Human Physiology II · Systems Physiology
Functional Anatomy of the Kidney
On this page 7 sections
In 30 seconds
The kidney's functional unit is the Nephron The microscopic filter-and-reclaim unit of the kidney Full entry →. Blood enters a high-pressure capillary tuft, the Glomerulus A ball of high-pressure capillaries Full entry →, where filtration begins, then continues through a second, low-pressure capillary network—Peritubular capillaries Low-pressure network around cortical tubules Full entry → in cortical nephrons or the Vasa recta Hairpin capillaries alongside loops of Henle Full entry → in juxtamedullary nephrons—that reclaims most of the filtered fluid. Where the distal tubule touches its own glomerulus, the Juxtaglomerular apparatus Contact point of distal tubule with its arterioles Full entry → senses tubular fluid composition and arteriolar stretch to regulate the filtration rate.
Why this matters
Reduced Renal blood flow Volume of blood delivered to the kidneys per minute Full entry →—from blood loss, dehydration, or heart failure—is the classic trigger that activates the renin–angiotensin–aldosterone system to raise blood pressure and retain salt and water. Imaging and clearance measurements that estimate renal blood flow or glomerular filtration depend on the anatomy described here: a substance must enter via the Afferent arteriole Vessel carrying blood into the glomerulus Full entry →, be filtered or secreted, and be measured in collected urine. Understanding which vessels are upstream versus downstream of the filter explains how a drug that constricts the Efferent arteriole Vessel carrying blood out of the glomerulus Full entry → can raise filtration pressure in the short term yet reduce overall renal blood flow. Clinical values and diagnostic thresholds vary by institution and jurisdiction; these notes support education and do not replace clinical instruction or supervision.
The college version
1. The Nephron: Cortical vs Juxtamedullary
Each kidney contains about one million nephrons, and each nephron has two parts: a Renal corpuscle Glomerulus + Bowman's capsule Full entry → (the glomerulus inside Bowman's capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct). About 85% are cortical nephrons, whose renal corpuscles sit in the outer cortex and whose short loops of Henle barely dip into the medulla. The remaining ~15% are juxtamedullary nephrons, whose corpuscles lie at the cortex–medulla border and whose long loops of Henle plunge deep into the medulla. That difference is not cosmetic: the long loops (plus their vasa recta) build the osmotic gradient that lets the kidney produce concentrated urine.
2. Renal Blood Flow and the Two Capillary Beds
Renal blood flow follows a precise, one-way series of vessels: renal artery → segmental arteries → interlobar arteries → arcuate arteries → interlobular (cortical radiate) arteries → afferent arteriole → glomerular capillaries → efferent arteriole → peritubular capillaries and/or vasa recta → interlobular veins → arcuate veins → interlobar veins → renal vein. The kidneys receive about 20–25% of cardiac output (roughly 1 L/min of blood) despite being only ~0.5% of body mass. The two capillary beds serve opposite goals: the glomerulus is a high-pressure filter, while the peritubular capillaries and vasa recta are low-pressure vessels specialized for reabsorption. The vasa recta are hairpin-shaped capillaries that run parallel to the loops of Henle in the medulla, preserving the medullary osmotic gradient.
3. The Juxtaglomerular Apparatus
Where the distal convoluted tubule passes between the afferent and efferent arterioles of its own nephron lies the juxtaglomerular apparatus (JGA). Its key cells are the Macula densa Salt-sensing patch of tubule cells Full entry →—a patch of specialized tubule cells that detect the concentration and delivery of sodium chloride in tubular fluid—and the Granular cells Renin-secreting cells of the afferent arteriole Full entry → (juxtaglomerular cells), modified smooth-muscle cells in the wall of the afferent arteriole that synthesize, store, and secrete the enzyme renin. Between them, extraglomerular mesangial cells provide structural support and cell-to-cell signaling.
How it works
- Blood enters under pressure through the renal artery and is distributed down the arterial tree to the afferent arteriole.
- The afferent arteriole feeds the glomerulus, where filtration produces a protein-free filtrate.
- Filtrate travels the renal tubule while blood exits via the efferent arteriole into the peritubular capillaries or vasa recta.
- Reabsorbed water and solutes move from tubule back into these capillaries and return to the circulation.
- The macula densa and granular cells of the JGA monitor the system and adjust arteriolar tone and renin release to keep filtration stable.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Renal corpuscle | Renal tubule | Corpuscle = filter (glomerulus + capsule); tubule = the reabsorption/processing pipe |
| Afferent arteriole | Efferent arteriole | Afferent carries blood into the glomerulus; efferent carries it out |
| Peritubular capillaries | Vasa recta | Peritubular serve cortical tubules; vasa recta are the hairpin medullary vessels |
| Macula densa | Granular cells | Macula densa senses NaCl in tubule fluid; granular cells secrete renin |
| Cortex | Medulla | Cortex = outer (filtration); medulla = inner (concentration gradient) |
Memory aids
"A before E, C before J, M-senses / G-secretes." Afferent before Efferent (blood order); Cortical loops are short, Juxtamedullary loops are long; the Macula densa senses salt while the Granular cells release renin.
Quick review
Topic Recap
The nephron is a two-capillary-bed machine. The glomerulus filters under high pressure; the peritubular capillaries and vasa recta reclaim under low pressure. Cortical and juxtamedullary nephrons differ mainly in loop length, which determines the kidney's ability to concentrate urine. The juxtaglomerular apparatus—macula densa sensing salt plus granular cells secreting renin—sits at the control point between the two beds and regulates filtration.
Knowledge Check
- Which vessel carries blood into the glomerulus?
- Which nephron type has a long loop of Henle extending deep into the medulla?
- What do granular (juxtaglomerular) cells secrete?
- What is the functional role of the vasa recta?
- Which cells of the juxtaglomerular apparatus detect NaCl delivery in tubular fluid?
Answers and Rationales
- The afferent arteriole—the efferent arteriole carries blood out; remembering "A before E" tracks blood flow order.
- The Juxtamedullary nephron Nephron with a long loop deep in the medulla Full entry →—its long loop (with the vasa recta) builds the gradient for concentrated urine.
- Renin—the enzyme that starts the RAAS cascade to raise blood pressure and retain sodium.
- The vasa recta act as a countercurrent exchanger that preserves the medullary osmotic gradient while still supplying blood to medullary tissue.
- The macula densa—a salt-sensing patch of the distal tubule in contact with the afferent arteriole.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of the kidney as a water-treatment plant. Dirty blood arrives through one pipe (the renal artery), clean blood leaves through another (the renal vein), and waste exits through a separate drain (the ureter). Each nephron is a tiny "filter-then-reclaim" unit: a strainer (the glomerulus) lets water and small molecules through, and a long set of pipes (the tubules) pulls the useful stuff back into the blood while dumping extra waste in.
It is like a coffee filter you can partially "un-filter." It stops being exact because a coffee filter works in only one direction (strain out solids), whereas the nephron works in both directions—it filters, then actively reabsorbs and secretes specific molecules using pumps and channels that cost energy, not passive sieving alone.
Simple Example
Drinking a liter of water does not produce a liter of urine. The kidneys filter roughly 180 liters of plasma-derived fluid per day but reabsorb almost all of it, excreting only the extra water and waste—so a large drink becomes just a few hundred milliliters of dilute urine.
Worked example
- Oxygenated blood enters the afferent arteriole under arterial pressure and is pushed into the glomerular capillary tuft; the high pressure forces fluid through the filtration barrier into Bowman's capsule (the first capillary bed).
- The blood that was not filtered exits through the efferent arteriole—narrower than the afferent arteriole, which helps keep glomerular pressure high.
- From the efferent arteriole, blood enters the low-pressure peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons), which wrap around the tubules and reclaim reabsorbed water and solutes (the second capillary bed).
- Tubular fluid flows down the nephron; when the macula densa detects too much sodium chloride (implying high flow), it signals the afferent arteriole to constrict, reducing glomerular filtration—closing the feedback loop that protects against excessive fluid and salt loss.
The direction of flow matters because filtration requires high pressure before the efferent arteriole, while reabsorption requires low pressure after it. A useful derived quantity is renal plasma flow: RPF = RBF × (1 - hematocrit), where RBF is renal blood flow (mL/min) and hematocrit is the fraction of blood volume occupied by red cells; this isolates the plasma (the part that is actually filtered).
Key takeaways
- High yield: The glomerulus is the first capillary bed (filtration); peritubular capillaries/vasa recta are the second (reabsorption).
- High yield: Two arterioles in series—afferent then efferent—are unique to the kidney and are what make glomerular pressure adjustable.
- Cortical nephrons (short loops) predominate; juxtamedullary nephrons (long loops) drive urine concentration.
- The vasa recta are the blood-vessel counterpart of the loop of Henle and run in the same hairpin arrangement.
- Granular cells secrete renin; macula densa cells sense NaCl delivery.
- Kidneys get ~20–25% of cardiac output, about 1 L/min.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Trace the path of blood through the kidney, from the renal artery to the renal vein, naming the vessels in order.
- Distinguish cortical nephrons from juxtamedullary nephrons and explain the functional consequence of each arrangement.
- Describe the juxtaglomerular apparatus, identifying the macula densa and granular cells and what each senses.
- Explain how the nephron's two capillary beds (glomerular and peritubular/vasa recta) divide the kidney's work into filtration versus reclamation.
Key vocabulary
- Nephron
- The microscopic filter-and-reclaim unit of the kidney
- Renal corpuscle
- Glomerulus + Bowman's capsule
- Glomerulus
- A ball of high-pressure capillaries
- Afferent arteriole
- Vessel carrying blood into the glomerulus
- Efferent arteriole
- Vessel carrying blood out of the glomerulus
- Peritubular capillaries
- Low-pressure network around cortical tubules
- Vasa recta
- Hairpin capillaries alongside loops of Henle
- Cortical nephron
- Nephron with a short loop of Henle
- Juxtamedullary nephron
- Nephron with a long loop deep in the medulla
- Juxtaglomerular apparatus
- Contact point of distal tubule with its arterioles
- Macula densa
- Salt-sensing patch of tubule cells
- Granular cells
- Renin-secreting cells of the afferent arteriole
- Renal blood flow
- Volume of blood delivered to the kidneys per minute
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