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Examines how the kidneys maintain water and electrolyte balance through filtration and reabsorption.
Every single day, your kidneys filter about 180 liters of fluid—enough to fill a large bathtub—yet you only excrete about 1.5 liters. How does your body decide with surgical precision exactly which drops of water to keep and which to throw away?
The nephron is the functional unit of the kidney, responsible for removing waste while conserving essential substances. The process begins at the glomerulus, a high-pressure capillary bed where ultrafiltration forces water and small solutes into Bowman's capsule. The resulting filtrate travels through the Proximal Convoluted Tubule (PCT), where nearly of glucose and amino acids are reabsorbed. As the filtrate descends the Loop of Henle, the surrounding high salinity of the renal medulla draws water out via osmosis. On the ascending limb, which is impermeable to water, and ions are actively transported out. This creates a concentration gradient that allows the kidney to produce concentrated urine later in the collecting duct.
Quick Check
Which specific part of the nephron is responsible for the majority of nutrient reabsorption (like glucose)?
Answer
The Proximal Convoluted Tubule (PCT).
When you are dehydrated, your blood's osmolarity increases. This is detected by osmoreceptors in the hypothalamus, triggering the release of Antidiuretic Hormone (ADH) from the posterior pituitary. ADH travels to the kidneys and binds to receptors on the collecting ducts. This signaling cascade causes aquaporins (water channels) to be inserted into the cell membranes. Because the surrounding medulla is so salty (thanks to the Loop of Henle), water rushes out of the duct and back into the blood. This results in a small volume of highly concentrated urine. Conversely, if you drink too much water, ADH levels drop, aquaporins are removed, and the excess water is excreted as dilute urine.
If a student's blood osmolarity rises from a normal to due to sweating, follow these steps to predict the body's response: 1. The hypothalamus detects the increase. 2. The posterior pituitary releases ADH. 3. Aquaporins increase in the collecting duct. 4. Water reabsorption increases, bringing blood osmolarity back toward .
Quick Check
If a drug blocked the action of aquaporins in the collecting duct, would urine volume increase or decrease?
Answer
Urine volume would increase because water cannot be reabsorbed back into the bloodstream.
While ADH responds to salt concentration, the Renin-Angiotensin-Aldosterone System (RAAS) responds to low blood volume or pressure. When the juxtaglomerular apparatus (JGA) senses a drop in pressure, it releases the enzyme renin. Renin converts a plasma protein called angiotensinogen into Angiotensin I, which is then converted by ACE (Angiotensin-Converting Enzyme) into Angiotensin II. This powerful hormone does two things: it causes systemic vasoconstriction to raise pressure immediately, and it triggers the adrenal glands to release aldosterone. Aldosterone acts on the Distal Convoluted Tubule (DCT) to reabsorb more . Since water follows salt, blood volume increases, further stabilizing blood pressure.
Imagine a patient has lost a significant amount of blood in an accident. Their blood pressure drops rapidly. Analyze the integrated response: 1. JGA detects low pressure and releases Renin. 2. Angiotensin II is produced, causing immediate vasoconstriction to keep blood flowing to vital organs. 3. Aldosterone is released, increasing reabsorption in the DCT. 4. Simultaneously, the drop in volume triggers ADH release to maximize water retention. 5. Result: The body minimizes fluid loss to while attempting to maintain a mean arterial pressure () sufficient for survival: .
What is the primary effect of Aldosterone on the nephron?
Which hormone is released directly in response to high blood osmolarity?
The ascending limb of the Loop of Henle is highly permeable to water.
Review Tomorrow
In 24 hours, try to sketch the RAAS pathway from memory, starting with the release of Renin and ending with the effect on blood pressure.
Practice Activity
Research why 'ACE inhibitors' are commonly prescribed for high blood pressure and explain their mechanism of action based on the RAAS pathway.