C H A P T E R 4 2
Introduction to the cardiovascular system
653
pressure on the venous end of the capillaries. The hydro
static pressure pushing fluid out of the capillary is soon
higher than the oncotic pressure that is trying to pull the
fluid back into the vessel, causing fluid to be lost into
the tissues. This shift of fluid accounts for the oedema
seen with HF. Pulmonary oedema results when the left
side of the heart fails; peripheral, abdominal and liver
oedema occur when the right side of the heart fails.
Cardiac:
Congestive heart failure
Cardiac:
Oedema
Other factors can contribute to a loss of fluid in the
tissues, including protein loss and fluid retention. Protein
loss can lead to a fall in oncotic pressure and an inability
to pull fluid back into the vascular system. Protein levels
fall in renal failure, when protein is lost in the urine,
and in liver failure, when the liver is no longer able to
produce plasma proteins. Fluid retention, which is often
stimulated by aldosterone and ADH as described earlier,
can increase the hydrostatic pressure so much that fluid
is pushed out under higher pressure and the balancing
pressure to pull it back into the vessel is not sufficient.
Drugs that are used to treat HF may affect the vascular
system at any of these areas in an attempt to return a
balance to the pressures in the system.
■■
Blood pressure is maintained by stimulus from the
sympathetic system and reflex control of blood
volume and pressure by the renin–angiotensin system
and the aldosterone–ADH system. Alterations in
blood pressure (hypotension or hypertension) can
upset the balance of the cardiovascular system and
lead to problems in blood delivery.
■■
Fluid shifts out of the blood at the arterial ends
of capillaries to deliver oxygen and nutrients to
the tissues. It moves out due to the hydrostatic or
fluid pressure of the arterial side of the system.
Fluid returns to the system at the venous end of the
capillaries because of the oncotic pull of proteins in
the vessels. Disruptions in these pressures can lead to
oedema or loss of fluid in the tissues.
KEY POINTS
↓
Blood flow
↓
Oxygenation
NEPHRON
Renin release
Activation of angiotensinogen
Angiotensin I
Lungs
converting enzyme (ACE)
Angiotensin I Angiotensin II
ADRENAL GLAND
Angiotensin III
↑
Aldosterone release
Retention of sodium with water
↑
Blood volume and pressure
↑
Blood flow to the kidneys
Erythropoietin
release
↑
RBC production
Powerful
vasoconstriction
↑
Blood pressure
↑
Blood flow
to the kidneys
Angiotensin-
FIGURE 42.9
The renin–angiotensin–aldosterone
system for reflex maintenance of blood pressure
control.
