Chapter 9 Circulatory systems
I.
Overview
1.
Developed by Robert Brown in
1827, Brownian motion considering the random movement of particles in a
solution: the time needed for a particle to diffuse across a given distance is
proportional to the square of the distance (t
α
x2).
2.
A small molecule such as
glucose in aqueous solution takes about 5 seconds to diffuse across 100 µm (the
size of an average cell) at 37oC, but takes more than 60 years to
diffuse across several meters (the distance from the heart to the feet and back
again in an average-sized human).
3.
Bulk flow (convective
transport) is the movement of a fluid as a result of a pressure or temperature
gradient to transport substances across long distances far faster than
diffusion. For example, human circulatory system can move 1 ml of blood from the
heart to the feet and back again in about 60 seconds, i.e., 3.1 x
107 times faster than diffusion.
II.
Unity and Diversity of
Circulatory Systems
1.
General characteristics of
circulatory systems
(1)
Despite the structural
diversity of animal circulatory systems, every animal circulatory system is
composed of 3 important components:
1)
One or more pumps to drive
fluid flow, often in combination with one-way valves to ensure unidirectional
flow
2)
A system of tubes, channels,
or other spaces through which the fluid can flow
3)
A fluid that circulates
through the system
(2)
Circulatory systems use
diverse pumping structures.
1)
Contractile chamber: Contractile chambers, such as
the vertebrate hearts, are found in both vertebrates and invertebrates.
2)
External pump: Some organs that are not
strictly associated with the circulatory system, such as skeletal muscles, can
be used to develop pressure gradients.
3)
Peristaltic contraction: Peristalsis is the rhythmic
waves of contraction of smooth muscle or tube in some invertebrates and the
early embryos/intestines of vertebrates.
(3)
Circulatory systems can be
open or closed.
1)
Closed circulatory system is a
circulatory system in which the circulatory fluid (blood) remains within
enclosed blood vessels throughout the circulation.
2)
Open circulatory system is a
circulatory system in which the circulatory fluid (blood) directly contacts with
tissues (sometimes called hemocoel).
(4)
Circulatory systems pump
several types of fluids.
1)
Interstitial fluid is the
component of extracellular fluid that exists between cells.
2)
Blood is the circulatory fluid
in animals with closed circulatory systems, and generally contains ions,
proteins, organic molecules, and various cell types suspended in plasma.
3)
In addition to the
cardiovascular systems, most vertebrates have a secondary circulatory lymphatic
system. Lymph is colorless, containing lymphocytes, and is similar to in
composition to blood except that it lacks red blood cells and large proteins.
4)
Hemolymph is the circulating
fluid of open circulatory systems in hemocoel.
5)
Thus, blood and hemolymph are
similar, while interstitial fluid and lymph are similar.
(5)
Vertebrate blood has 3 main
components:
1)
Plasma in ~55%
2)
Erythrocytes (red blood cells)
in ~45%
Hematocrit
is the fraction of erythrocytes in blood (20-65%).
3)
Other hemocytes (blood cells)
<1%, e.g., immune and blood-clotting cells
2.
Circulatory plans of the major
animal phyla (singular: phylum)
(1)
Animals such as sponges
(Phylum Porifera),
cnidarians (Phylum Cnidaria), and flatworms (Phylum Platyhelminthes) lack a circulatory system that transports an internal
fluid, but they have mechanisms for driving fluids around their bodies. The bulk
flow of these animals is in part of a combined respiratory, digestive, and
circulatory system.
1)
Sponges (Porifera) drive water through
their bodies using choanocytes.
2)
Cnidarians (Cnidaria) drive water from
the external medium through their mouths in a gastrovascular cavity using
muscular contractions.
3)
Flatworms
(Platyhelminthes) also have a
gastrovascular cavity lined with ciliated flame cells.
(2)
Most annelids
(Phylum Annelida) have closed
circulatory systems.
1)
Phylum Annelida is divided
into 3 main branches: Classes Polychaeta (e.g., tube worms), Oligochaeta (e.g.,
earthworms), and Hirudinea (e.g., leeches).
2)
Most polychaetes and all
oligochaetes have closed circulatory systems, while
leeches and some polychaetes have open
circulatory systems.
3)
Oligochaetes such as giant
earthworms with 6 m in length have 2 lateral heart/segment in anterior 9-13
segments.
(3)
Most mollusks
(Phylum Mollusca) have open
circulatory systems. Only the cephalopods (e.g., squid, octopus, and cuttlefish)
have closed circulatory systems.
(4)
Arthropod (Phylum Arthropoda) circulatory systems
vary in complexity.
1)
Brachiopod crustaceans (e.g.,
fairy shrimp; also known as sea monkeys) have a tubular heart.
2)
Decapod crustaceans (e.g.,
lobsters, crabs, and crayfish) have a vary muscular heart and blood vessels.
Ostium is a small opening in a muscular heart that can be opened or closed to
regulate flow.
3)
The hearts of most arthropods
are neurogenic that contract in response to signals from the nervous system,
instead of myogenic in vertebrates.
4)
Insects have open circulatory
systems.
(5)
Chordates (Phylum Chordata) have both open and
closed circulatory systems.
1)
Both urochordates (e.g.,
tunicates) and cephalochordates (e.g., lancelets) lack chambered heart and
instead have long tubular hearts in open circulatory systems.
2)
Vertebrates have closed
circulatory systems.
3.
The circulatory plan of
vertebrate
(1)
Vertebrate blood vessels have
complex walls.
1)
The vertebrate circulatory
plan: heart
à
arteries (e.g., aorta)
à
arterioles
à capillaries
à
venules
à
veins (e.g., venae cavae)
à
heart
2)
Lumen is a central open cavity
of hollow blood vessels in vertebrates.
3)
The walls of vertebrate blood
vessels are composed of 3 layers:
i)
Tunica externa is the
outermost layer, and contains collagen fibers.
ii)
Tunica media is middle layer,
and contains smooth muscles and elastin.
iii)
Tunica intima is the innermost
layer, and contains an inner lining vascular endothelium and subendothelial
layer (elastic fibers).
4)
Capillaries lack the tunica
externa and media and contain a single sheet of endothelial cells (tunica
intima). Based on the wall structure, capillaries are classified into 3 types:
i)
Continuous capillaries are the
least permeable (through cleft), located in skin, muscle, and blood-brain
barrier. The endothelial cells are connected via tight junctions. In brain
capillaries (blood brain barrier; BBB), only glucose and some amino acids can be
transported through.
ii)
Fenestrated capillaries
exhibit intermediate permeability (through pore), found in the renal glomerulus,
intestines, and endocrine glands. Fenestrated capillaries are similar to
continuous capillaries except that the cells of the vascular endothelium contain
numerous pores. Small molecules and fluids can pass easily through these pores.
iii)
Sinusoidal capillaries are the
most permeable (through gap/space), present in the liver and bone marrow. The
endothelial cells are loosely linked. Sinusoidal capillaries have fewer tight
junctions and more spaces between cells to allow large proteins to move across
the capillary wall.
(2)
Vertebrate circulatory systems
contain one or more pumps.
1)
Water-breathing fish (1 atrium
+ 1 ventricle) have a single-circuit circulatory system in which blood flows
from the heart through the gills to the body tissues and then back to the heart.
2)
Tetrapods (amphibians,
reptiles, birds, and mammals) have two circuits within their circulatory
systems. This change in circulatory pattern is associated with the colonization
of land, and the shift to the use of the lungs for gas exchange. The right side
of the heart pushes blood through the lungs in the pulmonary circuit of the
circulatory system, whereas the left side of the heart pushes blood through body
tissues in the systemic circuit of the circulatory system.
(3)
Many tetrapods have
incompletely separated pulmonary and systemic circuits. Unlike birds and
mammals, amphibians and most (noncrocodilian, e.g., turtles, snakes, and
lizards) reptiles (2 atria + 1 ventricle) have an incompletely divided heart. It
is possible to mix deoxygenated blood with oxygenated blood.
(4)
Mammals and birds have
completely separated pulmonary and systemic circuits. Mammals and birds have
completely separated the right and left sides of the heart (2 atria + 2
ventricles).
III.
Structure and Function of
Vertebrate Hearts
1.
Heart anatomy
(1)
Vertebrate hearts have 4 main
parts surrounded by a sac of pericardium.
(2)
A vertebrate heart contains
2
layers in pericardium:
1)
Fiber layer is the outer layer
of the pericardium.
2) Serosal layer consists of three parts:
i) Epicardium also called visceral pericardium is the inner layer of the serosal layer.
ii)
(5)
Coronary arteries of coronary
circulation supply blood to the vertebrate heart.
(6)
Septum separates the two
ventricles in bird and mammalian hearts.
(7)
The atrioventricular (AV)
valves are located between the atria and ventricles. The left AV is the bicuspid
valve (also called mitral valve), while the right AV is
the tricuspid valve.
(8)
The pulmonary semilunar valve
is located between the right ventricle and the pulmonary artery, while the
aortic semilunar valve is located between the left ventricle and the aorta.
2.
The cardiac cycle
(1)
The cardiac cycle is divided
into 2 phases: systole (contraction) and diastole (relaxation).
(2)
The right and left ventricles
develop different pressures.
3.
Control of cardiac contraction
(1)
All vertebrate and some
invertebrate hearts are myogenic. The pacemaker cells are excitable cells that
spontaneously fires action potentials in a rhythmic pattern.
(2)
Pacemaker cells initiate the
heartbeat, and pacemaker depolarization can spread via gap junctions in cardiac
muscles.
(3)
Cardiac action potentials have
an extended plateau phase.
(4)
The sinoatrial (SA) node is
located in an area of the right atrium and venae cavae, while the
atrioventricular (AV) node communicates the electrical signal to the ventricle.
(5)
The AV node transmits signals
through the His bundles, and the electrical signal then spreads into the
Purkinje fibers (a network of conducting pathways).
(6)
The integrated electrical
activity of the heart can be detected with the electrocardiogram (EKG for the
original German spelling, ECG for the English spelling). The small P wave shows
atrial depolarization, the large QRS complex shows ventricular depolarization
and artrial repolarization, and the T wave shows ventricular repolarization.
(7)
Cardiac output is the product
of heart rate and stroke volume.
1)
Cardiac output (CO) is the
amount of blood that heart pumps per unit time.
2)
Stroke volume (SV) is the
amount of blood that heart pumps with each beat (about 70 ml in a healthy 70-kg
man).
3)
Heart rate (HR) is the number
of times that the heart beats in a given period of time. Thus, CO = SV x HR.
(8)
The Frank-Starling effect is
an increase in the force of cardiac contraction (stroke volume increases) in
response to increasing venous return to the heart. Thus, the Frank-Starling
effect allows the heart to automatically compensate for increases in the amount
of blood returning to the heart.
IV.
Regulation of Circulatory
Function
1.
Regulation of flow
(1)
Myogenic autoregulation
maintains blood flow. Myogenic autoregulation is the regulation of blood flow
via contraction of vascular smooth muscle, and acts as a negative feedback loop
to maintain blood flow to a tissue at a constant level.
(2)
Metabolic activity and
paracrine signals influence blood flow.
(3)
The nervous and endocrine
systems regulate arteriolar diameter.
2.
Regulation of blood pressure
(1)
The ventricular pressure is
very high during ventricular systole.
(2)
Blood velocity is greatest in
the arteries, and lowest in the capillaries.
(3)
The total cross-sectional area
of the capillaries is the greatest in circulatory system.
(4)
The aorta acts as a pressure
reservoir in blood pressure during the cardiac cycle.
(5)
The veins act as a volume
reservoir.
(6)
Hypertension is a condition in
which arterial blood pressure is elevated above the normal level.
(7)
The lymphatic system returns
filtered fluids to the venous part of the circulatory system.
(8)
Tall animals must have
specialized circulatory systems. A giraffe has an extremely large and muscular
heart and the highest blood pressure (280/180 mm Hg for systolic/diastolic
pressure) known for any mammals. The blood pressure of giraffe is twice that of
a typical human. A resting giraffe also has a very high heart rate (HR = 170
beats per minute versus 70 bpm in human).