Briefly explain the formation of urine.
The formation of urine occurs through a triple process of filtration, reabsorption, and secretion. These
processes occur in the nephrons of the kidneys; the functional unit of the kidneys comprised of the renal
corpuscle and the renal tubules (Thibodeau, 2010). The first process, filtration, begins in the renal
corpuscles, which are made up of the Bowman capsules and the glomeruli encased within them. The
blood pressure in the glomeruli is high, and this pushes water and dissolved substances out into the
Bowman capsule (Thibodeau, 2010). This glomerular filtrate is produced at a rate of approximately
125ml/minute.
As this filtrate is moved out of the Bowman capsule and into the proximal convoluted tubule, the
process of reabsorption begins. This reabsorption continues from the proximal tubules through the Loop
of Henle, and on into the distal convoluted tubules. During this phase of urine formation, large amounts
of water and some glucose are reabsorbed by the blood through a process of osmosis between the
proximal tubules to the peritubular capillaries surrounding them (Thibodeau, 2010). Sodium (Na+) ions
are also actively transported back into the blood from these tubules, however, the volume is dependent
on intake: the greater the sodium intake, the less absorption takes place, resulting in a greater excretion
of Na+ into the urine (Thibodeau, 2010). Chloride ions, being negatively charged, are attracted to the
positive sodium ions, and are responsible in 'dragging' them out of the urine and back into the blood
(Thibodeau, 2010).
Secretion is the final stage of the formation of urine, and involves the balancing of hydrogen and
potassium ions, as well as certain drugs from the blood (Thibodeau, 2010). Secretion is the act of solutes
moving from the blood, into the urine. Various hormones play a role in assisting with the balance of
fluids in the blood, and consequently the urine. These hormones can trigger the retention or excretion
of water levels in the blood, which in turn effect blood pressure, blood volume, and urine output
(Thibodeau, 2010).
Name several substances eliminated or regulated by the kidney.
Some of the substances regulated or eliminated by the kidneys are water, sodium ions, chloride ions,
potassium and hydrogen ions, ammonia, and some drugs.
Explain the salt and water balance maintained by aldosterone and ADH.
ADH, or anti-diuretic hormone is responsible for regulating water reabsorption (Thibodeau, 2010) in the
kidney tubules. This hormone acts on the tubules and ducts by making them more permeable to water:
when present, water is reabsorbed by the blood, resulting in more water being retained by the body,
and subsequently less being eliminated in urine (Thibodeau, 2010).
Aldosterone, on the other hand, is important in the reabsorption of sodium (Na+) from the kidney
tubules, and also by increasing the tubular rate of water reabsorption. When sodium is reabsorbed into
the blood, water follows, so this would result in a decrease of urine volume as water would be held by
the body. Aldosterone secretion is regulated wholly by the kidneys: when blood volume/pressure
decrease to a below-normal level, the juxtaglomerular apparatus (JG) senses this and triggers the action
of the RAAS (renin-angiotensin-aldosterone system) (Thibodeau, 2010). This system causes the
constriction of blood vessels which results in increased blood pressure. This sensitivity also promotes
the secretion of aldosterone from the adrenal gland, further contributing to the increase in blood
pressure (Thibodeau, 2010).
A third hormone, produced in the atrial walls of the heart called atrial natriuretic hormone (ANH) has
the opposing action of aldosterone: it stimulates the kidney tubules to secrete Na+ ions, thus decreasing
water volume (lowering blood pressure) and therefore increasing urine output (Thibodeau, 2010).
Why is proper blood pressure necessary for proper kidney function?
Proper blood pressure is necessary for proper kidney function as the process of filtration is stimulated
by the blood pressure exerted on the vessels of the glomeruli; without this pressure, the filtration
process would not progress, resulting in toxic levels of waste substances being built up in the blood.
When various substances such as water, hydrogen, sodium, ammonia, potassium, chloride, and some
drugs, are not filtered, reabsorbed or secreted by the kidneys, a state of homeostasis within the body is
not achieved, which then can result in a plethora of complications and health issues.
The blood pressure and blood volume regulation is the responsibility of the juxtaglomerular apparatus
(JG); through the sensors of this apparatus, enzymes are released by the JG that stimulates the RAAS to
constrict blood vessels, thus raising the blood pressure levels (Thibodeau, 2010).
Dayle
References:
Thibodeau, G., & Patton, K. (2010). The Human Body in Health and Disease (5th Ed.) St. Louis, USA:
Mosby, Inc.