Download MCB 32, FALL 2000

MCB 32, FALL 2000
CARDIOVASCUAR SYSTEM ANATOMY AND PHYSIOLOGY
Reading: Chapters 8 and Chapter 9
I.
Diffusion and bulk flow
Diffusion delivers substances over short distances (10-200 µm). If distances are
longer than this, then diffusion is a very slow process.
Bulk flow required for rapid delivery of substances over long distances like those
in the cardiovascular system, e.g., delivery of nutrients from the gastrointestinal tract to
the tissues of the body and delivery of wastes from the tissues to the kidneys.
II.
Blood composition
Blood is composed of cells (approx 50%) and plasma (approx 50%). Red blood
cells are produced in bone marrow and are basically bags of hemoglobin, which is
responsible for binding oxygen in the lungs, delivering and then releasing it to the tissues.
This function will be discussed later in the Respiration portion of the course. White cells
are involved in specific and non-specific defenses of the body. This will be discussed
during Immunology. Plasma contains all the nutrients and waste products circulating in
the blood stream, and also plasma proteins (many of these are antibodies that are
available to fight infectious agents. Plasma composition of salts and water are largely the
same as those in the tissues, i.e., high [Na] and [Cl] low [K].
III.
General anatomy of the circulatory system: Fig. 8.1
Systemic (L heart, aorta, arteries, capillaries, veins, vena cava) and pulmonary (R
heart, pulmonary artery, arterioles, lung capillaries, veins, pulmonary vein) circulations.
Series and parallel arrangements of blood vessels: blood flows in series through
systemic and pulmonary circulations, while it flows in parallel to the muscles and the
kidneys and gastroinestinal tract. The body must be able to control the distribution of the
blood so that the metabolism of each tissue can be provided with proper blood supply.
IV.
Heart anatomy: Fig. 8.6
Left and right hearts in chest cavity, bounded by diaphragm
Pericardium connective tissue covering
Left atrium (blood collection from pulmonary circulation) and ventricle
(propulsion; thick wall due to large pressures involved). Right atrium (blood collection
from vena cava and systemic circulation) and ventricle (propulsion; thinner wall due to
lower pressures in pulmonary circulation)
Coronary blood flow refers to blood that flows in arteries, capillaries and veins
that go directly to the heart itself. Little of the heart’s nutrients come from the blood
inside the heart. Coronary supply is critical to operation of heart, since interruption of
blood supply leads to inadequate supply of oxygen. If coronary vessel become blocked,
this leads to death of some portion of the heart tissue and a heart attack.
Valves: connective tissue with attached chordae tendinae to prevent eversion.
Valves move only passively in response to pressures on each side. One-way operation to
assure correct diretional movement of blood during pulsatile contraction and relaxation of
heart. Make sounds as they close. Sometimes close incompletely, leading to murmur
(turbulent blood flow is noisy, in contrast to laminar flow)
Note that the left and right hearts perform quite different functions. Left heart
pumps blood to many different organs and does so by generating high pressures. Right
heart pumps blood only to the lungs and does so by generating only low pressures. Left
heart is therefore thicker than right heart.
V.
SA node is the cardiac pacemaker; conduction system assures orderly
electrical excitation: Fig. 8.6
Coordinated activity of heart is necessary. If electrical activity occurred
randomly, contraction would similarly be random, and there would be no possibility for
efficient contraction of atria followed by the ventricles.
Electrical activity begins in sino-atial node, which has the capability to generate
its own action potentials, at rate of heart beat.
AP  gap junctions  electrical excitation followed by contraction throughout
the atria.
AP  AV node and Purkinje fibers, specialized set of fibers with less contractile
ability but good transmission capability.
Purkinje fibers  ventricular musculature
VI. Frequency of action potentials from SA node can be modulated by autonomic
nervous system: Fig. 8.7
Sympathetic nerves release norepinehrine onto the cells of the heart (including
those in the SA node – see below), which speeds rate at which action potentials are
generated in the cells, increasing the rate of contraction.
Parasympathetic nerves (primarily the vagus) release acetylcholine onto the cells
of the heart (including the SA node – see below), which slows rate of action potentials,
slowing rate of contraction of heart.
VII.
Coordinated activity of the heart: Fig. 8.10
SA node action potential  atria contract Atrial systole
Action potential --> AV node and conducting system  ventricles contract
Pressure in ventricle rises  atrio-ventricular valves shut (first heart sound), with
no change in volume (isometric or isovolumetric contraction)
Pressure in ventricle > aortic pressure, valve opens, blood ejected (isotonic
contraction), arterial pressure rises to systolic level. Ejection phase. Ventricular systole
End of action potential, ventricular musculature relaxes (ventricular diasotle) 
aortic valve closes (second heart sound)
Blood begins to fill atria and ventricles
VIII. Control of heart rate by autonomic nerves: Fig. 8.8
Control center: medulla oblongata. This region of the brain stem receives
information from the peripheral blood pressure monitors, including the pressure
receptors in the aorta and large arteries and also in the heart itself.
Sympathetic nerves  increase rate of depolarization  increase frequency of
AP in SA node. This response occurs during “fight or flight” situations, or during
exercise.
Parasympathetic nerves  decrease rate of depolarization  reduce frequency of
AP in SA node. This response occurs during times when cardiovascular system
needs to reduce blood pressure and maintain vegetative state.
IX.
Control of cardiac output
CO (liters/min) = stroke volume (ml/beat) x heart rate (beats/min)
Normal human CO = 5 l/min = 72 ml/beat x 70 beats/min
Control by changing heart rate (sympathetic increase, parasympathetic decrease)
and stroke volume.
Stroke volume can be changed by altering contractility of heart muscle, usually
increased by sympathetic nerves. This increases amount of force generated by the
myosin and actin, often through changes in cell [Ca].
Stroke vol also changed by alterations of size of heart (length tension diagram of
muscle). This in turn controlled by venous return.
X.
Structure and functions of blood vessels: Figs 9.3 – 9.9
All are lined with endothelium, thin layer of cells connected by tight junctions,
function: provide smooth surface for blood flow.
Amount and types of connective tissue and smooth muscle vary with type of
vessel.
Arteries
Large diameter compared to other vessels, lots of connective tissue to support
high pressures present inside
One artery per organ
Store P energy of heart, for smooth distribution of blood during diastole. This is
shown well in Fig. 9.2.
Arterioles
Smaller, great deal of muscle compared to size of vessels
Sites of controlled resistance, regulated by nerves, hormones and, most
importantly, by paracrine factors produced by the tissues themselves. Thus, there
can be localized regulation of blood flow.
Arterioles are the vessels that are most important for controlling pressures and
flows in CV system.
Three mechanisms for regulating blood flow through arterioles:
1. Myogenic autoregulation: mechanism for maintenance of constancy
2. Metabolic control by paracrines: lactic acid or other metabolites are produced
during periods of intense metabolic activity. Accumulation of these products
causes smooth muscle to relax, leading to increased blood flow to the affected
tissue. No nerves required for this response.
3. Sympathetic control: increase constriction to increase pressure
Veins
Veins are low resistance (easy to force blood through them) reservoirs (have large
volume of blood in them). Veins are larger in diameter than arterioles and also
have less connective tissue and smooth muscle because pressures are low.
Valves (Fig. 9.5) assure one-way movement of blood back to the heart.
Muscle and respiratory pumps refer to the increased blood flow through the veins
that occur during exercise: rhythmic changes in contraction and relaxation of the
skeletal muscles, and also the increased depth and rate of breathing, which
reduces pressure surrounding veins near the heart and thereby increases blood
flow back to the heart.
Capillaries: Fig. 9.4
Small diameter, low rate of flow, very large number (nearly one per cell of body)
Delivery of nutrients to tissues and elimination of wastes (CO2, lactic acid etc)
occurs by diffusion.
Water exchange across capillaries.
Fluid exchange across the capillaries is by filtration (pressure due to heart’s
pumping forces fluid out of capillaries into tissue spaces) and by osmosis (large
plasma proteins pull water into the capillaries). Figs 9.15 and 9.16
Imbalance of forces often leads to edema (collection of excess tissue fluid), e.g.,
in heart patients.
Lymph vessels collect excess tissue fluid and proteins
These are blind-ended capillary-like structures that are very permeable to proteins
and fluids. They collect the excess tissue fluid, which then circulates through
larger and larger vessels, ultimately back into the venous system. The flow of
fluid though the lymph vessels appears to occur like that through the veins, with
valves and muscle pumps.