Download Topic 16: INTERNAL CIRCULATION MECHANISMS (CONVECTIVE

Topic 18: INTERNAL CIRCULATION MECHANISMS (CONVECTIVE MOVEMENT OF
INTERNAL FLUIDS). (lectures 27-28)
OBJECTIVES:
1. Be able to differentiate between intracellular, interstitial and extracellular
compartments.
2. Know the basic factors that impact blood flow (volume of blood moved per unit time).
3. Be able to compare and contrast open vs. closed circulatory systems with respect to
their structures, functional characteristics and in which organisms they are found.
4. Be able to describe the evolutionary progression form fish to amphibian to
mammalian hearts in terms of structure and functioning.
5. Know the overall pathway of blood flow.
6. What are the basic events in the cardiac cycle?
7. Be able to describe how the rate and force of heart contraction are regulated.
8. Understand the functional anatomy of veins.
Organisms have three kinds of fluids which can be characterized on the basis of where
these fluids reside.
1. intracellular- fluid found inside of cells; materials here move by passive diffusion
2. interstitial – fluid found between cells; materials generally move here by passive
diffusion
3. extracellular- fluid found is special compartments like tubes (blood and lymphatic
vessels; tubes carrying urine; ducts of glands); materials typically move by
convection; that is the bulk movement of fluid
How is bulk transport of extracellular fluid (and interstitial) fluids achieved?
Simple organisms- fig. 41.11; Hydra have gastrovascular cavities; bulk movement of
fluid is by ciliary activity on the epidermal cells.
Larger, more complex and active organisms require a specialized circulatory system
consisting of a pump (usually a heart or heart-like organ) and vessels (or at least
sinuses) through which blood can flow.
What are the forces that determine the amount of blood flowing through a tube?
Hydrostatic pressure- water is an incompressible substance so that when a force is
applied to it, it does not change in volume but the pressure increases inside; this is
known as hydrostatic pressure. When the fluid is blood we refer to the pressure as
blood pressure. Blood pressure is generally expressed in terms of mm Hg.
Blood flow (volume of blood transported per unit time) = P/R where
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P = the difference in blood pressure between two points; the higher the
pressure difference, the greater the blood flow
R = resistance to flow; resistance increases as the radius of the vessel
decreases and the length increases
The heart generates a very high blood pressure by compressing the blood; this
produces a P which drives the blood forward. Efficient circulatory systems in active
animals generate very high blood pressures.
There are two fundamental types of circulatory systems; fig. 42.2
(1) Open – blood is pumped through vessels into large, ill-defined cavities or sinuses; in
effect, the blood is equivalent to the interstitial fluid and is often referred to as
hemolymph. Characteristic of all arthropods and most molluscs.
(2) Closed- blood is physically contained within specialized vessels and is not in direct
contact with the cells. Characteristic of vertebrates, annelids and cephalopd
molluscs (squids, octopus)
Functional differences between open vs. closed
Blood volume
Blood pressure
Blood flow
OPEN
CLOSED
Large (as much as 50% Small (8% body volume)
of the total body volume)
Low (1-13 mm Hg)
Generally high (50-315
mm Hg)
Sluggish
Fast
What this means is that animals cannot be active and achieve large size if they have an
open circulatory system
1. vertebrates have closed systems
2. squid, the largest and most active of aquatic invertebrates, have a closed system
unlike other molluscs
3. insects are an exception; they are small but they achieve the highest activity levels
(flight) of any organism yet they have an open system; we’ll see why later
There is a general correlation between increased activity levels and complexity and
performance of the circulatory (ca. squid vs. clams).
Case example- the vertebrate circulatory system.
There has been a progressive evolution of the vertebrate circulatory plan to
accommodate and enhance activity levels; fig. 42.3. Basic elements: multi-chambered
heart (atria, ventricles), aorta, arteries, arterioles, capillaries, venules and veins.
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1. fish- simple circuit in which deoxygenated blood is passed by the branchial
circulation to the gills for oxygenation and returns through the systemic circulation
where the blood releases oxygen and takes up carbon dioxide. The disadvantage of
this system is that the capillaries of the gills offer a great deal of resistance to flow.
Thus, blood pressure is low after the gills and flow to systemic circulation is sluggish.
2. Amphibian- establishment of distinct pulmonary (lung) and systemic circulations;
there are two atria and one ventricle so that some oxy- and deoxy blood mix in the
ventricle which reduces the efficiency of the system.
3. Mammals (as well as birds)- four chambered hearts; two separate blood pumps;
right side is pulmonary and left side is systemic. No mixing of oxy- and deoxy- blood.
Left heart is larger than right heart because it must generate higher pressure to force
blood throughout the body.
The functional anatomy of the four chambered heart and circulatory plan.
Fig. 42.4- major path of blood flow: right atrium, right ventricle, pulmonary artery, lungs,
pulmonary veins, left atrium, left ventricle, aorta, systemic circulation, veins, posterior
and anterior vena cava (REPEAT)
Fig. 42.5- valves (AV, semi-lunar) are present which prevent the retrograde (backward)
flow of blood
The cardiac cycle (fig. 42.6)Diastole- relaxation of a heart chamber
Systole- contraction of the walls of the chamber
(1) atrial and ventricular diastole- filling of both chambers (AV valve open; semi-lunar
valves closed.
(2) atrial systole, ventricular diastole- filling of ventricle
(3) atrial diastole, ventricular systole- emptying of the ventricle; semi-lunar valves open
and AV valves close
Contraction of the ventricle generates a high degree of pressure which ejects the blood
from the heart and drives it through the entire circulatory system.
Systolic blood pressure- pressure at the peak of ventricular systole
Diastolic blood pressure- pressure during ventricular relaxation
Fig. 42.8- functional anatomy; note that veins have unidirectional valves which prevent
backflow of blood; pressure in veins is quite low
Fig. 42.14- plasma vs. cellular elements; red blood cells dominate the cellular elements;
more when we talk about oxygen transport in blood.
Control of the heart:
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(1) there is an intrinsic automatic rhythm that originates in cells in a pacemaker region
known as the sino-atrial node (SA node)
(2) this intrinsic rhythm is modified by nerves (vagus nerve- slows down the rate;
cardiac accelerator nerve= increases rate) as well as a circulating hormone known
as epinephrine (increases heart rate)
(3) the rate and force of heart contraction can be modified to meet the needs of the
organism; rate rises during exercise, falls during sleep.
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