Chapter 10: Homeostasis and the Renal System

Your body temperature right now is approximately 37 degrees C (98.6 degrees F). It was the same temperature yesterday. It will be the same tomorrow. Whether you are running a marathon in the desert or skiing in a blizzard, your core temperature barely budges. The same is true for your blood pH (7.35-7.45), your blood glucose (~70-100 mg/dL fasting), your blood osmolarity (~285-295 mOsm/L), and dozens of other variables.

This is not an accident. Your body is constantly measuring, adjusting, and correcting. That process - maintaining a stable internal environment despite changing external conditions - is homeostasis.

Why Homeostasis Matters for the MCAT

Homeostasis is not just one topic. It is the organizing principle behind every organ system. The endocrine system uses hormones to maintain blood glucose. The respiratory system adjusts breathing rate to maintain blood pH. The cardiovascular system changes heart rate to maintain blood pressure. The renal system adjusts urine output to maintain fluid balance.

Every time the MCAT asks “what happens when X changes,” the answer almost always involves a homeostatic mechanism restoring the variable to its set point.

The Three Components of Every Homeostatic System

Every feedback loop has three parts:

1. Sensor (receptor) - detects the current value of the variable. Example: osmoreceptors in the hypothalamus detect blood osmolarity.

2. Control center (integrator) - compares the current value to the set point and decides what to do. Example: the hypothalamus compares detected osmolarity to the normal range.

3. Effector - carries out the correction. Example: the posterior pituitary releases ADH, which tells the kidneys to reabsorb more water.

Negative Feedback: The Default Mode

Negative feedback is the dominant control mechanism in the body. The principle: the output of a process opposes the original stimulus, pushing the variable back toward the set point.

“Negative” does not mean “bad.” It means the response negates (reverses) the change that triggered it.

How it works:

1. A variable deviates from its set point
2. The sensor detects the deviation
3. The control center activates the effector
4. The effector produces a response that opposes the original change
5. The variable returns toward the set point
6. As the variable normalizes, the stimulus weakens, and the response diminishes

Classic examples:

Positive Feedback: The Rare Amplifier

Positive feedback is the opposite: the output amplifies the original stimulus rather than opposing it. This creates an escalating cycle that continues until an external event breaks the loop.

Positive feedback is rare because runaway amplification is dangerous. The body uses it only when a process must be driven rapidly to completion.

Key positive feedback examples for the MCAT:

• Oxytocin during labor - cervical stretch triggers oxytocin release, which causes stronger contractions, which increases cervical stretch. Loop breaks when the baby is delivered.
• LH surge during ovulation - rising estrogen from the dominant follicle triggers a massive LH surge (positive feedback on the anterior pituitary). Loop breaks when ovulation occurs and progesterone restores negative feedback.
• Blood clotting cascade - activated platelets release signals that activate more platelets. Loop breaks when the clot seals the vessel.

Negative vs. Positive Feedback Comparison

Set Points Can Shift

Set points are not permanently fixed. During a fever, the hypothalamus raises the body temperature set point (e.g., from 37 degrees C to 39 degrees C). Now the body “thinks” 37 degrees C is too cold, so it shivers and vasoconstricts to warm up. When the fever breaks, the set point drops back to normal, and you suddenly feel hot and start sweating.

This concept explains why fever is not a failure of homeostasis - it is homeostasis working perfectly around a new, temporarily elevated set point. Pyrogens (from pathogens or immune cells) cause prostaglandin release, which acts on the hypothalamus to raise the set point.

Before you can understand how the kidney works, you need a clear mental map of what it looks like. The kidney’s structure is not random - every anatomical feature exists because of the function it serves. The cortex is where filtration begins. The medulla is where urine gets concentrated. The pelvis is where urine collects before leaving. Once you connect structure to function, the physiology becomes much easier.

Gross Anatomy: The Kidney

Each person has two kidneys, bean-shaped organs roughly the size of a fist, located retroperitoneally (behind the peritoneum) against the posterior abdominal wall. The right kidney sits slightly lower than the left because the liver pushes it down.

Three regions of the kidney:

Renal cortex - the outer layer. Contains the glomeruli, Bowman’s capsules, proximal convoluted tubules (PCT), and distal convoluted tubules (DCT). This is where blood filtration begins and where most reabsorption occurs.

Renal medulla - the inner region. Contains the loops of Henle and collecting ducts, organized into cone-shaped structures called renal pyramids. The medulla creates the osmotic gradient that allows urine concentration. The tips of the pyramids (renal papillae) point inward toward the renal pelvis.

Renal pelvis - the funnel-shaped collecting area at the center. Urine drains from the collecting ducts through the papillae into minor calyces, which merge into major calyces, which empty into the renal pelvis. From there, urine flows into the ureter.

The Urinary Tract: From Kidney to Exit

Once urine leaves the renal pelvis, it follows a simple path:

Renal pelvis -> Ureter -> Urinary bladder -> Urethra -> outside the body

• Ureters - muscular tubes (~25-30 cm long) that use peristalsis to move urine to the bladder. One from each kidney.
• Urinary bladder - hollow muscular organ lined with transitional epithelium (stretches as it fills). The detrusor muscle (smooth muscle) contracts during urination.
• Urethra - final passage to the exterior. In females, it is short (~4 cm) and carries only urine. In males, it is longer (~20 cm) and carries both urine and semen (at different times).

Two sphincters control urine release:

• Internal urethral sphincter - smooth muscle, involuntary (autonomic control)
• External urethral sphincter - skeletal muscle, voluntary (somatic control)

The Nephron: Functional Unit of the Kidney

Each kidney contains over 1 million nephrons. The nephron is where all the action happens - filtration, reabsorption, secretion, and excretion. Understanding the nephron’s anatomy is essential for every renal question on the MCAT.

Walk through the nephron, segment by segment:

1. Renal corpuscle (in the cortex)

• Glomerulus - a ball of fenestrated capillaries where blood is filtered. High pressure forces fluid and small solutes out of the blood.
• Bowman’s capsule - a cup that surrounds the glomerulus and collects the filtrate. Lined with podocytes whose foot processes form filtration slits.

2. Proximal convoluted tubule (PCT) (in the cortex)

• The workhorse of reabsorption. Reabsorbs ~65% of filtered Na+, water, all glucose, all amino acids, and most bicarbonate.
• Cells have a brush border (microvilli) to maximize surface area.

3. Loop of Henle (dips into the medulla)

• Descending limb - permeable to water, impermeable to solutes. Water leaves by osmosis as the filtrate descends into the increasingly salty medulla.
• Ascending limb - impermeable to water, actively pumps out NaCl (especially the thick ascending limb). This is what creates the medullary osmotic gradient.

4. Distal convoluted tubule (DCT) (in the cortex)

• Fine-tuning station. Responds to aldosterone (reabsorbs Na+, secretes K+) and PTH (reabsorbs Ca2+). Site of additional H+ secretion for acid-base balance.

5. Collecting duct (runs from cortex through medulla)

• Receives filtrate from multiple nephrons. Permeability to water is controlled by ADH - more ADH means more aquaporin channels, more water reabsorbed, more concentrated urine.

Nephron Vasculature

The kidney’s blood supply is unusual and highly testable. Pay close attention to the arrangement:

Renal artery -> segmental arteries -> interlobar arteries -> arcuate arteries -> cortical radiate (interlobular) arteries -> afferent arteriole -> glomerulus -> efferent arteriole -> peritubular capillaries (or vasa recta for juxtamedullary nephrons) -> venous system

Key features:

• Afferent arteriole - brings blood TO the glomerulus. “Afferent” = arriving.
• Efferent arteriole - carries blood AWAY from the glomerulus. “Efferent” = exiting.
• The glomerulus is a capillary bed sandwiched between TWO arterioles (not an arteriole and a venule). This is unique in the body and allows precise pressure control.
• Peritubular capillaries - surround the PCT and DCT in cortical nephrons. Low-pressure, high-surface-area vessels that reabsorb solutes and water from the tubular cells back into the blood.
• Vasa recta - long, straight capillaries that run alongside the loops of Henle in juxtamedullary nephrons. They maintain the medullary osmotic gradient by acting as countercurrent exchangers.

Two Types of Nephrons

Filtration is the first step of urine formation. It is a bulk, nonselective process - the glomerulus does not “choose” what to filter. Instead, it acts like a sieve: anything small enough passes through, and anything too large stays behind. The selectivity comes later, in the tubules, where the kidney carefully reclaims what the body needs.

Understanding what drives filtration, what determines how much gets filtered, and what the filtration barrier looks like is critical for MCAT success.

The Filtration Membrane

The filtration barrier between blood and Bowman’s capsule has three layers:

1. Fenestrated endothelium of glomerular capillaries - has pores (fenestrations) that allow most plasma components to pass but block blood cells.

2. Basement membrane - a gel-like layer of glycoproteins with a net negative charge. This charge repels most plasma proteins (which are also negatively charged at physiological pH, especially albumin).

3. Podocytes - specialized cells that wrap around the capillaries with foot processes (pedicels). The gaps between the foot processes are called filtration slits. These provide the final size barrier.

What Gets Filtered vs. What Stays Behind

Clinical connection: If protein or blood appears in the urine, something is wrong with the filtration membrane. Proteinuria (protein in urine) indicates damage to the basement membrane or podocytes. Hematuria (blood in urine) indicates damage to the capillary endothelium.

Starling Forces: What Drives Filtration

Filtration is driven by pressure. Specifically, the net filtration pressure (NFP) is determined by the balance of four forces - collectively called Starling forces:

Forces FAVORING filtration (pushing fluid out of capillaries):

• Glomerular hydrostatic pressure - the blood pressure inside the glomerular capillaries. This is the main driving force and is much higher than in most capillary beds.

Forces OPPOSING filtration (pushing fluid back or keeping it in):

• Bowman’s capsule hydrostatic pressure - the pressure of fluid already in Bowman’s space pushing back.
• Glomerular oncotic pressure - the osmotic pull of plasma proteins in the blood (proteins cannot cross, so they pull water back).

The net filtration pressure is the balance of these forces. Under normal conditions, hydrostatic pressure greatly exceeds the opposing forces, so the net flow is always from blood into Bowman’s space.

Glomerular Filtration Rate (GFR)

GFR is the volume of filtrate produced per minute by all glomeruli in both kidneys.

Normal GFR: ~125 mL/min = ~180 L/day

That is an enormous volume - far more than your total blood volume. You only excrete ~1-2 L as urine because the tubules reabsorb 99% of the filtrate.

Factors that increase GFR:

• Increased glomerular hydrostatic pressure (higher blood pressure)
• Afferent arteriole dilation (more blood enters)
• Efferent arteriole constriction (blood backs up, raising pressure)
• Decreased plasma protein concentration (less oncotic opposition)

Factors that decrease GFR:

• Decreased blood pressure (shock, hemorrhage)
• Afferent arteriole constriction (less blood enters)
• Efferent arteriole dilation (less backup, lower pressure)
• Increased plasma protein (dehydration concentrates proteins)
• Urinary obstruction (increases Bowman’s capsule pressure)

Filtration Fraction

Measuring GFR: Clearance

You cannot watch filtrate form, so GFR is estimated by measuring how fast the kidneys clear a substance from plasma. A substance that is freely filtered, not reabsorbed, and not secreted has a clearance that equals GFR.

Afferent vs Efferent: Why Direction Matters

Because the glomerulus sits between two arterioles, the kidney can control GFR with precision. Constriction at either end has opposite effects:

• Afferent constriction -> less blood arrives -> glomerular pressure falls -> GFR drops.
• Efferent constriction -> blood backs up in the glomerulus -> glomerular pressure rises -> GFR rises (up to a point).

Angiotensin II preferentially constricts the efferent arteriole, which protects GFR when renal perfusion is low. This is why ACE inhibitors and ARBs can drop GFR in patients with already-compromised renal blood flow: remove angiotensin II, and the efferent arteriole relaxes, dropping glomerular pressure.

Autoregulation of GFR

The kidneys maintain a stable GFR even when systemic blood pressure fluctuates (within the range of ~80-180 mmHg MAP). Two mechanisms accomplish this:

1. Myogenic mechanism - when blood pressure rises, the increased stretch on afferent arteriole walls triggers them to constrict (smooth muscle response). This prevents the pressure increase from reaching the glomerulus. When pressure drops, the arteriole relaxes.

2. Tubuloglomerular feedback (TGF) - the macula densa cells in the DCT (part of the juxtaglomerular apparatus) monitor NaCl concentration in the filtrate. If GFR increases, more NaCl reaches the macula densa. The macula densa signals the afferent arteriole to constrict, reducing blood flow and lowering GFR back to normal.

The Juxtaglomerular Apparatus (JGA)

The JGA is where the DCT brushes against its own afferent arteriole. Two cell types do the work:

• JG cells in the afferent arteriole wall secrete renin when blood pressure drops or sympathetic nerves fire.
• Macula densa cells in the DCT wall sense tubular NaCl and signal the JG cells.

That physical adjacency is the point - a local sensor (macula densa) sitting next to a local effector (JG cells), no hormones required for the feedback.

After filtration dumps 180 liters of fluid per day into Bowman’s space, the tubules must reclaim almost all of it. If they did not, you would lose your entire plasma volume in about 30 minutes. Reabsorption is the process of moving useful substances from the tubular filtrate back into the blood. Secretion is the reverse - moving additional waste from the blood into the filtrate for excretion.

Each nephron segment has a specific job. The MCAT expects you to know what happens where.

The Big Picture: Three Processes

1. Filtration (glomerulus) - bulk movement of fluid and small solutes from blood into Bowman’s space. Nonselective.

2. Reabsorption (tubules) - movement of useful substances from filtrate back into peritubular capillary blood. Selective and highly regulated.

3. Secretion (tubules) - movement of additional waste, drugs, and ions from peritubular blood into the filtrate. Provides a second chance to remove substances that were not filtered or need faster elimination.

Proximal Convoluted Tubule (PCT): The Workhorse

The PCT reabsorbs the majority of everything. It is the most metabolically active segment of the nephron, packed with mitochondria and covered in microvilli (brush border) to maximize surface area.

What the PCT reabsorbs:

• ~65% of filtered Na+ and water
• ~100% of filtered glucose (via sodium-glucose co-transporters)
• ~100% of filtered amino acids
• ~80-90% of filtered bicarbonate (HCO3-)
• Most phosphate, lactate, and citrate
• Urea (partially - ~50% is passively reabsorbed)

What the PCT secretes:

• H+ (important for bicarbonate reabsorption)
• Organic acids (uric acid, drug metabolites)
• Organic bases (creatinine, certain antibiotics)
• NH3/NH4+ (for acid-base buffering)

How sodium reabsorption works in the PCT:

The Na+/K+ ATPase on the basolateral membrane (blood side) pumps Na+ out of the cell and into the blood, keeping intracellular Na+ low. This creates a gradient that drives Na+ into the cell from the tubular lumen through various co-transporters on the apical membrane (lumen side):

• Na+-glucose co-transporter - drags glucose into the cell along with Na+
• Na+-amino acid co-transporters - drags amino acids with Na+
• Na+/H+ exchanger - trades Na+ in for H+ out (helps reclaim bicarbonate)

Water follows Na+ passively through aquaporins and paracellular pathways. The PCT reabsorbs water and solutes in roughly equal proportions, so the filtrate leaving the PCT is still isotonic (~300 mOsm/L) - just much reduced in volume.

Transport Maximum (Tm) and Glucose

Glucose reabsorption uses sodium-glucose co-transport carriers in the PCT. These carriers have a maximum transport rate - the transport maximum (Tm).

At normal blood glucose (~100 mg/dL), all filtered glucose is reabsorbed. But if blood glucose exceeds ~180-200 mg/dL (the renal threshold), the carriers become saturated. Glucose that exceeds the Tm cannot be reabsorbed and appears in the urine (glucosuria).

This is exactly what happens in uncontrolled diabetes mellitus. The excess glucose in the urine also drags water with it by osmosis, causing the classic symptoms of polyuria (excessive urination) and polydipsia (excessive thirst).

Loop of Henle: Creating the Medullary Gradient

The loop of Henle has two limbs with opposite permeability properties. This is the key to understanding urine concentration.

Descending limb:

• Permeable to water
• Impermeable to solutes (NaCl)
• As filtrate descends into the increasingly salty medulla, water leaves by osmosis
• Filtrate becomes progressively more concentrated (up to ~1200 mOsm/L at the hairpin turn)

Thin ascending limb:

• Impermeable to water
• Passively permeable to NaCl (salt diffuses out)
• Filtrate becomes less concentrated as salt leaves

Thick ascending limb:

• Impermeable to water
• Actively pumps NaCl out via the Na+/K+/2Cl- co-transporter
• This is the segment that actively creates the medullary osmotic gradient
• Filtrate becomes dilute (~100 mOsm/L) by the time it reaches the DCT

Distal Convoluted Tubule (DCT): Fine-Tuning

The DCT handles the last bit of adjustable Na+ and Ca2+ reabsorption:

• Thiazide-sensitive Na+/Cl- cotransporter - reabsorbs a small fraction of filtered Na+ (the site of thiazide diuretics).
• PTH - increases Ca2+ reabsorption here.
• Aldosterone begins to act on the late DCT but exerts its main effect on the collecting duct.

The DCT also continues H+ secretion for acid-base regulation.

Segment-by-Segment Summary Table

The countercurrent multiplier is one of the most elegant mechanisms in all of physiology - and one of the most commonly tested on the MCAT. It explains how the kidney creates a concentrated environment in the medulla that allows the collecting duct to produce concentrated urine. Without this gradient, you could never conserve water during dehydration.

The concept is straightforward once you see it clearly: two parallel tubes running in opposite directions, doing different things, amplifying a small difference into a large gradient.

How It Works: Step by Step

The countercurrent multiplier depends on two facts:

1. The descending limb is permeable to water but NOT to salt
2. The thick ascending limb actively pumps salt out (via Na+/K+/2Cl- co-transport) but is impermeable to water

These two limbs run side by side in opposite directions (countercurrent flow).

The single effect: At any horizontal level, the thick ascending limb pumps NaCl into the medullary interstitium. This raises the interstitial osmolarity by a small amount (~200 mOsm/L above the filtrate).

The multiplication: Because the descending limb is permeable to water, water leaves the descending limb (drawn out by the salty interstitium), concentrating the filtrate inside. This concentrated filtrate then flows around the hairpin turn into the ascending limb, where more salt is pumped out. Each “cycle” adds to the gradient.

The result:

• Cortex: ~300 mOsm/L (isotonic with plasma)
• Outer medulla: ~600 mOsm/L
• Inner medulla: ~900 mOsm/L
• Deepest medulla (papilla): ~1200 mOsm/L

Why “Countercurrent” and Why “Multiplier”?

Countercurrent - the fluid flows in opposite directions in the two limbs. Descending filtrate flows down; ascending filtrate flows up. This is critical because it means the ascending limb is always adjacent to a slightly less concentrated section of the descending limb, allowing continuous salt extraction to have a cumulative effect.

Multiplier - the single effect (a ~200 mOsm/L difference at any one level) gets multiplied along the length of the loop into a much larger total gradient (300 to 1200 mOsm/L).

If flow were concurrent (same direction in both tubes), the gradient would equilibrate and you would get a uniform, modest difference. The opposite flow directions are what allow the small single effect to be amplified.

The Vasa Recta: Countercurrent Exchanger

The vasa recta are the capillaries that run alongside the loops of Henle in juxtamedullary nephrons. They have a critical job: supply blood to the medulla WITHOUT washing away the osmotic gradient.

The vasa recta accomplish this through countercurrent exchange:

• As blood descends into the salty medulla, water leaves and solutes enter (blood equilibrates with the surrounding interstitium)
• As blood ascends back toward the cortex, water re-enters and solutes leave (blood re-equilibrates)
• Net effect: blood delivers oxygen and nutrients to the medulla while maintaining the gradient

Urea Recycling: The Gradient Booster

Urea contributes about 50% of the medullary osmotic gradient in the inner medulla. Here is how:

1. Urea is filtered at the glomerulus and partially reabsorbed in the PCT
2. In the inner medullary collecting duct (under ADH stimulation), urea transporters allow urea to diffuse out into the medullary interstitium
3. Some of this urea enters the thin ascending limb of the loop of Henle (urea recycling)
4. The urea travels through the nephron again and re-enters the medulla through the collecting duct

This recycling keeps urea concentrated in the inner medulla, adding to the osmotic gradient that drives water reabsorption from the collecting duct.

What Happens If the Gradient Is Destroyed?

Loop diuretics block the Na+/K+/2Cl- co-transporter in the thick ascending limb. Without active NaCl pumping, the medullary gradient collapses. The collecting duct has nothing to drive water reabsorption against, so large volumes of dilute urine are produced. This is why loop diuretics are the most powerful class of diuretics - they attack the gradient at its source.

The collecting duct is where the final decision is made: will you produce dilute urine or concentrated urine? Everything upstream - the glomerulus, PCT, loop of Henle, DCT - has been preparing the filtrate. The collecting duct uses the medullary gradient (built by the countercurrent multiplier) to determine how much water to reclaim based on hormonal signals.

The key hormone is ADH (antidiuretic hormone, also called vasopressin). ADH is the single most important regulator of urine concentration.

ADH: The “Don’t Pee” Hormone

Where ADH comes from:

1. Osmoreceptors in the hypothalamus detect increased blood osmolarity (dehydration)
2. The hypothalamus synthesizes ADH
3. ADH is transported down axons to the posterior pituitary
4. The posterior pituitary releases ADH into the bloodstream
5. ADH travels to the kidneys and binds receptors on collecting duct principal cells

What ADH does at the collecting duct:

1. ADH binds receptors on the basolateral membrane of principal cells
2. This activates a cAMP signaling cascade
3. Aquaporin water channels are inserted into the apical (lumen-facing) membrane
4. Water flows from the tubular lumen through aquaporins, through the cell, and out the basolateral side
5. Water enters the hypertonic medullary interstitium and is picked up by the vasa recta

Two Scenarios: Hydrated vs. Dehydrated

When you are well-hydrated (low ADH):

• Blood osmolarity is low, so little ADH is released
• Collecting duct remains impermeable to water (few aquaporins)
• The dilute filtrate (~100 mOsm/L from the ascending limb) passes through the collecting duct without losing water
• Result: large volume of dilute urine (~50-100 mOsm/L)

When you are dehydrated (high ADH):

• Blood osmolarity is high, so ADH is released
• Collecting duct becomes permeable to water (aquaporins inserted)
• Water is reabsorbed as the collecting duct passes through the increasingly concentrated medulla
• Result: small volume of concentrated urine (up to ~1200 mOsm/L)

Cell Types in the Collecting Duct

The collecting duct has two main cell types:

Principal cells - respond to ADH (insert aquaporins) and aldosterone (Na+ reabsorption, K+ secretion). These handle water and electrolyte balance.

Intercalated cells - handle acid-base balance.

• Type A intercalated cells - secrete H+ into the lumen and reabsorb HCO3- into the blood. Active during acidosis.
• Type B intercalated cells - secrete HCO3- into the lumen and reabsorb H+. Active during alkalosis.

Diabetes Insipidus: When ADH Fails

Diabetes insipidus (DI) is a condition where ADH signaling fails, resulting in massive volumes of dilute urine (up to 18-20 L/day) and severe dehydration.

Central DI - the hypothalamus/posterior pituitary does not produce enough ADH. Caused by head trauma, tumors, or surgery.

Nephrogenic DI - the kidneys do not respond to ADH (defective receptors or aquaporin channels). ADH levels are actually HIGH, but the collecting duct cannot respond.

The key distinction: in central DI, the problem is upstream (no hormone produced). In nephrogenic DI, the problem is downstream (hormone is present but the effector is broken). This is the same “primary vs. secondary” reasoning pattern you learned for endocrine disorders.

SIADH: When There Is Too Much ADH

Syndrome of Inappropriate ADH secretion (SIADH) is the opposite of DI. Too much ADH is released (often from lung tumors, brain injury, or certain drugs), causing excessive water reabsorption. The result:

• Very concentrated, low-volume urine
• Dilutional hyponatremia (blood Na+ drops because excess water dilutes it)
• Patients may develop confusion, seizures, or cerebral edema from the low sodium

Three hormones dominate renal regulation: ADH, aldosterone, and ANP. Each responds to a different stimulus, acts on a different part of the nephron, and produces a different effect. But they all serve the same ultimate goal - maintaining blood volume, blood pressure, and electrolyte balance.

The MCAT tests these hormones constantly. If you know the trigger, target, and effect of each hormone, you can reason through almost any renal physiology question.

ADH (Antidiuretic Hormone / Vasopressin)

Trigger: Increased blood osmolarity (detected by hypothalamic osmoreceptors) or decreased blood volume/pressure (detected by baroreceptors in the carotid sinus and aortic arch)

Source: Synthesized in the hypothalamus, released from the posterior pituitary

Target: Receptors on collecting duct principal cells

Effect: Inserts aquaporin-2 channels into the apical membrane, increasing water reabsorption. Also promotes urea recycling in the inner medullary collecting duct.

Net result: Decreased urine volume, increased urine concentration, decreased blood osmolarity

Inhibited by: Low blood osmolarity, alcohol, ANP

Aldosterone

Trigger: Angiotensin II (from RAAS), high blood K+, low blood Na+, and ACTH (minor role)

Source: Zona glomerulosa of the adrenal cortex

Target: Principal cells of the DCT and collecting duct

Effect: Increases expression of sodium channels on the apical membrane and Na+/K+ ATPase on the basolateral membrane. This increases Na+ reabsorption and K+ secretion.

Net result: Na+ and water retention (water follows sodium), K+ excretion, increased blood volume and blood pressure

Inhibited by: ANP, high blood Na+, low K+

ANP (Atrial Natriuretic Peptide)

Trigger: Atrial stretch from increased blood volume (the atria of the heart release ANP when they are overfilled)

Source: Cardiac atrial myocytes

Target: Multiple sites - kidneys, adrenal glands, blood vessels, hypothalamus

Effects:

• Increases GFR (dilates afferent arteriole, constricts efferent arteriole)
• Inhibits Na+ reabsorption in the collecting duct
• Inhibits renin release (suppresses RAAS)
• Inhibits aldosterone secretion
• Inhibits ADH release
• Causes vasodilation

Net result: Increased Na+ and water excretion, decreased blood volume and blood pressure

Head-to-Head Comparison

How the Three Hormones Interact

Dehydration scenario:

1. Blood osmolarity rises and blood volume drops
2. ADH is released - collecting duct reabsorbs water
3. RAAS is activated - aldosterone causes Na+ (and water) retention
4. ANP is suppressed (atria are not stretched)
5. Result: concentrated, low-volume urine; blood volume and pressure restored

Overhydration scenario:

1. Blood volume rises, blood osmolarity drops
2. ADH is suppressed - collecting duct stays impermeable
3. RAAS is suppressed - less aldosterone, less Na+ retention
4. ANP is released - promotes Na+ and water excretion, further suppresses RAAS and ADH
5. Result: dilute, high-volume urine; blood volume and pressure decrease

RAAS is the body’s emergency blood pressure rescue system. When blood pressure drops - from dehydration, hemorrhage, or heart failure - the kidneys detect the problem and launch a hormonal cascade that raises blood pressure through multiple simultaneous mechanisms. Understanding every step of this cascade is essential for the MCAT.

The RAAS Cascade: Step by Step

Step 1: Trigger - low blood pressure detected
The JG cells of the afferent arteriole sense reduced renal perfusion (low blood pressure). Three signals activate renin release:

• Direct detection of low pressure by baroreceptors in the afferent arteriole
• Low NaCl detected by the macula densa (signals low GFR)
• Sympathetic nervous system activation (beta-1 receptors on JG cells)

Step 2: Renin release
JG cells secrete renin (an enzyme) into the blood.

Step 3: Angiotensinogen to Angiotensin I
Renin cleaves angiotensinogen (a large protein continuously produced by the liver) into angiotensin I. Angiotensin I is inactive - it is just an intermediate.

Step 4: Angiotensin I to Angiotensin II
Angiotensin-converting enzyme (ACE), located primarily on the endothelial surface of pulmonary capillaries (lungs), converts angiotensin I into angiotensin II. ACE also degrades bradykinin (a vasodilator), which explains the dry cough side effect of ACE inhibitors.

Step 5: Angiotensin II - the effector
Angiotensin II is one of the most potent vasoconstrictors in the body, playing a central role in blood pressure regulation. It has multiple simultaneous effects:

Pharmacological Targets in the RAAS

The MCAT commonly describes drugs that target specific steps in the RAAS cascade:

ACE inhibitors - block the conversion of angiotensin I to angiotensin II. Less vasoconstriction, less aldosterone release, blood pressure drops. ACE also normally degrades bradykinin, so blocking ACE causes bradykinin accumulation - this explains the dry cough side effect.

ARBs (angiotensin receptor blockers) - block angiotensin II receptors. Same blood pressure-lowering effects as ACE inhibitors but without the cough (bradykinin is still degraded normally).

Negative Feedback in RAAS

The RAAS cascade has built-in negative feedback:

1. When blood pressure normalizes, the JG cells sense adequate pressure and stop releasing renin
2. Angiotensin II directly inhibits further renin release (short-loop feedback)
3. Elevated blood pressure activates baroreceptors that suppress sympathetic drive to the JG cells
4. ANP (released when blood volume is high) inhibits renin, aldosterone, and ADH

Blood pH must stay between 7.35 and 7.45. A pH below 7.35 is acidosis; above 7.45 is alkalosis. Either extreme can be fatal. Two organ systems share the job of maintaining this narrow range: the lungs handle the fast response (seconds to minutes), and the kidneys handle the slow but powerful response (hours to days).

The MCAT tests acid-base physiology frequently and expects you to identify the disorder, determine the cause, and predict the compensation.

The Bicarbonate Buffer System

The body’s most important extracellular buffer is the bicarbonate system. The chemistry behind this buffer (Henderson-Hasselbalch equation) is covered in general chemistry - here we focus on how the body regulates it:

How the Kidneys Regulate pH

The kidneys have three mechanisms for acid-base regulation:

1. Bicarbonate reabsorption (PCT)

• The PCT reabsorbs ~80-90% of filtered HCO3-
• This is not direct reabsorption - HCO3- cannot cross the apical membrane
• Instead: H+ is secreted into the lumen (via Na+/H+ exchangers), combines with filtered HCO3- to form CO2 + H2O (catalyzed by carbonic anhydrase on the brush border)
• CO2 diffuses into the PCT cell, is reconverted to HCO3- (intracellular carbonic anhydrase), and HCO3- exits on the basolateral side into the blood
• Net effect: each H+ secreted “reclaims” one HCO3-

2. H+ secretion (collecting duct)

• Type A intercalated cells actively secrete H+ via H+ ATPase and H+/K+ ATPase
• This generates NEW bicarbonate (not just reclaiming filtered HCO3-)
• Each H+ secreted generates one new HCO3- that enters the blood
• This is the kidney’s mechanism for correcting acidosis

3. Ammonium (NH4+) excretion

• The PCT produces NH3 (ammonia) from glutamine
• NH3 diffuses into the tubular lumen and combines with H+ to form NH4+ (ammonium)
• NH4+ is trapped in the lumen (charged, cannot diffuse back) and excreted
• This allows the kidney to excrete large amounts of H+ without dropping urine pH below ~4.5 (NH3 acts as a urinary buffer)

The Four Acid-Base Disorders

How to Read an Arterial Blood Gas (ABG)

The MCAT may give you ABG values and ask you to identify the disorder:

Step 1: Look at pH. Is it acidotic (< 7.35) or alkalotic (> 7.45)?

Step 2: Determine the primary cause.

• If CO2 is abnormal and matches the pH direction, it is respiratory
• If HCO3- is abnormal and matches the pH direction, it is metabolic

Step 3: Check for compensation.

• Is the other value changing in the expected compensatory direction?

Normal values:

• pH: 7.35-7.45
• pCO2: 35-45 mmHg
• HCO3-: 22-26 mEq/L

Example: pH 7.30, pCO2 55 mmHg, HCO3- 28 mEq/L

• pH is low (acidosis)
• CO2 is high (respiratory cause - hypoventilation)
• HCO3- is slightly elevated (renal compensation - kidneys retaining bicarbonate)
• Diagnosis: respiratory acidosis with partial renal compensation

Common Causes of Each Disorder

Metabolic acidosis:

• Diabetic ketoacidosis (excess ketone production)
• Lactic acidosis (anaerobic metabolism)
• Renal failure (cannot excrete H+ or regenerate HCO3-)
• Severe diarrhea (loss of HCO3- in stool)

Metabolic alkalosis:

• Prolonged vomiting (loss of HCl from stomach)
• Excessive antacid use
• Hyperaldosteronism (excess H+ secretion in collecting duct)

Respiratory acidosis:

• COPD, pneumonia, or any cause of hypoventilation
• Opioid overdose (suppresses respiratory drive)

Respiratory alkalosis:

• Hyperventilation (anxiety, pain, high altitude)

Humans are endotherms - we generate our own body heat and maintain a stable core temperature regardless of the environment. The hypothalamus acts as the body’s thermostat, integrating temperature information from peripheral and central thermoreceptors and coordinating the appropriate heating or cooling response.

Thermoregulation is a classic example of negative feedback, and the MCAT frequently tests your ability to trace the loop from stimulus to response.

The Hypothalamic Thermostat

The hypothalamus receives temperature input from:

• Central thermoreceptors in the hypothalamus itself (monitor blood temperature directly)
• Peripheral thermoreceptors in the skin (detect environmental temperature changes early)

The hypothalamus compares incoming data to the set point (~37 degrees C) and activates the appropriate effectors.

Cooling Mechanisms (When Body Temp Is Too High)

1. Cutaneous vasodilation - arterioles in the skin dilate, increasing blood flow to the body surface. Heat radiates from the blood through the skin to the environment. This is why you look flushed when hot.

2. Sweating - eccrine sweat glands secrete water and electrolytes onto the skin surface. As sweat evaporates, it removes heat (evaporative cooling). This is the most effective cooling mechanism in humans. In high humidity, sweating is less effective because the air is already saturated with water vapor.

3. Behavioral responses - seeking shade, removing clothing, reducing activity. These are voluntary but highly effective.

Heating Mechanisms (When Body Temp Is Too Low)

1. Cutaneous vasoconstriction - arterioles in the skin constrict, reducing blood flow to the surface. Heat is retained in the core. This is why you look pale when cold.

2. Shivering - involuntary rapid contraction of skeletal muscles generates heat as a byproduct of ATP hydrolysis. Shivering can increase heat production by 5-fold.

3. Non-shivering thermogenesis - brown adipose tissue generates heat by uncoupling oxidative phosphorylation. Uncoupling proteins in the inner mitochondrial membrane allow H+ to flow back across the membrane without passing through ATP synthase - the energy is released as heat instead of making ATP. Infants have significant brown fat; adults retain smaller amounts.

4. Piloerection - contraction of arrector pili muscles at the base of hair follicles. In furred animals, this traps an insulating layer of air. In humans, it produces goosebumps - a vestigial response with minimal insulating value.

5. Thyroid hormone - chronic cold exposure stimulates thyroid hormone production (T3/T4), which increases basal metabolic rate and heat generation over days to weeks.

6. Behavioral responses - adding clothing, seeking warmth, curling up (reducing surface area).

Four Mechanisms of Heat Transfer

Heat moves between your body and the environment by four physical mechanisms:

Countercurrent Heat Exchange

In the extremities (arms, legs), arteries and veins run parallel and close together. Warm arterial blood heading to the fingers transfers heat to the cooler venous blood returning to the core. This means:

• Less heat reaches the extremities (conserving core temperature)
• The returning venous blood is pre-warmed before reaching the core

This is the same countercurrent principle as the loop of Henle, but for heat instead of solutes. It is especially important in cold environments and in aquatic mammals (whales, penguins) that have highly developed countercurrent heat exchangers in their flippers.

Fever: A Shifted Set Point

Fever is NOT a failure of thermoregulation. It is the hypothalamus deliberately raising its set point.

How fever works:

1. Infection triggers immune cells to release pyrogens (e.g., IL-1, IL-6, TNF-alpha)
2. Pyrogens stimulate the hypothalamus to produce prostaglandin E2 (PGE2)
3. PGE2 raises the hypothalamic set point (e.g., from 37 degrees C to 39 degrees C)
4. The body now “thinks” it is too cold and activates warming mechanisms (shivering, vasoconstriction)
5. Body temperature rises to the new set point and stabilizes

When the fever breaks:

1. The infection is controlled, pyrogen levels drop
2. PGE2 decreases, and the set point returns to 37 degrees C
3. The body now “thinks” it is too hot and activates cooling mechanisms (sweating, vasodilation)
4. Temperature falls back to normal

Anti-inflammatory drugs reduce fever by inhibiting cyclooxygenase (COX), which blocks PGE2 synthesis, lowering the set point back toward normal.

Endotherms vs. Ectotherms

Osmoregulation is the maintenance of blood osmolarity within a narrow range (~285-295 mOsm/L). Because water moves freely across cell membranes by osmosis, changes in extracellular osmolarity directly affect cell volume. If blood becomes too dilute, cells swell. If blood becomes too concentrated, cells shrink. Either extreme disrupts cellular function - especially in the brain, where swelling or shrinking can be rapidly fatal.

The kidneys, working with ADH and the thirst mechanism, are the primary regulators of osmolarity.

Body Fluid Compartments

Total body water makes up approximately 60% of body weight in an average adult. This water is distributed between two main compartments:

Intracellular fluid (ICF) - ~$\frac{2}{3}$ of total body water. The fluid inside all cells. The major intracellular cation is K+.

Extracellular fluid (ECF) - ~$\frac{1}{3}$ of total body water. Subdivided into:

• Plasma (~$\frac{1}{4}$ of ECF) - the fluid portion of blood within blood vessels
• Interstitial fluid (~$\frac{3}{4}$ of ECF) - the fluid between cells, outside blood vessels

The major extracellular cation is Na+. Because Na+ is the primary extracellular solute, sodium levels largely determine ECF osmolarity and volume.

Osmolarity vs. Tonicity

These terms are related but not identical:

Osmolarity - the total concentration of all solutes in a solution (mOsm/L). Includes ALL solutes, whether or not they can cross the membrane.

Tonicity - the effective osmotic pressure of a solution relative to a cell. Only counts non-penetrating solutes (those that cannot cross the membrane). Tonicity determines whether cells will swell, shrink, or stay the same.

Urea can cross cell membranes freely, so it contributes to osmolarity but NOT to tonicity. A solution with high urea concentration may be hyperosmolar but isotonic (cells will not change volume because urea equilibrates across both sides).

The Osmoregulation Feedback Loop

When blood osmolarity rises (dehydration, salt intake):

1. Osmoreceptors in the hypothalamus detect the increase
2. Two responses are triggered simultaneously:
   • ADH release from the posterior pituitary - increases water reabsorption in collecting ducts
   • Thirst - drives water intake (behavioral response)
3. Blood osmolarity decreases back toward normal
4. As osmolarity normalizes, ADH secretion and thirst decrease

When blood osmolarity falls (excess water intake):

1. Osmoreceptors detect the decrease
2. ADH release is suppressed
3. Collecting ducts remain impermeable to water
4. Large volumes of dilute urine are produced
5. Blood osmolarity increases back toward normal

Sodium and Volume Regulation

Because Na+ is the dominant extracellular solute, sodium balance is tightly linked to ECF volume:

• More Na+ in the ECF -> water follows by osmosis -> ECF volume increases -> blood pressure rises
• Less Na+ in the ECF -> water leaves by osmosis -> ECF volume decreases -> blood pressure falls

This is why aldosterone (which reabsorbs Na+) increases blood volume, and why ANP (which excretes Na+) decreases blood volume. It is also why excessive salt intake contributes to hypertension.

Edema: When Fluid Leaves the Vessels

Edema is the accumulation of excess fluid in the interstitial space (swelling). It occurs when the balance of Starling forces at the capillary level is disrupted:

Causes of edema:

• Increased capillary hydrostatic pressure - CHF, venous obstruction, standing for long periods
• Decreased plasma oncotic pressure - liver disease (low albumin production), kidney disease (protein lost in urine), malnutrition (kwashiorkor)
• Increased capillary permeability - inflammation, burns, allergic reactions (histamine increases permeability)
• Lymphatic obstruction - lymphedema from surgery, infection (elephantiasis), or tumor

Electrolyte Disorders: Clinical Connections

When your body breaks down amino acids and nucleic acids, the nitrogen-containing groups must be removed and excreted. Nitrogen cannot simply be stored - its waste products are toxic. Different organisms have evolved different strategies for handling this nitrogen, and understanding these strategies reveals important principles about water conservation, toxicity, and evolution.

For the MCAT, the key concepts are: what nitrogenous wastes are, how the urea cycle works, and how the kidney excretes them.

The Three Nitrogenous Waste Products

1. Ammonia (NH3)

• Produced directly from amino acid deamination
• Extremely toxic to the nervous system (even at low concentrations)
• Very water-soluble
• Must be diluted in large volumes of water for safe excretion
• Organisms that excrete ammonia directly are called ammonotelic (most aquatic animals - fish, aquatic invertebrates)

2. Urea

• Produced in the liver via the urea cycle (from two ammonia molecules + one CO2)
• Much less toxic than ammonia (~100,000x less toxic)
• Water-soluble
• Requires moderate water for excretion
• Organisms that excrete urea are called ureotelic (mammals, adult amphibians, sharks)

3. Uric acid

• Produced from purine (adenine, guanine) metabolism
• Least toxic of the three
• Poorly water-soluble (semi-solid paste)
• Requires very little water for excretion
• Organisms that excrete uric acid are called uricotelic (birds, reptiles, insects)

The Urea Cycle (Liver)

The urea cycle converts toxic ammonia into urea in the liver. This is the primary detoxification pathway for nitrogen in humans.

Key points:

• Occurs partly in the mitochondria and partly in the cytoplasm of hepatocytes
• Combines 2 NH3 (actually one free NH4+ and one from aspartate) + 1 CO2 to produce 1 urea molecule
• Consumes 3 ATP equivalents per cycle
• Urea is released into the blood, filtered at the glomerulus, and partially reabsorbed (about 50% in the PCT, with some recycled in the inner medulla)

Clinical Markers of Kidney Function

The MCAT often tests how kidney function is assessed:

Blood urea nitrogen (BUN) - measures urea in the blood. Elevated BUN suggests reduced kidney function (the kidneys are not clearing urea efficiently) or increased protein catabolism (more urea is being produced).

Serum creatinine - creatinine is a waste product of creatine phosphate metabolism in skeletal muscle. It is freely filtered at the glomerulus and NOT reabsorbed (making it an excellent marker of GFR). Elevated creatinine = reduced GFR = kidney impairment.

BUN/Creatinine ratio - helps distinguish the cause of elevated BUN:

• Normal ratio: ~10-20:1
• High ratio (>20:1): suggests prerenal cause (dehydration, CHF - urea is reabsorbed more because flow through tubules is slow)
• Normal ratio with both elevated: suggests intrinsic renal disease (both accumulate equally)

Comparative Nitrogenous Waste Excretion

What Happens When Waste Excretion Fails

When kidney function declines significantly, waste products accumulate in the blood:

• Uremia - elevated urea and other waste products cause fatigue, nausea, confusion, and eventually coma
• Metabolic acidosis - kidneys cannot excrete H+ or regenerate HCO3-
• Hyperkalemia - kidneys cannot excrete K+
• Fluid overload - kidneys cannot excrete excess water

Similarly, if the liver fails, ammonia from amino acid catabolism cannot be converted to urea. The resulting hyperammonemia is toxic to the brain because ammonia crosses the blood-brain barrier and disrupts neurotransmission.