BIOL 1911 Chapter Notes - Chapter 42.3: Capillary, Exocytosis, Circulatory System
29 Apr 2018
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42.3 – Patterns of blood pressure and flow reflect the
structure and arrangement of blood vessels
What are the forces and structures that influence blood flow in the vessels of each circuit?
Blood Vessel Structure and Function
• The vertebrate circulatory system relies on blood vessels to deliver oxygen and nutrients and remove wastes throughout
the body
• Blood vessels exhibit a close match of structure and function
• All bloods contain a central lumen (cavity) lined with an endothelium, a single layer of flattened epithelial cells
• Smooth endothelial layer minimizes resistance to fluid flow
• Tissue layers that surround the endothelium differ among capillaries, arteries and veins
o Reflect distinct adaptations to the particular functions of these vessels
• Capillaries
o Smallest blood vessels – diameter only slightly greater than that of a red blood cell
o Have very thin walls – have just an endothelium and a surrounding extra cellular layer called basal lamina
o Exchange of substances between the blood and interstitial fluids occurs only in capillaries because of their thin
walls permit it
• Arteries and veins
o Walls that consist of two layers of tissue surrounding the epithelium
o Outer layer formed by connective tissue that contains elastic fibers and collagen
▪ Elastic fibers allow vessel to stretch and recoil
▪ Collagen provides strength
o Inner layer contains smooth muscle are more elastic fibers
• Arteries
o Thick, strong and elastic walls to accommodate blood pumped at high pressure by the heart
o Walls bulge out as blood enters and recoil as heart relaxes between contractions
▪ This behavior has an important role in maintaining blood pressure and flow to capillaries
o Smooth muscles in the walls of arteries and arterioles help regulate the path of blood flow
▪ Signals from nervous system and circulating hormones act on smooth muscles of these vessels ->
cause dilation or constriction that modulates blood flow to different parts of the body
• Veins
o Do not need thick walls because they convey blood back to the heart at lower pressures
o Wall of a vein is about a third as thick as that of an artery
o Veins contain valves that maintain a unidirectional flow of blood despite the low blood pressure in these
vessels
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Blood Flow Velocity
• Blood slows as it moves from arteries to arterioles to capillaries
• There are an enormous number of capillaries (7 billion) in the human body
o Arteries convey blood to all of these capillaries
o the total cross section area is much greater in capillary beds than in the arteries or any other part of the
circulatory system
• blood travels 500 times more slowly in the capillaries than in the aorta as a result of the enormous increase in cross-
sectional area
• Blood speeds up as it enters the venules and veins, which have smaller total cross-sectional areas
Blood Pressure
• Blood flows from areas of higher pressure to areas of lower pressure
• Contraction of a heart ventricle generates blood pressure, which exerts a force in all directions
o The part of the force directed lengthwise in an artery causes the blood to flow away from the heart because it is
the site of highest pressure
o The part of the force exerted sideways stretches the wall of the artery
• Recoil of the elastic arterial walls following ventricular contraction plays a critical role in maintaining blood pressure
and blood flow throughout the cardiac cycle
• Blood enters the millions of tiny arterioles and capillaries -> narrow diameter of these vessels generates substantial
resistance to flow
• By the time the blood enters the heart, this resistance dissipates much of the pressure generated by the pumping heart
Changes in Blood Pressure During the Cardiac Cycle
• Arterial blood pressure is highest when heart contacts during ventricular systole -> this pressure is called systolic
pressure
o Each ventricular contraction causes a spike in blood pressure that stretches the walls of the arteries
o This results in a pulse – the rhythmic bulging of the artery walls with each heartbeat
o Pressure surge partly due to narrow opening of arterioles that impede exit of blood from arteries
o When heart contracts, blood enters arteries faster than it can leave -> vessels stretch to a wide diameter from
the rise in pressure
• The elastic walls of the arteries snap back during diastole, which results in a lower but still substantial blood pressure
called diastolic pressure when ventricles are relaxed
o The blood pressure is still substantial because the heart contracts again before enough blood has flowed into
the arteries to completely relieve pressure in the arteries
o Arteries remain pressurized throughout the cardiac cycle -> blood continuously flows into arteries and
capillaries
Regulation of Blood Pressure
• Homeostatic mechanisms regulate arterial blood pressure by altering the diameter of arterioles
• Vasoconstriction occurs when muscles in arteriole walls contract, causing arterioles narrow
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