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uitgebreide Samenvatting hoofdstuk 15 silverthorn human physiology
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Voorbeeld van de inhoud
Fleur sam Ch 15Artery=slagader, vein= ader. Arteriole= arteriolen, venule=venule.
Vasovagal syncope (syncope= fainting), a benign and common emotional reaction
to blood, hypodermic needles, or other upsetting sights. Normally, homeostatic
regulation of the cardiovascular system maintains blood flow, or perfusion, to the
heart and brains. In vasovagal syncope, signals from the NS cause a sudden
decrease in blood pressure, and the individual faints from a lack of O2 to the brain.
Right heart lungs left heart rest of body back to right heart.
Blood leaving the left heart enters systemic arteries, shown as expandable elastic
region. Pressure produced by contraction of the left ventricle is stored in the elastic
walls of arteries and slowly released through elastic recoil. This mechanism
maintains a continuous driving pressure for blood flow during ventricular relaxation.
Bc of this, the arteries are known as the pressure reservoir of the circulatory system.
Downstream from the arteries, small vessels called
arterioles create a high-resistance outlet for arterial
blood flow. Arterioles direct distribution of blood flow to
individual tissues by selectively constricting and
dilating, so they are known as the site of variable
resistance.
Arteriolar diameter is regulated both by local factors,
such as tissue oxygen concentrations, and by the
autonomic NS and hormones.
When blood flows into the capillaries, their leaky
epithelium allows exchange of materials between the
,plasma, the interstitial fluid and the cells of the body. At the distal end of the
capillaries, blood flows into the venous side of the circulation. The veins act as
volume reservoir from which blood can be sent to the arterial side of the circulation if
blood pressure falls too low.
From the veins, blood flows back to the right heart.
Total blood flow through any level of the circulation is equal to cardiac output. Bv. If
cardiac output is 5L/min, blood flow through all the systemic capillaries is 5L/min. in
the same way, blood flow through the pulmonary side of the circulation is equal to
blood flow through the systemic circulation.
15.1 THE BLOOD VESSELS
The walls of blood vessels are composed of layers of smooth muscle, elastic
connective tissue and fibrous connective tissue.
The inner lining of all blood vessels is a thin layer of endothelium, a type of
epithelium. endothelium was always thought to be a passive barrier, but it can also
secrete paracrine signal molecules and play important roles in regulation of
blood pressure, vessel growth and absorption of materials.
In most vessels, layers of connective tissue and smooth muscle surround the
endothelium. The endothelium and its adjacent elastic connective tissue together
make up the tunica intima, usually called the simple intima. The thickness of the
smooth muscle and connective tissue layers varies in different vessels.
The smooth muscle of blood vessels is known as vascular smooth muscle.
Most blood vessels contain smooth muscle, arranged in either circular or spiral
layers. Vasoconstriction narrows the diameter of the vessel lumen and vasodilation
widens it.
In most blood vessels, smooth muscle cells maintain a state of partial contraction at
all times, creating the condition known as muscle tone. Contraction of smooth
muscle, like that of cardiac muscle, depends on the entry of Ca2+ from the ECF
through Ca2+ channels. Signal molecules, including neurotransmitters, hormones
and paracrine signals, influence vascular smooth muscle tone. Many vasoactive
paracrine molecules are secreted either by endothelial cell lining blood vessels or by
tissues surrounding the vessels.
The aorta and major arteries are characterized by walls that are both stiff (stijf/stevig)
and springy (=veerkrachtig)
Arteries have thick, smooth muscle and large amounts of elastic and fibrous
connective tissue. Bc of the stiffness of the fibrous tissue, substantial amounts of
energy are required to stretch the walls of an artery outward, but that energy can be
stored by the stretched elastic fibers and released through elastic recoil.
Arteries and arterioles are characterized by a divergent pattern of blood flow. As
major arteries divide into smaller and smaller arteries, the character of the wall
changes, becoming less elastic and more muscular.
, The walls of the arterioles contain several layers of smooth muscle that contract and
relax under the influence of various chemical signals.
Arterioles, along with capillaries and small postcapillary vessels called venules, form
the microcirculation. Regulation of blood flow through the microcirculation is an active
area of physiological research.
Some arterioles branch into vessels known as metarterioles. True arterioles have a
continuous smooth muscle layer, but the wall of a metarteriole is only partially
surrounded by smooth muscle.
Blood flowing through metarterioles can take 1 of two paths:
- if muscle rings called precapillary sphincters are relaxed, blood flows into adjoining
capillary beds.
- If the precapillary sphincters are all constricted, metarteriole blood bypasses the
capillaries and goes directly to the venous circulation.
Capillaries are the smallest vessels in the cardiovascular system. They are the
primary site of exchange between the blood & the interstitial fluid. A small amount of
exchange takes place in postcapillary venules at the distal ends of the capillaries, but
this is insignificant.
To facilitate the exchange of materials, capillaries lack smooth muscle and elastic or
fibrous tissue reinforcement. Instead, their walls consist of a flat layer of endothelium,
one cell thick, supported on an acellular matrix called the basal lamina (basement
membrane).
Many capillaries are closely associated with cells known as pericytes. In most
tissues, these highly branched contractile cells surround the capillaries, forming a
mesh like outer layer between the capillary endothelium and the interstitial fluid.
Pericytes contribute to the tightness of capillary permeability: the more pericytes, the
less leaky the capillary endothelium. Bv. Cerebral capillaries and those in the retina
are surrounded by pericytes and glial cells and connected with tight junctions. In both
locations, these complex cell arrangements form the selectively permeable barriers
known as the blood-brain barrier and the blood-retinal barrier.
Pericytes secrete factors that influence capillary growth and some of them can
differentiate into new endothelium or smooth muscle cells.
Loss of pericytes around the capillaries of the retina is a hallmark of the disease
diabetic retinopathy, a leading cause of blindness.
Blood from the capillaries flow into small vessels called venules. The smallest
venules are similar to capillaries, with a thin exchange epithelium and little
connective tissue. They are distinguished from capillaries by their convergent pattern
of flow.
Smooth muscle begins to appear in the walls of larger venules. From venules, blood
flows into veins that become larger in diameter as they travel toward the heart.
Finally, the largest veins, the venae cavae, empty into the right atrium.
To assist venous flow, some veins have internal one-way valves. These valves, like
those in the heart, ensure that blood passing the valve cannot flow backward. Once
blood reaches the vena cava, there are no valves.
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