Identify the superficial veins of the forearm and arm, and describe their anatomical location.
The superficial veins include the cephalic, basilic, median, and
median cubital veins.
The cephalic vein and basilic veins arise
from the dorsal venous network. The cephalic ascends
the lateral
forearm, whereas the basilic ascends the medial aspect. The median
cubital vein
bridges these two veins; it runs obliquely across
the cubital fossa from the cephalic to a more
proximal union with
the basilic vein. The median vein ascends the center of the forearm
before
passing the medial side of the cubital fossa.
Describe how blood flow is regulated (a) to whole organs, (b) within
tissues, and (c) to
capillary beds.
Blood flow to groups of organs or to individual organs is regulated
by the muscular
arteries. Sympathetic vasomotor fibers
innervating their walls constrict or dilate the arteries
to
divert or enhance blood flow. A similar regulatory system
occurs within tissues via
vasoconstriction of arterioles.
Sympathetic stimulation can cause vasoconstriction of arterioles
to
increase systemic blood pressure. Alternatively, local
chemical conditions within metabolically
active tissues can cause
local vasodilation of the arterioles. These chemical factors also
cause
relaxation of the precapillary sphincters allowing greater
perfusion of capillary beds within the
tissues.
Define and explain the significance of vascular anastomoses.
A vascular anastomosis is a "coming together" of either
arteries or veins. Vascular
anastomoses are alternative pathways
for blood flow. These pathways are called collateral
channels.
Arterial anastomoses often occur around joints providing alternative
pathways for
blood to flow when movement of a joint impinges upon
flow through other vessels. The
anastomosis ensures adequate
perfusion of the tissue. Venous anastomoses provide
multiple
pathways for drainage. Blockage of a single vein rarely
blocks blood flow or leads to tissue
death.
Define the term portal system, and describe the significance of portal-systemic anastomoses.
A portal system consists of two separate capillary beds between the
arterial and venous
ends of the circuit. The capillary beds are
joined to each other in sequence by a portal vein. In the
hepatic
portal system, destruction of the hepatic sinusoids can lead to
blockage of blood flow
through the liver, resulting in increased
blood pressure throughout the liver and GI tract.
Venous
anastomoses along the GI tract provide a "safety
valve" through which venous blood can drain,
bypassing the
blocked liver and relieving the hypertension.
Describe the function of the fetal shunts from the pulmonary circuit.
The foramen ovale is a shunt between the right and left atria of the
heart. This shunt
allows blood to bypass the entire pulmonary
circuit. Only about half of the blood entering the
right atrium
is diverted through the foramen ovale. Much of the remainder of the
blood that
proceeds through the right ventricle and into the
pulmonary trunk then enters the second shunt.
This ductus
arteriosus connects the pulmonary trunk directly to the arch of the
aorta. This blood
then continues to the systemic circuit. Only a
small quantity of the initial blood entering the heart
goes to
the developing lungs.