Emily Bullas – Urinalysis Assignment
Urine Production and Control
Urine is produced by the kidneys during the filtration of the blood, to remove waste products and
excess water from the body. The kidneys use tiny functional units known as nephrons, shown in
figure 1, to filter the blood. The process begins when the blood diffuses from the glomerulus, a
network of high-pressure capillaries, into the Bowman’s capsule. The water, ions and small
molecules which diffuse into the capsule are known as the filtrate. Large molecules such as blood
cells and proteins cannot fit through and would tear apart the nephron if filtered. In the proximal
convoluted tubule, around 60% of the water is reabsorbed back into the blood, as well as potassium
ions, sodium ions, 90% of bicarbonate and practically 100% of the glucose. At the same time,
ammonia and any drugs in the body are secreted into the nephron to be removed in the urine. Next,
in the descending limb of the loop of Henle, water flows out of the nephron by osmosis, due to the
high salt content of the surrounding medulla of the kidney. Because of the loss of water, the filtrate
is highly concentrated at this point. On the ascending limb, NaCl is actively transported into the
medulla of the kidney – this should cause water to flow out via osmosis, but the ascending limb is
impermeable to water. Throughout the distal convoluted tubule, NaCl is reabsorbed, while
potassium and hydrogen ions are secreted into the nephron. This is also where final adjustments to
filtrate, such as the pH and concentration of ions, are made. In the collecting duct, the filtrate flows
towards the ureters. Some NaCl and urea are also absorbed. There are pores known as aquapores on
the collecting ducts which allow water to be reabsorbed when anti-diuretic hormone is excreted.
Figure 1: Structure of the Nephron (11)
Anti-diuretic hormone (ADH) is one of the hormones which controls urine output. It works by
opening the aquapores on the collecting duct of the nephron, allowing more water to be reabsorbed
in times of dehydration. This hormone works in a negative feedback loop, as seen in figure 2 below.
When the water content of the blood gets too low, usually due to excessive sweating or salt intake,
the hypothalamus detects the change and instructs the pituitary gland to produce more ADH. This
causes the aquapores on the collecting duct to open, reabsorbing more water, leaving less in the
urine and therefore producing a lower volume of more concentrated urine, until the normal water
content of the blood is restored. When there is too much water in the blood, usually because too
much water has been drunk, the hypothalamus detects the change and inhibits ADH production by
the pituitary gland, closing aquapores on the collecting duct and allowing less water to be
reabsorbed the kidneys, leading to a larger volume of less concentrated urine to be produced, until
the normal water content of the blood is restored.
, Figure 2: ADH Negative Feedback Loop (12)
ADH is also released is through stimulation by atrial natriuretic peptide (ANP). When the blood
volume and pressure increase, the stretching atria release ANP, which causes vasodilation to prevent
blood pressure getting dangerously high. As the vessels expand, it promotes natriuresis, the
excretion of sodium, causing water to be reabsorbed and triggering the release of ADH from the
pituitary gland. This stabilises the blood volume and pressure.
The Renin-Angiotensin-Aldosterone System (RAAS) is a system of hormones and enzymes which also
works to control urine output, as shown in figure 3 below. When baroreceptors in the arteries detect
a drop in the blood pressure (less water is in the blood), the kidneys produce the enzyme renin.
Renin reacts with angiotensin, a hormone produced by the liver, to produce angiotensin I. The lungs
then produce an enzyme called angiotensin-converting enzyme (ACE), which converts angiotensin I
into angiotensin II. This hormone works on the blood vessels, causing systematic vasoconstriction, as
well as causing the adrenal glands to produce aldosterone. This hormone works on the kidneys to
increase sodium as well as the brain to release ADH, increasing water reabsorption in the collecting
ducts, as well as triggering thirst signals which causes the person to drink more water. The hormone
angiotensin II has a short half-life of only 1-2 minutes, before it is broken down into angiotensin III
and IV, which have the same purpose but are less effective.
Figure 3: Renin-Angiotensin-Aldosterone System (13)
