Maintenance of the Body
Nutrients, gases, and other solutes smaller than plasma proteins
cross the capillary wall by diffusion; larger molecules are actively
transported via pinocytotic vesicles or caveolae. Water-soluble
substances move through the clefs or Fenestrations; Fat-soluble
substances pass through the lipid portion oF the endothelial cell
Bulk flow oF fluids at capillary beds determines the distribution
oF fluids between the bloodstream and the interstitial space. It
reflects the relative effects oF hydrostatic and osmotic pressures
acting at the capillary (outward minus inward pressures). In
general, fluid flows out oF the capillary bed at the arterial end and
reenters the capillary blood at the venule end.
Cardiovascular; Topic: Autoregulation and Capillary Dynamics,
pp. 14–38.
Lymphatic vessels collect the small net loss oF fluid and protein
into the interstitial space and return it to the cardiovascular
Circulatory shock occurs when blood perFusion oF body tissues
is inadequate. Most cases oF shock reflect low blood volume
(hypovolemic shock), abnormal vasodilation (vascular shock), or
pump Failure (cardiogenic shock).
Circulatory Pathways:
Blood Vessels of the Body
The Two Main Circulations of the Body
(p. 721)
Te pulmonary circulation transports O
-poor, CO
-laden blood
to the lungs For oxygenation and carbon dioxide unloading.
Blood returning to the right atrium oF the heart is pumped by
the right ventricle to the lungs via the pulmonary trunk. Blood
issuing From the lungs is returned to the lef atrium by the
pulmonary veins. (See ±able 19.3 and ²igure 19.19.)
Te systemic circulation transports oxygenated blood From the
lef ventricle to all body tissues via the aorta and its branches.
Venous blood returning From the systemic circuit is delivered to
the right atrium via the venae cavae.
Systemic Arteries and Veins: Differences in Pathways
and Courses
(p. 721)
All arteries are deep while veins are both deep and superficial.
Superficial veins tend to have numerous interconnections. Dural
venous sinuses and the hepatic portal circulation are unique
venous drainage patterns.
Principal Vessels of the Systemic Circulation
(pp. 721–745)
±ables 19.3 to 19.13 and ²igures 19.20 to 19.30 illustrate and
describe vessels oF the systemic circulation.
Developmental Aspects of Blood Vessels
(p. 745)
Te Fetal vasculature develops From embryonic blood islands and
mesenchyme and Functions in blood delivery by the Fourth week.
²etal circulation differs From circulation afer birth. Te
pulmonary and hepatic shunts and special umbilical vessels are
normally occluded shortly afer birth.
Blood pressure is low in inFants and rises to adult values. Age-
related vascular problems include varicose veins, hypertension, and
atherosclerosis. Hypertension and associated atherosclerosis are the
most important causes oF cardiovascular disease in the aged.
Higher brain centers (cerebrum and hypothalamus) may modiFy
neural controls oF BP via medullary centers.
Hormones that increase BP by promoting vasoconstriction
include epinephrine and NE (these also increase heart rate and
contractility), ADH, and angiotensin II (generated in response
to renin release by kidney cells). Hormones that reduce BP by
promoting vasodilation include atrial natriuretic peptide, which
also causes a decline in blood volume.
Te kidneys regulate blood pressure by regulating blood volume.
Rising BP directly enhances filtrate Formation and fluid losses
in urine; Falling BP causes the kidneys to retain more water,
increasing blood volume.
Indirect renal regulation oF blood volume involves the renin-
angiotensin-aldosterone mechanism, a hormonal mechanism.
When BP Falls, the kidneys release renin, which triggers the
Formation oF angiotensin II. Angiotensin II causes (1) release
oF aldosterone, stimulating salt and water retention, (2)
vasoconstriction, (3) release oF ADH, and (4) thirst.
Cardiovascular System; Topic: Blood Pressure Regulation, pp. 1–31.
Pulse and blood pressure measurements are used to assess
cardiovascular efficiency.
Te pulse is the alternating expansion and recoil oF arterial walls
with each heartbeat. Pulse points are also pressure points.
Blood pressure is routinely measured by the auscultatory method.
Normal BP in adults is 120/80 mm Hg (systolic/diastolic).
Chronic hypertension (high blood pressure) is persistent BP
readings oF 140/90 or higher. It indicates increased peripheral
resistance, which strains the heart and promotes vascular
complications oF other organs, particularly the eyes and kidneys.
It is a major cause oF myocardial inFarction, stroke, and renal
disease. Risk Factors are high-Fat, high-salt diet, obesity, diabetes
mellitus, advanced age, smoking, stress, and being a member oF
the black race or a Family with a history oF hypertension.
Hypotension, or low blood pressure (below 90/60 mm Hg), is
rarely a problem except in circulatory shock.
Cardiovascular System; Topic: Measuring Blood Pressure,
pp. 11–12.
Blood Flow Through Body Tissues:
Tissue Perfusion
(pp. 711–720)
Blood flow is involved in delivering nutrients and wastes to and
From cells, gas exchange, absorbing nutrients, and Forming urine.
Blood flows Fastest where the cross-sectional area oF the vascular
bed is least (aorta), and slowest where the total cross-sectional
area is greatest (capillaries). Te slow flow in capillaries allows
time For nutrient-waste exchanges.
Autoregulation is the local adjustment oF blood flow to
individual organs based on their immediate requirements. It
involves myogenic controls that maintain flow despite changes
in blood pressure, and local chemical Factors. Vasodilators
include increased CO
, H
, and nitric oxide. Decreased O
concentrations also cause vasodilation. Other Factors, including
endothelins, decrease blood flow.
Cardiovascular; Topic: Autoregulation and Capillary Dynamics,
pp. 1–13.
In most instances, autoregulation is controlled by the
accumulation oF local metabolites and the lack oF oxygen.
However, autoregulation in the brain is controlled primarily by a
drop in pH and by myogenic mechanisms; and pulmonary circuit
vessels dilate in response to high levels oF oxygen.
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