Circulatory system II

F. Human Heart

1. Structure

- consists of two atria (receiving chambers) and two ventricles (pumping chambers).
Atria are separated from the ventricles by atrioventricular valves (= AV valves).
Semilunar valves found between ventricles and arteries leading away from heart.
- heart has its own "coronary circulation" branching off from aorta and re entering
right atrium

2. Cardiac cycle [Fig. 39.10]

- consists of a phase of vetricular contraction (systole) followed by a phase of
relaxation (diastole)
- Diastole - as the atria begin to fill, the ventricles relax. Pressure of returning blood
forces the AV valves to open and blood fills the ventricles. At the end of diastole
the atria contract to "top off" the ventricles.
- Systole - the ventricles contract (the AV valves close with a thump) and the
pressure in the ventricles opens the semilunar valves leading into the aorta and
pulmonary arteriy. When the semilunar valves close, another "thump" is
sounded. Thus, the heart beat:

3. Cardiac output = Stroke Volume X Heart Rate (bpm)

- Stroke Volume = volume of blood pumped by one ventricle during one beat
(approx. 75 ml/beat)
- Output at rest = 5 liters/minute
- Output during heavy exercise = 20 - 30 liters/minute

4. Contraction mechanism

a. Cardiac muscle cells are connected electrically [Fig. 39.11a]
- Because of electrical gap juctions between cardiac muscle cells (intercalated discs)
muscle action potentials can propagate from one cell to the other. Thus, cardiac
muscle cells contract in nearly in unison.
b. Cardiac muscle cells contract spontaneously [Fig. 39.11b]
- Some cells contract at a more rapid rate than others. The intrinsic rhythmicity of
the heartbeat cycle is initiated by the fastest pacemaker tissue of the heart = the
sinoatrial node (SA node).
- The SA node is located in the wall of the right atrium. It initiates electrical signals
about 70-80 times per minute. Once initiated the electrical signal spreads over both
atria, slows briefly at the AV node, and then spreads rapidly over the ventricles.

5. Control of heartrate, strength of contraction

-The nervous system can only adjust the rate and strength of the heartbeat. The
nerves to the heart (sympathetic and parasympathetic) do not initiate each heartbeat;
they can speed the heart up and increase the force of contraction, or slow it down
and reduce the force of contraction.

H. Blood pressure in the vascular system

1. Pressure drop [Fig 39.12]

- Blood pressure drops along the way from the heart to the veins, due to energy loss
from the increase of resistance. Arteries have higher pressure than capillaries, and
capillaries have higher pressure than veins.
[Blood flow = Constant x { P(pressure drop)/R(resistance) }
This equation can be applied to individual vessels, or to whole sections of
circulatory system. Resistance is inversely related to cross sectional area of vessels:
The greater the cross-sectional area, the lower the resistance. Peripheral resistance
in the systemic circuit is determined mainly by the total cross-sectional area of

2. Arterial blood pressure

- Arteries are elastic and muscular. Arteries dampen the high pressure "pulses"
coming from the heart. The pressure in the arteries fluctuates with each beat, but
the average pressure does not drop much because the diameter of arteries is so large
they offer little resistance to flow.

3. Arterioles - A large increase in resistance

- Arterioles are considerably smaller in diameter. They present a substantial
increase in resistance to flow, and there is thus a large drop in pressure from the
upstream to the downstream ends of arterioles. The rate of flow (velocity) is also
slower than in the arteries because the total cross-sectional area is greater.

4. Control of blood pressure and blood distribution

- Smooth muscles in arteriole walls are muscles that can adjust the diameter of the
- Neural input and hormones induce vasoconstriction or vasodilation by causing
smooth muscles in the arterioles to contract or relax
- By controlling the diameter of arterioles the flow of blood to different capillary
beds can be controlled
- General vasoconstriction leads to an overall increase in resistance. To maintain
flow the heart must pump harder and blood pressure rises
- The medulla of the brain monitors signals from sensory receptors in certain
arteries and the heart. It sends out commands that maintain resting blood pressure
at a relatively constant level.
- If resting blood pressure increases, arterioles are induced to open (vasodilation)
and the heart rate is induced to slow.
- If resting blood pressure decreases, arterioles are induced to constrict
(vasoconstriction) and the heart rate is made to increase.
[= another example of negative feedback control]

I. Capillaries and veins

1. Capillary beds (= diffusion zones)

- Capillaries have thin walls = one layer of flat endothelial cells
- Small diameter --> blood cells flow through in single file
--> resistance of individual capillaries is high; however, the combined
diameters of the capillaries are much greater than the diameters of arterioles leading
into them. Thus, the pressure drop is not as great. Also, the flow rate (velocity) is
much lower due to the larger total cross-sectional area.
- Low velocity and thin walls allow for efficient diffusion of blood gases and
dissolved solutes into and out of capillaries. No cell lies more than .01 mm from
the nearest capillary.
- Hydrostatic pressure difference (capillary pressure - interstitial fluid pressure)
drives some water and solutes out of capillaries at the upstream end, but large
proteins (esp. albumin) provide net osmotic gradient that reverses this flow at the
downstream end. [Fig. 39.15]
Consquently, there is only a small net filtration of fluid which is returned to the
blood by the lymphatic system.

2. Veins (= blood volume reservoirs)

- capillaries merge into venules
- venules merge into veins = large-diameter vessels with highly distensible walls
- veins contain valves to prevent backflow; since there is virtually no pressure drop
along the length of veins, pressure is provided by contraction of skeletal muscles
which push on the walls of veins when they contract and bulge. Thus, blood flow
back to the heart depends upon regular contraction of skeletal muscle.

J. Hemosatsis (not homeostasis)

- repair of damage to blood vessels; entire process includes, vessel spasm, platelet
plug formation, and blood coagulation.
- vessel spasm: smooth muscles contract to stop flow temporarily
- platelet plug formation: platelets clump together to form a temporary plug; they
also release substances that prolong spasm and attract more platelets
- blood coagulation: blood converts to a gel and forms a clot; as fluid is forced out
of clot, it retracts into a compact mass and seals the breach in the vessel

K. Blood typing

- blood contains antibodies that recognize surface markers on blood cells; if
incompatible blood types are mixed during a transfusion, the antibodies cause the
cells with the "wrong" surface markers to clump togther = agglutination.

1. ABO blood typing [Fig. 39.18]

- Type A blood has A markers on cells which are ignored by antibodies. If B type
blood from a donor is mixed, the antibodies in the A type blood causes
- Similarly, if a type B recipient receives type A blood from a donor, the recipient's
antibodies cause agglutination of the A type cells.
- AB blood has cells with both A and B markers. The antibodies of an AB person
ignore both, so that person can receive blood from A type, B type or AB type donors
(= universal recipient)
- Type O blood has neither A or B markers on the cell surface to be attacked by
antibodies of recipients (= universal donor). However, a person with type O blood
has antibodies that recognize both A markers and B markers as foreign. O type
blood causes agglutination of cell with A or B markers.
[If you have type O blood, what blood type(s) can you receive in a transfusion
without harm?]

2. Rh blood typing [Fig. 39.19]

- If you are Rh+ , your blood cells have the Rh marker and your antibodies ignore it.
If, however, you are "Rh negative" your immune system will treat the Rh marker
as foreign.
- A mother with Rh negative blood is at risk in her second pregnancy if her first
child was the product of fertilization by an Rh positive father. --> The first child
produces cells with the Rh marker which mingle with the mother's blood. The
mother's immune system produces antibodies against the Rh marker.
- If the second fetus is Rh positive, the mother's antibodies will attack the child's
red blood cells

L. Lymphatic system

1. Functions

- Takes up water and plasma proteins that have leaked out of capillary beds and
returns them to the blood circulation
- Transports fats absorbed from the small intestine
- Transports foreign particles and cell debris to lymph nodes for disposal

2. Vascular portion

- Lymph capillaries (overlapping endothelial cells at tip),
- Lymph vessels (lead to nodes; contain valves to prevent backflow)

3. Lymphoid organs, tissues

- lymph nodes, spleen, thymus (plus tonsils, adenoids and other patches of tissue in
- nodes = sites with many lymphocytes and macrophages
- spleen = largest lymphoid organ; filters blood, holding site for lymphocytes;
reservoir for red blood cells, macrophages; site for destruction of worn out RBCs
- thymus = site for maturation, multiplication of lymphocytes; hormone
production to regulate immune process