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The Circulatory System
Circulation and Blood
Functions of the Circulatory System
1.
2.
3.
a. To transport oxygen from the lungs to the
tissue cells of the body for cellular respiration.
b. To transport CO2 from the tissue cells of
the body to the lungs for excretion.
To distribute nutrients (due to digestion) from
the intestinal capillaries to all cells of the
body.
To transport:
a. Metabolic (nitrogenous) wastes to the
kidneys, including urea.
b. Toxic substances to the liver.
4.
5.
6.
7.
To distribute hormones to the tissues/organs
on which they act.
To regulate body temperature:
i. Donation of heat
ii. Flow shunting
To prevent blood loss through blood clotting.
To protect the body from pathogens
(viruses/bacteria) due to the circulation of
antibodies and white blood cells.
DEFINITIONS: (see fig. 13.7 p. 246)
Systemic Circulation –
Blood pumped by the LEFT side of the
heart, which services the entire
body except the lungs.
Pulmonary Circulation –
Blood pumped by the RIGHT side of
the heart, which services only the
lungs.
‘Services’ = provides O2 & nutrients, while carrying away
CO2 and other wastes.
The Major Components of the
Human Circulatory System
I.
II.
III.
Blood Vessels (5 types)
Blood
Heart
Blood Vessels (refer to fig. 13.1 p. 240)
i.
Arteries
•
Carry blood AWAY from the heart.
The thickest of all vessel-types; they possess three
layers of tissue:
i. Inner epithelial layer (aka endothelium)possesses elastic fibres and promote smooth flow.
ii. Middle smooth muscle layer (contracts or
relaxes to regulate blood flow and pressure)
* the thickest layer
iii. Outer fibrous (elastic) connective tissue which
serves a protective function as well as allowing the
artery to stretch and recoil.
•
Vein
Artery
The walls of major arteries (eg. Aorta) are so thick
that they must be supplied by their own blood vessels.
► Arteries in the systemic circuit carry oxygenated blood,
whereas arteries in the pulmonary circuit carry
►
deoxygenated blood.
►
Notice the smaller inner diameter of arteries compared
to that of veins – due to the thicker middle muscle
layer in arteries.
ii.
•
•
•
•
Arterioles
Small arteries (same structure, but smaller) into which arteries
have been divided (just visible to naked eye).
It is easier for blood to enter arteries than it is for it to exit
them, due to the narrower nature of arterioles  creates
noticeable blood pressure during both heart contraction
(systole) and relaxation (diastole), because the heart contracts
again before enough blood has flowed into the arterioles to
completely relieve the pressure in the arteries.
This causes artery walls to elastically snap back and forth
(reason for our pulse). This impedance by the arterioles is
known as peripheral resistance.
As a consequence of elastic arteries working against peripheral
resistance, there exists noticeable blood pressure even during
diastole, thus continuously driving blood into arterioles and
eventually capillaries.
►
Blood flow into arterioles, and
eventually capillaries, is
controlled in two ways
(through nervous/endocrine
signals) (see fig. 13.2 p. 241):
 Smooth muscles lining arterioles
constrict, thus allowing less
blood to enter; however, the
arteriole does not fully close, so
some blood enters…
 The ‘back-up’ plan involves
precapillary sphincter muscles
contracting or relaxing in order
to respectively close or open
access into capillary beds; if
closed off, blood flows to
venules through a thoroughfare
channel so that it can reach
more ‘useful’ areas quicker.
Why restrict access to
arterioles/capillaries???
► Example
scenarios:
 Cold weather; want blood (with heat) to flow to core of
body, not periphery…blood gets shunted to core through
the ‘closing off’ of the peripheral arterioles/sphincter
muscles.
 Exercising; want blood (with O2 and nutrients) to flow to
skeletal muscles and heart, not the digestive tract or other
non-necessary places…blood gets shunted to muscles.
Good or bad to exercise after eating and why?
 Relaxing; blood shunted to digestive tract to pick up
nutrients etc…
iii. Capillaries
• Very tiny vessels with walls that are one cell thick
(comprised of endothelium with a basement
membrane), which allow for efficient exchange of
substances.
• Present in all bodily regions  thus, a cut anywhere
will draw blood.
• Small diameter allowing for ‘single file’ passage of red
blood cells (again, helps with efficient exchange of, in
this case, oxygen and CO2).
• Surround cells/tissues/organs like a ‘spider web’ or
‘basket’.
• Capillaries are, at most, 0.2 μm away from any cell in
the body (also aids the exchange of substances).
• Certain capillary beds may be open or closed
depending on demands & subsequent flow shunting.
In general, only about 5-10% of the body’s blood is in the capillaries at any
one time.
iv. Venules
• Same structure as a vein (see below), only smaller.
• Collect blood from the capillaries and/or the
thoroughfare channels and join/enlarge to form veins.
v. Veins
• Thin-walled compared to arteries.
-- this provides veins with a larger interior
diameter than arteries.
Thinner
muscle layer
•
•
•
•
•
Same three layers of tissue as arteries, but the
middle smooth muscle layer is thinner.
Carry blood TOWARDS the heart.
There exists a lower blood pressure in veins
since they are further from the heart, and
because of the larger interior diameter.
VALVES (one-way) assist with the upward
(against gravity) movement of blood back to
the heart (valves prevent the backflow of
blood). Malfunctioning valve  varicose vein.
Generally, 70% of the body’s blood is in the
veins…acts as somewhat of a blood resevoir.
ONE-WAY
VALVES
► Veins
are located closer to the surface of the body
than arteries, and they are surrounded by skeletal
muscle.
► The contraction of these skeletal muscles aid in blood
flow through the veins (ie. The skeletal muscles are
the “hearts” for the veins).
► In the systemic circuit, veins carry deoxygenated
blood.
► In
the pulmonary circuit, veins carry oxygenated blood
Against
Gravity
Blood Pressure and Blood Velocity
(fig. 13.9 p.248)
Blood Pressure (BP): The hydrostatic pressure that blood
exerts against the wall of a vessel.
-
-
-
highest in arteries due to their receiving of blood from the
heart and due to the peripheral resistance created by the
smaller arterioles.
that said, the BP within arteries varies with respect to the
heart contracting (systole) and relaxing (diastole) 
systolic pressure is higher than diastolic pressure.
BP begins to drop in arterioles as the blood simply gets
further from the heart’s push, and it ‘spreads out’ more.
BP in the capillaries is somewhat ‘medium’ in that even
though the blood is far from the heart’s pump, the vessel
openings are small and the walls are thin allowing for a
greater hydrostatic pressure against them.
-
-
By the time the blood reaches the veins, its
pressure is not affected much by the heart due
to its travel (and coupled ‘slow-down’) through
tiny-diameter arterioles and capillaries.
Thus, very low BP in veins (the lowest, in fact):
- Blood is furthest from heart;
- Blood experienced extreme resistance within
arterioles/capillaries;
- Veins possess a very large (relative to arteries)
interior diameter.
*BP can also increase with higher blood volume!
Blood Velocity: the speed of blood moving
through vessels.
blood velocity is highest in the arteries due to the heart’s
pump;
- Blood velocity is lowest in the capillaries due to the
single-file RBC flow through them and the massive
‘spreading-out’ of the blood to the millions of capillary
beds in the body;
- Blood velocity picks up again (but not to the arterial
level) in veins due to their large interior diameter
(‘freeway’) and due to the action of skeletal muscles to
propel the blood back to the heart.
* The cross-sectional area (area of vessel wall in contact
with blood) of the vessels is greatest in capillaries and
lowest in arteries and veins.
-
Normal BP = 120 mm Hg/80 mm Hg
(systolic/diastolic).
Major Blood Vessels (fig. 13.8 p. 247)
Red vessels:
usually
arteries except
for pulmonary
circuit. Carry
oxygenated
blood.
Blue vessels:
usually veins
except for
pulmonary
circuit. Carry
de-oxygenated
blood.
Aorta – carries oxygenated blood out from the Left
Ventricle of the heart and services the entire
systemic circuit by eventually branching into various
arteries. *Houses special nerves cells (Aortic Bodies)
that sense H+, CO2, and O2 levels in blood.
2. Coronary Arteries and Veins – Arteries: branch
off of the aorta and service the actual heart muscle
(these vessels are seen on the surface of the heart)
(*Blood in the heart’s chambers does not actually
service the heart). Coronary Veins: carry ‘spent’
blood back to the heart’s chambers.
3. Carotid Arteries – branch off of the aorta to
service the brain/head region. Highly specialized 
contain special nerve cells (Carotid Bodies):
1.
i. Chemoreceptors that detect O2, H+, and
CO2 content in the blood.
ii. Pressure Receptors that detect blood
pressure changes.
-- the carotid artery can be used to measure
one’s pulse.
4. Jugular Veins – opposite of carotid arteries.
Carry blood from the brain/head region back
to the heart. Possess no valves since gravity
aids the flow.
5. Subclavian Arteries/Veins – service the
arms. Within the right subclavian vein there
is a union between the lymphatic system and
the circulatory system.
Mesenteric Arteries – carry blood from the
aorta to the intestines (gut). Subdivide into
villi capillaries.
7. Hepatic Portal Vein – carries blood from
the intestines to the liver.
*Hepatic = liver-related
8. Hepatic Vein – carries blood from the liver
back to the heart.
9. Renal Arteries/Veins – service the kidneys.
*Renal = kidney-related
10. Iliac Arteries/Veins – service the legs.
6.
Anterior (Superior) and Posterior (Inferior)
Vena Cava – collect/receive all of the blood from
the various veins of the systemic circuit and conduct
it back into the right atrium of the heart.
Anterior (Superior) Vena Cava – collects blood from
above the heart.
Posterior (Inferior) Vena Cava – collects blood from
below the heart.
12. Pulmonary Arteries/Veins – only major vessels of
the pulmonary circuit. Pulmonary arteries carry
deoxygenated blood from the heart to the lungs.
Pulmonary veins carry oxygenated blood from the
lungs back to the heart.
* Pulmonary Trunk – first vessel to receive blood
(bound for the lungs) from the heart. Splits into two
pulmonary arteries (one for each lung).
11.
Aorta
The aorta and the coronary system…heart not shown.
Blood
Made up of, and will separate into, two
components:
1. Plasma – comprises 55% of the blood
volume
2. Formed Elements (Cells) – comprise 45% of
the blood volume.
- includes Red Blood Cells (RBCs), White
Blood Cells (WBCs), and Platelets (which
aren’t whole cells, but cell fragments).
*fig. 13.10 p. 249.
►
Plasma
- 90-92% water  maintains blood volume and
pressure; transports substances due to its
flowing and polar nature. Primarily absorbed
by small and large intestines. *Regulates temperature too!
- 7-8% Plasma Proteins  maintain blood
O.P./volume, etc. Produced by the liver. Too
large to leave blood through capillaries.
- Albumins: maintain BP and blood volume; transport
bilirubin.
- Immunoglobulins: antibodies that fight infection
(pathogens); transport cholesterol.
- Fibrinogen/Prothrombin: aid in blood clotting.
-
<1% Other stuff:
- Salts/Electrolytes (minerals): maintain OP and BP,
pH, and aid in metabolism in many ways; absorbed
primarily in small intestine.
- Gases: oxygen/carbon dioxide from lungs/tissues
respectively;
- Nutrients: fats, glucose, amino acids from small
intestine;
- Nitrogenous wastes: urea and uric acid from liver;
- Hormones to aid bodily processes;
- Vitamins from small intestine to aid in enzymatic
reactions.
Formed Elements
Red Blood Cells (RBCs) – fig. 13.11 p. 250
- aka erythrocytes, RBCs are the most numerous of the
-
blood cells; there exist about 25 trillion in our (on average)
5 L of blood (in fact, RBCs comprise 99% of blood cells).
Structure promotes Function:
- RBC is a biconcave disk (flatter in center) allowing them
to thread through capillaries very efficiently and providing
them with a large SA to Volume ratio.
- Lack nuclei and mitochondria to help limit size – without
nuclei, RBCs live (on avg.) 120 days and are then
destroyed (by phagocytic cells through phagocytosis) in
the liver or spleen. Without mitochondria, RBCs
metabolize anaerobically so that they do not use the very
oxygen that they carry, in order to make their ATP
energy.
Large SA:Volume promotes RBCs’ main function: to
transport O2 in the blood and have it diffuse into/out
of them at the lungs and tissues, respectively. (RBCs
also carry CO2 and H+, but to a lesser extent than O2).
- Each RBC houses about 250 million molecules of
hemoglobin, an iron-containing protein responsible
for carrying O2 (primarily) – hemoglobin is purple-red
without O2 and bright red with O2. Hemoglobin is
made up of four tertiary proteins (two alpha, two
beta), four heme groups, and four Fe2+ ions. It can
associate with, and carry, four O2 molecules at any
one time (more in Respiration Unit).
- Hemoglobin’s iron is sent to the bone marrow to be
reused when RBCs are destroyed; heme becomes
bilirubin (released in bile pigments); and the alpha
and beta chains are hydrolyzed and the amino acids
reused.
-
Manufacture of RBCs (fig. 13.12 p. 251)
- RBCs are formed in the BONE MARROW of large
bones:
- bones of the chest (ribs), upper arms (humerus),
upper legs (femur), hips, skull.
- multipotent stem cells in the bone marrow become
erythroblasts (RBC precursor), which lose their nuclei,
gain hemoglobin, and mature or differentiate into an
erythrocyte (RBC).
- Certain hormonal/nervous signals ‘tell’ stem cells how
to develop as they have the ability to form any type of
blood cell.
Control of RBC Production
Oxygen-sensitive chemoreceptors in various locations
(medulla oblongata, aortic/carotid bodies, renal artery,
hepatic vein) sense low O2 levels in the blood.
- Stimulus sent to kidney to produce the hormone
erythropoietin (EPO), which stimulates production of
RBCs in bone marrow, and acts to slow the rate of
RBC destruction – this allows for more O2 to be carried
in the blood as we exhale plenty of it.
- Once O2 levels are restored, chemoreceptors send
negative feedback message to kidney to cease release
of erythropoietin.
- Some causes of lower than average O2 levels:
exercise, loss of blood, high altitudes, poor
hemoglobin/RBC production/formation (anemia)
-
- Most common cause of anemia is iron deficiency.
Blood Typing
A couple of definitions:
Antigen – a foreign substance that elicits a defensive
response from the immune system. In the case of
RBCs, an antigen (if present) is a protein that is
displayed on the surface of the cell and serves as an
ID tag for that cell.
Antibody – an antigen-binding protein, produced by
certain WBCs, that bind to certain antigens, ‘tagging’
them for destruction by phagocytic WBCs.
FYI: Anti-gen = Antibody
generating
ABO System
See table 14.2 p.
278 for Pop’n
distribution
O most
common, then
A, then B, then
AB…
Key Points: If antibody B (say) comes into contact with antigen B, it will
tag each B antigen for destruction (the blood clumps and causes flow
problems).
Antibodies exist only in the plasma.
Donated blood is made up of primarily RBCs only.
So…a person with AB blood can receive blood from anyone; whereas, a
person with type O blood can donate to anyone.
AB = Universal Recipient; O = Universal Donor.
Another Antigen: Rh protein (see fig. 14.13, p. 279 – excellent figure!)
An Rh antigen is present in people with Rh+ blood (no antibody), and is not
present in people with Rh- blood (also, no antibody). If a person who is Rh- is
exposed to Rh+ blood, antibodies will then be produced that will tag each Rh+ RBC
for destruction…happens often during a pregnancy where a mother is Rh- and her
fetus is Rh+…the fetus’ RBCs move across the placenta (late in the pregnancy or
during delivery, when the placenta begins to break down) and stimulate the mother
to produce Rh antibodies which can then cross the placenta (usually in a
subsequent pregnancy) to tag and destroy the fetus’ RBCs. Solved by injection of
anti-Rh antibodies during, or just after, delivery of ‘first’ child, which destroy any
Rh+ RBCs that entered the mother’s system, leading to prevention of production of
Rh antibodies in mother and ‘saving’ second fetus if he/she is Rh+ as well.
White Blood Cells (WBCs) (fig. 13.10 p. 249)
- aka Leukocytes
Can be granular or agranular.
- Very large in size (have a nucleus) relative to RBCs and
platelets.
- In general, WBCs fight infection and resist disease by
aiding in the development of immunity.
- Produced in the bone marrow from the same stem cells as
RBCs, but follow a different developmental pathway.
Two Classes and Five Types:
Class I: Granular Leukocytes (filled with vesicles of enzymes
that ‘defend’ against ‘invaders’)
a. Basophils – release histamines that cause allergic
reactions (clotting of area, dilation of vessels to allow
neutrophils/monocytes to arrive).
-
b. Eosinophils – attack parasites by releasing substances
that kill them.
c. Neutrophils – attack and engulf foreign invaders,
destroying themselves in the process (pus); aka
phagocytes. Most numerous (60-70% of WBCs).
Class II – Agranular Leukocytes
a. Monocytes (Macrophages) – like neutrophils except
that they possess pseudopodia (‘arms’) that can
extend out to capture invaders. As well, they may
live through an encounter and even act to engulf
dead neutrophils.
b. Lymphocytes (T and B) – produce antibodies that
tag specific invaders for destruction.
Colony-stimulating Factors (CSFs)
are secreted by ‘living’ WBCs to
promote the WBC developmental
pathway, leading to an increase in
WBC production (akin to EPO for
RBCs).
RBC = R
Agranular WBC (Monocyte) = N
Granular WBC (Eosinophil) = E
Platelets (fig. 13.14 p. 254) – Cell Fragments
- regulated by hormone thrombopoietin, which is
released by the liver and/or kidneys when platelet
counts are low.
- They lack a nucleus; are fragments of
megakaryocytes, which are derived from bone marrow
stem cells.
- Play a major role in blood clotting; when a blood
vessel is broken, it must be repaired. In order for the
tissue to regenerate, the blood flow through the cut
must be stopped; a clot serves this function.
- When a cut occurs, platelets congregate and stick to
the irregular surface created by the cut.
- If it is a minor cut, this congregation clogs the hole.
- If it is a major cut, a sequence of events takes place:
-
-
-
Platelets and damaged tissue cells release enzyme
Prothrombin Activator, which, along with Ca2+ ions in the
plasma, acts to convert the plasma protein prothrombin to
the protein thrombin.
The liver produces prothrombin with help from Vitamin K
(a lack of K in diet leads to hemorrhagic disorders).
Thrombin then acts as an enzyme to convert the plasma
protein fibrinogen to fibrin, a thread (filament)-like protein
that winds around the platelet congregation to stabilize it.
Fibrin threads also capture RBCs that act to further plug
holes in the clot.
The fibrin web eventually contracts (like actin fibers) to
pull the tissue back together (forms a scab).
Tissue repair occurs beneath scab.
Once repairs are complete, the clot is released and
destroyed by the enzyme plasmin (present in blood).
Types of Body Fluids
Name
Blood
Composition
Formed elements
and plasma
Water, proteins,
Plasma
salts, etc.
Plasma minus
Serum
fibrinogen (after
clotting)
Tissue (ECF) fluid Plasma minus
proteins
Tissue Fluid in
Lymph
lymphatic vessels
Capillary Exchange – fig. 13.15 p. 255
The diffusion of water, hormones, O2, nutrients, CO2, and
other wastes occurs between the capillaries and the ECF
(and eventually, body cells).
► Capillaries are very close (at most 0.2 micrometers) to
body cells, and their walls are one-cell thick (easy exchange).
► Water is the transfer medium for the substances diffusing;
it is the osmotic gradient that is followed. Thus, tonicity
(Osmotic Pressure (OP)) and Blood Pressure (BP) within
capillaries are important to analyze.
► Ultimate goals: to move O2 and nutrients from blood into
ECF, and eventually into cells; and to move CO2 and other
wastes from ECF (originally from cells) into capillaries.
►
Picture two regions of the capillary: the arterial end and
the venous end.
► At the arterial end, the BP > OP (in fact, BP = 30 mmHg
and OP = 21 mmHg), so there is a net movement (9 mm
Hg) of water and its contained stuff (O2/nutrients) out of
blood into the ECF. The movement of O2 and nutrients
follows their own conc. gradients. Most water, O2,
nutrients eventually enter cells.
► At the venous end, since the plasma proteins were unable
to move out of the capillary, and due to the movement of
water, OP > BP (in fact, OP = 21 mmHg and BP = 15 mm
Hg). Thus, there is a net movement (6 mmHg) of water
and its contained stuff (CO2/other wastes – following their
own conc. gradient) from the tissue cells/ECF into the
capillary for eventual disposal from/by the body.
► The excess water (3 mmHg diff.) is taken up by lymph
capillaries…this excess can be greater if the [plasma
proteins] is lower than average.
►
Ignore these
numbers… pay
attention to the
premise only…
Now…see the Lymphatic System…