Renal medulla

Key takeaways

Renal medulla belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Renal medulla to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Renal medulla from memory before moving on to harder problems.

Reference excerpt

The renal medulla (Latin: medulla renis 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the segmental arteries which then branch to form interlobar arteries. The interlobar arteries each in turn branch into arcuate arteries, which in turn branch to form interlobular arteries, and these finally reach the glomeruli. At the glomerulus the blood reaches a highly disfavourable pressure gradient and a large exchange surface area, which forces the serum portion of the blood out of the vessel and into the renal tubules. Flow continues through the renal tubules, including the proximal tubule, the loop of Henle, through the distal tubule and finally leaves the kidney by means of the collecting duct, leading to the renal pelvis, the dilated portion of the ureter. The renal medulla contains the structures of the nephrons responsible for maintaining the salt and water balance of the blood. These structures include the vasa rectae (both spuria and vera), the venulae rectae, the medullary capillary plexus, the loop of Henle, and the collecting tubule. The renal medulla is hypertonic to the filtrate in the nephron and aids in the reabsorption of water. Blood is filtered in the glomerulus by solute size. Ions such as sodium, chloride, potassium, and calcium are easily filtered, as is glucose. Proteins are not passed through the glomerular filter because of their large size, and do not appear in the filtrate or urine unless a disease process has affected the glomerular capsule or the proximal and distal convoluted tubules of the nephron. Though the renal medulla only receives a small percentage of the renal blood flow, the oxygen extraction is very high, causing a low oxygen tension and more importantly, a critical sensitivity to hypotension, hypoxia, and blood flow. The renal medulla extracts oxygen at a ratio of ~80% making it exquisitely sensitive to small changes in renal blood flow. The mechanisms of many perioperative renal insults are based on the disruption of adequate blood flow (and therefore oxygen delivery) to the renal medulla.

Interstitium The medullary interstitium is the tissue surrounding the loop of Henle in the medulla. It functions in renal water reabsorption by building up a high hypertonicity, which draws water out of the thin descending limb of the loop of Henle and the collecting duct system. Hypertonicity, in turn, is created by an efflux of urea from the inner medullary collecting duct.

Pyramids

Renal pyramids (or malpighian pyramids or Malpighi's pyramids named after Marcello Malpighi, a seventeenth-century anatomist) are cone-shaped tissues of the kidney. In humans, the renal medulla is made up of 10 to 18 of these conical subdivisions. The broad base of each pyramid faces the renal cortex, and its apex, or papilla, points internally towards the pelvis. The pyramids appear striped because they are formed by straight parallel segments of nephrons' Loops of Henle and collecting ducts. The base of each pyramid originates at the corticomedullary border and the apex terminates in a papilla, which lies within a minor calyx, made of parallel bundles of urine collecting tubules.

Papilla The renal papilla is the location where the renal pyramids in the medulla empty urine into the minor calyx in the kidney. Histologically it is marked by medullary collecting ducts converging to form a papillary duct to channel the fluid. Transitional epithelium begins to be seen.

Clinical significance Some chemicals toxic to the kidney, called nephrotoxins, damage the renal papillae. Damage to the renal papillae may result in death to cells in this region of the kidney, called renal papillary necrosis. The most common toxic causes of renal papillary necrosis are NSAIDs, such as ibuprofen, acetylsalicylic acid, and phenylbutazone, in combination with dehydration. Perturbed renal papillary development has also been shown to be associated with onset of functional obstruction and renal fibrosis. Renal papillary damage has also been associated with nephrolithiasis and can be quantified according to the papillary grading score, which accounts for contour, pitting, plugging and Randall's plaque.

Image gallery

See also Medullipin Kokko and Rector Model, a theory to explain how a gradient is generated in the inner medulla Renal sinus Medullary interstitium Renal capsule

References This article incorporates text in the public domain from page 1221 of the 20th edition of Gray's Anatomy (1918)

External links Anatomy figure: 40:03-02 at Human Anatomy Online, SUNY Downstate Medical Center Anatomy photo:40:06-0107 at the SUNY Downstate Medical Center - "Posterior Abdominal Wall: Internal Structure of a Kidney" Histology image: 15901loa – Histology Learning System at Boston University - "Urinary System: neonatal kidney" posteriorabdomen at The Anatomy Lesson by Wesley Norman (Georgetown University) (renalpelvis)

Illustrations

Worked examples

Example 1 — a first encounter with Renal medulla

Start with the simplest possible case. Write down what Renal medulla claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Renal medulla before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Renal medulla ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Renal medulla

In research: Renal medulla appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Renal medulla in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Renal medulla is common in secondary-school and first-year university syllabi. It links to neighbouring topics Kidney anatomy, so understanding it makes those chapters shorter.
In everyday life: Look for Renal medulla outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Renal medulla in 20 minutes

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Frequently asked questions

What is Renal medulla in simple terms?

The renal medulla (Latin: medulla renis 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids.

Why does Renal medulla matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Renal medulla?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Renal medulla.