Biomedical Engineering Reference
In-Depth Information
Fig. 14.11
The urinary bladder and urethra under the infl uence of ADH
3. The kidneys respond to changes in blood pH by altering the excretion of
acidic or basic ions in the urine. If the body becomes more acidic, the kidneys
excrete acidic hydrogen ions (H
+
) and conserve basic bicarbonate ions (HCO
3
−
).
If the body becomes more basic, the kidneys excrete basic bicarbonate ions and
conserve acidic hydrogen ions.
Together, these three mechanisms maintain tight control over the pH of the body.
Renin-Angiotensin-Aldosterone System
The long-term control of blood pressure is via the Renin-Angiotensin-Aldosterone
(RAA) system. This system is also one of the body's compensatory mechanisms to
a fall in blood pressure. The kidneys release renin into the bloodstream and this
converts angiotensinogen to angiotensin I which in turn is converted to angiotensin II
by angiotensin converting enzyme in the capillaries of the lungs. Under the infl u-
ence of Angiotensin II, aldosterone levels increase. This increases blood sodium
levels by decreasing the amount of salt excreted by the kidneys. Retaining salt
instead of excreting it into urine increases the osmolarity of the blood and so the
blood volume. As the volume increases, so does the blood pressure. Angiotensin II is
also a potent vasoconstrictor which raises blood pressure by increasing vascular
resistance (Fig.
14.12
).
