels.
What Are the Properties of Blood Vessels?
Blood returning from the
upper body is assisted by
gravity.
Blood returning from below
the heart is assisted by
skeletal muscle contractions
that squeeze the veins.
One-way valves in the
veins prevent back-flow.
How Is the Circulatory System Controlled and Regulated?
The cardiovascular control center in the medulla
controls heart rate and vessel constriction.
Information on blood
composition and
pressure comes from
baroreceptors (stretch
receptors) and
chemoreceptors in the
aorta and carotid
arteries.
What Are the Properties of Blood?
Blood is a connective tissue made of cells in a liquid
extracellular matrix, called plasma.
Packed-cell volume, or hematocrit, is the percent of
blood that is red blood cells.
Normal hematocrit is
42– 46%, but can
vary: higher in people
who live at high
altitudes because low
O2 stimulates
production of more
red blood cells.
What Are the Properties of Blood?
Mature erythrocytes, or red blood cells, are
biconcave, flexible discs packed with hemoglobin.
They transport respiratory gases; the shape gives
them a large surface area for gas exchange, and
flexibility enables them to squeeze through narrow
capillaries.
Red blood cells are generated in the bone marrow.
Erythropoietin, a hormone released in the kidney in
response to hypoxia, controls red blood cell
production.
What Are the Properties of Blood?
Bone marrow stem cells also produce megakaryocytes
that break off cell fragments called platelets.
Platelets initiate blood clotting when activated by
collagen exposed in damaged blood vessels.
They release chemical clotting factors that activate
other platelets and also form a sticky plug at the
damaged site.
What Are The Major Diseases?
Atherosclerosis is “hardening of the arteries.”
The endothelial lining of
arteries is damaged by
high blood pressure,
smoking, high-fat diet,
or microorganisms.
Plaque forms at
damage sites.
What Are the Major Diseases?
The coronary arteries supply blood to the heart muscle.
Atherosclerosis in coronary arteries reduces blood flow; marked
by chest pain and shortness of breath.
Coronary thrombosis—a thrombus that forms in a coronary
artery can lead to a heart attack, or myocardial infarction.
An embolus is a piece of a thrombus that breaks loose.
• It may cause an embolism if it lodges in a smaller blood
vessel.
• Arteries narrowed by plaque are likely places for an
embolism.
• If the embolism is in the brain, the cells fed by that artery will
die, causing a stroke.
Why Do Animals Need a Circulatory System?
Some animals do not need circulatory systems:
• Single-celled organisms exchange materials
directly with the environment.
• Some aquatic multicellular organisms are small or
thin, so that all cells are close to the environment to
allow exchange.
• Other aquatic organisms have branched
gastrovascular systems that bring the external
environment inside the animal (e.g., sponges).
• Large animals without a circulatory system tend to be
inactive or sessile.
Why Do Animals Need a Circulatory System?
Open circulatory systems (fluid leaves the circulatory
system and moves between the cells) are found in
arthropods and mollusks.
Closed circulatory systems:
• Blood is kept separate from the interstitial fluid.
• Blood is pumped through the vascular system by one
or more hearts.
Earthworms have a closed system.
How Have Vertebrate Circulatory Systems Evolved?
In fishes, the heart pumps blood
to the gills and then the rest of
the body in a single circuit.
Turtles, snakes, and lizards
have three-chambered hearts.
Birds and crocodiles have four
completely separated chambers.
Amphibians circulatory system.
Birds and mammals have
four-chambered hearts and
completely separate pulmonary
and systemic circuits.
What Physical Factors Govern Respiratory Gas Exchange?
Animals must exchange the respiratory gases O2 and
CO2.
Gas exchange systems are made up of:
• Specialized surfaces where gas can move between
the body and the environment
• Mechanisms that ventilate the environmental side
and perfuse the internal side.
Passive diffusion is the only means of gas exchange
across these surfaces.
How Do Human Lungs Work?
Air enters the human lung
through the mouth or nasal
passage, which join in the
pharynx.
Below the pharynx, the
trachea leads to the lungs—
at the beginning is the
larynx, or voice box.
How Do Human Lungs Work?
The trachea branches into two bronchi, then into
bronchioles, and then into alveoli—the sites of gas
exchange.
The combined surface area of the alveoli is about
70 m2.
Alveoli have thin walls and are surrounded by
capillaries. The diffusion path between blood and air
is less than two micrometers.
The Human Respiratory System
How Do Human Lungs Work?
Human lungs are suspended inside the thoracic
cavity.
The diaphragm is a sheet of muscle at the bottom of
the cavity.
The pleural membrane covers each lung and lines the
thoracic cavity.
The pleural space contains fluid to help the membranes
slide past each other during breathing.
A surfactant reduces the surface tension of a liquid.
How Do Human Lungs Work?
Inhalation begins when the diaphragm contracts while
exhalation begins when the diaphragm stops
contracting and relaxes.
The intercostal
muscles between
the ribs can also
change the volume
of the thoracic
cavity.
Abdominal muscles
also participate.
What Adaptations Maximize Respiratory Gas Exchange?
A spirometer measures the amount of air breathed in
and out.
Tidal volume (TV) is the amount of air that moves in
and out per breath when at rest.
Inspiratory (IRV) and expiratory (ERV) reserve
volumes are the additional amounts of air that we can
forcefully inhale or exhale.
Vital capacity (VC) = TV + IRV + ERV.
Athletes generally have high vital capacity. VC
decreases with age because lung tissue stiffens.
What Adaptations Maximize Respiratory Gas Exchange?
Residual volume (RV) = air that can’t be expelled
from the lungs.
Allows calculation of the functional residual volume
(FRV) which equals ERV + RV.
How Is Breathing Regulated?
Breathing is controlled by neural circuits in the brain
stem.
Axons of respiratory motor
neurons in the medulla
form the phrenic nerve,
which innervates the
diaphragm and initiates
inhalation.
Neurons in the pons help
regularize the basic
respiratory rhythm.
How Does Blood Transport Respiratory Gases?
Blood plasma carries some O2 in solution, but most
vertebrate blood contains molecules that bind reversibly
to O2.
O2 is picked up where the partial pressure is high and is
released where the partial pressure is lower.
Red blood cells contain the protein hemoglobin, which
has 4 polypeptide subunits.
Each polypeptide surrounds an iron-containing heme
group that can bind a molecule of O2.
How Does Blood Transport Respiratory Gases?
The change in affinity is reflected in the increased
steepness of the O2 binding curve.
The influence of O2 binding by one subunit on the O2
affinity of the other subunits is called positive
cooperativity.
How Does Blood Transport Respiratory Gases?
Blood must also carry CO2 away from tissues.
CO2 is highly soluble and readily diffuses into the
blood plasma, where it is converted into bicarbonate
ions (HCO3–).
In endothelial cells and red blood cells carbonic
anhydrase speeds up the conversion.
How Does Blood Transport Respiratory Gases?
Myoglobin in muscle cells is a single polypeptide
molecule that can bind one molecule of O2.
It has a higher affinity
for O2, and binds it at
low PO2 values when
hemoglobin molecules
would release their O2.
It provides a reserve for
high metabolic demand
for O2.
What Physical Factors Govern Respiratory Gas Exchange?
Diffusion rate of O2 in
water is very slow.
This limits the size and
shape of invertebrate
species without internal
systems for gas exchange.
They are mostly small;
some have flat bodies to
increase surface area; or
thin bodies built around a
central cavity.
What Adaptations Maximize Respiratory Gas Exchange?
Adaptations to maximize respiratory gas exchange:
• Increase surface area
• Maximize partial pressure gradients
• Minimize diffusion path length
• Minimize diffusion that takes place in an aqueous
medium
What Adaptations Maximize Respiratory Gas Exchange?
Insects have a tracheal respiratory
system.
Fish gills use countercurrent
flow to maximize gas
exchange.
What Adaptations Maximize Respiratory Gas Exchange?
Bird lungs have unidirectional air flow, instead of
bidirectional as in mammals.
Interconnected air sacs receive inhaled air but are not
sites of gas exchange.
Air enters through the trachea, which divides into
primary bronchi, which extend to the posterior air
sacs and branch into secondary bronchi.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved