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Cardiovascular System
Lecture III
Blood pressure
Systemic arterial pressures, are generated by the forceful contractions of the heart's left
ventricle.
Healthy resting arterial pressures, are relatively low, mean systemic pressures typically
being under 100 mmHg, about 1.8 lbf/in², above surrounding atmospheric pressure (about
760 mmHg or 14.7 lbf/in² at sea level).
To withstand and adapt to the pressures within, arteries are surrounded by varying
thicknesses of smooth muscle, which have extensive elastic and inelastic connective
tissues.
The pulse pressure, i.e. Systolic vs. Diastolic difference, is determined primarily by the
amount of blood ejected by each heart beat, stroke volume, versus the volume and
elasticity of the major arteries.
Over time, elevated arterial blood sugar (see Diabetes Mellitus), lipoprotein cholesterol,
and pressure, smoking, and other factors are all involved in damaging both the
endothelium and walls of the arteries, resulting in atherosclerosis. Diabetes Mellitus also
leads to capillary damage.
Arteriole
An arteriole is a blood vessel that extends and branches out from an artery and leads to
capillaries.
Arterioles have thick muscular walls and are the primary site of vascular resistance. The
mean blood pressure in the arteries supplying the body is a result of the interaction
between the cardiac output (the volume of blood the heart is pumping per minute) and the
vascular resistance, usually termed total peripheral resistance by physicians and
researchers.
The up and down fluctuation of the arterial blood pressure is due to the pulsatile nature of
the cardiac output and determined by the interaction of the stroke volume versus the
volume and elasticity of the major arteries.
The muscular contraction of arterioles is targeted by drugs that lower blood pressure
(antihypertensives), for example the dihydropyridines (nifedipine and nicardipine), which
block the calcium conductance in the muscular layer of the arterioles, causing relaxation.
This decreases the resistance to flow into peripheral vascular beds, lowering overall
systemic pressure.
Capillary
The word capillary is used to describe any very narrow tube or channel through which a
fluid can pass. See capillary action for details.
Capillaries are the smallest of a body's blood vessels, measuring 5-10 μm. They connect
arteries and veins, and most closely interact with tissues. Capillaries have walls
composed of a single layer of cells, the endothelium. This layer is so thin that molecules
such as oxygen, water and lipids can pass through them by diffusion and enter the tissues.
Waste products such as carbon dioxide and urea can diffuse back into the blood to be
carried away for removal from the body. Capillary permeability can be increased by the
release of certain cytokines.
The endothelium also actively transports nutrients, messengers and other substances.
Large molecules may be too big to diffuse across endothelial cells. In some cases,
vesicles contained in the capillary membrane use endocytosis and exocytosis to transport
material between blood and the tissues.
In an immune response, the endothelial cells of the capillary will upregulate receptor
molecules, thus "catching" immune cells as they pass by the site of infection and aiding
extravasation of these cells into the tissue.
The "capillary bed" is the network of capillaries supplying an organ. The more
metabolically active the cells, the more capillaries it will require to supply nutrients. The
capillary bed usually carries no more than 25% of the amount of blood it could contain,
although this amount can be increased through autoregulation (e.g. active muscle cells)
by constricting smooth muscle.
Venule
A venule is a small blood vessel that allows blood to return from the capillary beds to the
larger blood vessels called veins. Venules have three layers: An inner endothelium
composed of squamous epithelial cells that act as a membrane, a middle layer of muscle
and elastic tissue and an outer layer of fibrous connective tissue. The middle layer is
poorly developed so that venules have thinner walls than arterioles.
Vein
In biology, a vein is a blood vessel, which returns blood from the microvasculature to the
heart. Veins form part of the circulatory system. The vessels carrying blood away from
the heart are known as arteries.
Veins have one-way valves to prevent backflow caused by gravity.
In systemic circulation, de-oxygenated blood from the capillary blood vessels is taken by
veins to the right part of the heart. Differently, in the pulmonary circulation oxygenated
blood from the lungs is taken to the left part of the heart by pulmonary veins. Another
special case is portal circulation where the portal vein transports blood rich in products of
digestion from the intestines to the liver.
NAMES OF IMPORTANT VEINS:
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Pulmonary veins
Portal vein
Superior vena cava
Inferior vena cava
Femoral vein
Great saphenous vein
Veins are used medically as points of access to the blood stream, permitting the
withdrawal of blood specimens (venipuncture) for testing purposes, and enabling the
infusion of fluid, electrolytes, nutrition, and medications. The latter is called intravenous
delivery. It can be done by an injection with a syringe, or by inserting a catheter (a
flexible tube).
If an intravenous catheter has to be inserted, for most purposes this is done into a
peripheral vein (a vein near the surface of the skin in the hand or arm, or less desirably,
the leg.) Some highly concentrated fluids or irritating medications must flow into the
large central veins, which are sometimes used when peripheral access cannot be obtained.
Catheters can be threaded into the superior vena cava for these uses: if long term use is
thought to be needed, a more permanent access point can be inserted surgically.
The precise location of veins is much more variable from person to person than that of
arteries.
VENAE CAVAE
The superior and inferior venae cavae are the veins that return the blood from the body
into the heart. They both empty into the right atrium.
The inferior vena cava travels up alongside the abdominal aorta with blood from the
lower part of the body.
The superior vena cava is above the heart, and forms from a convergence of the left and
right brachiocephalic veins that contain blood from the head and the arms. The vena cava
carries blood from the body to the right atrium of the heart.
PULMONARY ARTERIES
The pulmonary arteries carry blood from the heart to the lungs. They are the only arteries
(other than umbilical arteries in the fetus) that carry deoxygenated blood.
In the human heart, the pulmonary trunk begins at the base of the right ventricle. It is
short and wide - approximately 5 cm (2 inches) in length and 3 cm (1.2 inches) in
diameter. It then branches into two pulmonary arteries, which connect to the base of each
lung.
Role in disease
Pulmonary hypertension occurs alone and as a consequence of a number of lung diseases.
It can be a consequence of heart disease (Eisenmenger's syndrome) but equally a cause
(right-ventricular heart failure); it also occurs as a consequence of pulmonary embolism
and scleroderma. It is characterized by reduced exercise tolerance. Severe forms,
generally, have a dismal prognosis.
PULMONARY VEINS
The pulmonary veins carry oxygen rich blood from the lungs to the left atrium of the
heart. They are the only veins in the adult human body that carry oxygenated blood.
There are four of them:
Right inferior
Right superior
Left inferior
Left superior
MAJOR BLOOD VESSELS
HEAD: ARTERIES: carotid - common carotid - internal carotid (ophthalmic, retinal,
anterior cerebral, middle cerebral, posterior communicating) - external carotid (facial,
maxillary, superficial temporal artery) - posterior cerebral - anterior communicating basilar - circle of Willis - middle meningeal | VEINS: jugular - vein of Galen
ARMS: ARTERIES: axillary (superior thoracic, thoracoacromial, lateral thoracic,
subscapular, anterior circumflex humeral, posterior circumflex humeral) - brachial radial - ulnar - dorsal scapular | VEINS: axillary - brachial - radial - ulnar - median
cubital - basilic - cephalic
THORAX: ARTERIES: aorta - brachiocephalic - bronchial - thoracic (lateral thoracic,
internal thoracic) - subclavian - vertebral - axillary - pulmonary | VEINS: venae cavae
(superior - inferior) - brachiocephalic - subclavian - portal - ductus venosus - azygos pulmonary
ABDOMEN: ARTERIES: celiac artery - marginal - artery of Adamkiewicz gastroduodenal - gastroepiploic - left gastric - umbilical - mesenteric (superior - inferior) |
iliac (common - external - internal) - Internal pudendal - renal - hepatic - common hepatic
- splenic | VEINS: mesenteric (inferior, superior) | iliac (common - external) - renal hepatic - splenic
LEGS: ARTERIES: dorsalis pedis - femoral - peroneal - popliteal - profunda femoris tibial (anterior, posterior) | VEINS: femoral - saphenous (great, small) - peroneal popliteal - profunda femoris - tibial (anterior tibial, posterior tibial)
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