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The Cardiovascular System
blood vessels
Blood Circulation
Blood is carried in a closed system of
vessels that begins and ends at the heart
Human Cardiovascular System
The 3 major types of vessels
Arteries – carry blood away from the heart
Veins – carry blood toward the heart
Capillaries – contact tissue cells (Serving cellular needs)
Capillaries
· Very narrow (10 µm diameter, the red blood cells that
travel through capillaries are 6 µm in diameter).
Capillaries are made of thin endothelial cells (one layer thick)
Capillary Blood Flow
• Blood flow across the capillary bed is regulated
by a sphincter muscle on the arteriole side.
Whenever there is little need to supply blood to
a given capillary bed, the sphincter closes and
blood bypasses the capillary bed via an arteriovenal shunt.
• On the arterial side of the capillaries, blood is
under high pressure, whereas the blood at the
venal side is under low pressure.
Hydrostatic & Osmotic Pressures
• The high hydrostatic pressure on the arterial
side squeezes water and nutrients out of the
capillaries.
• Water leaving the capillaries builds up the
osmotic pressure because the blood components
have become more concentrated.
• Towards the venal end, water and waste
materials are sucked into capillaries by the
osmotic pressure.
Capillary Dynamics
Capillary Exchange
Gases, nutrients, and wastes are exchanged between blood in
the capillaries of tissues in 3 ways:
• 1. Diffusion
•
a.
most common
•
b.
substances including O2, CO2, glucose, & hormones
•
c.
lipid-soluble substances pass directly through
endothelial cell membrane;
•
d.
water-soluble substances must pass through
fenestrations or gaps between endothelial cells.
• 2.
Vesicular transport (endo/exocytosis);
• 3.
Bulk flow (filtration and absorption).
Arteries & arterioles
• (High pressure & Low resistance)
The blood vessels before the capillary beds have
to bear high hydrostatic pressures created by the
heart
Arteries and smaller arterioles are thick walled
tubes made of smooth muscle (to resist high
pressure), containing a layer of collagen fibers (to
give elasticity), and lined with endothelial cells (to
protect red blood cells from mechanical damage).
Tunica’s
Tunica’s
Veins & venules
(Low Pressure & Low Resistance)
• Contain valves to prevent the
backflow of blood
• Varicose veins result due to a failure
of these valves
Venous Blood Flow
HEMODYNAMICS: THE PHYSIOLOGY OF
CIRCULATION
Blood pressure = pressure exerted by blood on the wall
of blood vessels.
• In clinical use, we most commonly refer to mean (systemic)
arterial blood pressure (MABP), because the blood
pressure in the veins is essentially insignificant.
• The mean arterial blood pressure (MABP) rises to its
maximum during systole (contraction) and falls to its lowest
during diastole (relaxation).
• In a normal adult at rest, the MABP = 120 mmHg/80 mmHg
Factors that Influence Arterial Blood
Pressure
• Heart Action (cardiac output)
• Blood Volume (increase in blood volume increases
BP)
• Peripheral Resistance (resistance; R = opposition to
blood flow usually due to friction)
• So we can say CO = MABP/R
Blood Flow Friction
This friction depends on 3 things:
• 1. Blood viscosity
• 2. Total blood vessel length
• 3. Blood Vessel Radius
Regulation of BP
“Blood Flow”
• F = ΔP/ R
• ΔP= change in pressure between
arterial & venous ends
•
R= Peripheral Resistance
Hormonal Control
Several hormones affect BP by:
• acting on the heart
• altering blood vessel diameter
• adjusting blood volume.
Hormones that increase BP
Epinephrine and norepinephrine
Increases CO (rate & force of contraction) & causes
vasoconstriction of arterioles.
Antidiuretic hormone (ADH)
Causes vasoconstriction of arterioles during diuresis and
during hemorrhage.
Angiotensin II
Causes vasoconstriction of arterioles and causes the
secretion of aldosterone
Aldosterone
Increases Na+ and water reabsorption in the kidneys.
Hormones that decrease BP
Atrial natriuretic peptide (ANP)
Causes vasodilation of arterioles and
promotes the loss of salt and water in
urine.
Histamine
Causes vasodilation of arterioles (plays a
key role in inflammation)
Regulation of Blood Pressure
and Blood Flow:
• Neural Regulation:
• The cardiovascular (CV) center is located in the medulla
oblongata
•
CV Center Input:
• Nerve impulses are sent to the CV center from three areas:
• 1. Higher brain centers
• 2. Baroreceptors (or pressoreceptors) which
• detect changes in BP in aorta and carotid arteries
• 3. Chemoreceptors that detect changes in key blood
chemical concentrations (H+, CO2, and O2)
•
Regulation of Blood Pressure
and Blood Flow:
• Neural Regulation:
•
CV Center Output:
• Nerve impulses are sent from the CV center to
either:
• 1. The Sinoatrial (SA) Node of heart
• 2. The smooth muscle of peripheral blood vessels
(i.e. arterioles).
•
Regulation of Blood Pressure
and Blood Flow:
•
Negative-Feedback Regulation:
• If BP is too high:
• 1.
Increase is detected by baroreceptors in the carotid
artery or aorta
• 2. Baroreceptors send an impulse to CV center
• 3. CV center interprets that message and sends a signal to
the SA Node and arterioles
• 4. The SA Node decreases heart rate
• 5. The arterioles dilate
• If BP is too low...
• 1.
SA Node increases hr;
• 2. Constriction of arterioles