Urinary System

Objectives

Identify major structural feature of kidney including cortex and medulla, arcuate arteries, afferent and efferent arterioles, glomerulus, peritubular capillaries, medullary rays, and components of nephron.
Identify proximal convoluted tubules (PCTs), distal convoluted tubules (DCTs), Bowman capsule, loop of Henle, and collecting ducts.
Identify components of juxtaglomerular apparatus and describe their function.
Describe blood flow through the kidney and describe how blood flow relates to urine production and flow.
Identify distinctive features of ureter, urinary bladder, and female urethra.

Overview

Nephron
Histology: A Text and Atlas © Wolters Kluwer 2020

The most effective way to understand renal function is to learn the blood flow and urine flow patterns. The renal arteries branch directly from the aorta and carry a massive blood supply to the kidneys. Blood flows into interlobar arteries, across the arcuate arteries (at the corticomedullary junction), and then flows rapidly into interlobular arteries in the medullary rays. As the interlobular arteries ascend toward the superficial cortex, they send off numerous afferent arterioles that supply cortical glomeruli. A glomerular filtrate is produced in the glomeruli, and passes into the urinary space. The glomerular filtrate now passes through the PCT, loop of Henle, DCT, and collecting tubules. The composition of the glomerular filtrate is gradually changed as it passes through the nephron, with constituents being mostly removed and returned to the blood via the peritubular capillary network and vasa recta. In this process, the nitrogenous wastes are concentrated in urine and excreted down the ureters, to the urinary bladder, and out the urethra. After processing, blood leaves the branches of the efferent arterioles, and enters a venous drainage system that essentially mirrors the arterial blood supply of the kidney. Eventually, blood cleansed of its nitrogenous wastes is returned to the interlobular veins, arcuate veins, interlobar veins, renal veins, and inferior vena cava.

The juxtaglomerular apparatus consists of three cellular components:

Juxtaglomerular cells-pressure sensors in wall of afferent arteriole
Macula densa- urine composition sensors in wall of DCT
Mesangial cells- that connect these two cell types and provide a communication link

It is essential for regulating the interplay between blood pressure, glomerular filtration rate, and urine composition. The urinary passages, from the renal pelves, minor and major calyces, ureters, urinary bladder, and upper portion of the urethra are lined by transitional epithelium and surrounded by smooth muscle. They are essentially conduits for passive transfer and active (by smooth muscle contraction) elimination of urine.

Renal Corpuscle. Histology: A Text and Atlas, 2020. © Wolters Kluwer

Slides

Use the slides linked below to review the content of the urinary system.

Slide 63, Infant Kidney, H&E. There are 8 annotation boxes.

In this slide, you should first identify the cortex (outer portion), medulla (inner portion) and lumen of a major calyx.

Within the cortex, you can scan the slide to view portions of the tissue that contains pars radiata (or medullary rays) and portions that consist of glomeruli, blood vessels, and convoluted tubules known as pars convoluta. Zoom into the pars convoluta to find renal corpuscles, glomeruli, afferent/efferent arterioles, Bowman’s capsule, proximal convoluted tubules (PCT) and distal convoluted tubules (DCT). The PCTs and DCTs are arranged in a pattern known as the cortical labyrinths between the renal corpuscles which looks similar to the arrangement of the medullary rays. When examining renal corpuscles, the macula densa of the juxtaglomerular apparatus (JGA) is visible at the vascular pole.

When viewing the medulla, look for the parallel arrays of collecting/processing tubes known as the loops of Henle along with the vasa recta (blood vessels). The loops of Henle have thin and thick limbs which you can see by identifying the squamous epithelium of the thin limbs and cuboidal epithelium of the thick limbs.

Slide 62, Adult Kidney, H&E. There are 10 annotation boxes.

This slide has most of a renal pyramid from an adult kidney. The renal papilla (pointed end of tissue) projects into a minor calyx (white space at point of tissue). An interlobar artery branches to an arcuate artery (lumen in middle of tissue), which defines the corticomedullary junction. Repeat the drill from the last slide on this slide. Here, the JGA is much less obvious than in the last slide, so you shouldn’t spend a lot of time searching for it. When you are finished with this slide and the infant kidney slide above, you should be able to identify the following structures and understand their relations:

Renal corpuscle
Glomerulus
Bowman capsule
PCT and DCT
Loop of Henle
Collecting tubules and ducts
Papillary
Arcuate artery
Medullary rays and interlobular arteries
Afferent/Efferent arterioles (can not be easily distinguished in sections)
Vasa recta
Peritubular capillary network
Juxtaglomerular apparatus

Slide 64, Ureter, H&E. There are 3 annotation boxes.

This is a slightly oblique section through the ureter. Start at low magnification at the lumen (center of image) and observe the layers of the ureter. There is a mucosa, muscularis externa, and adventitia. There is no submucosa or muscularis mucosae present. Think about why this layer is adventitia and not serosa. Transitional epithelium is found in the mucosa. This is a stratified epithelium (5-10 layers of cells) with an apical layer of slightly fatter (“pillowy”) cells bulging a little into the lumen known as umbrella cells. There is a loose CT domain beneath the epithelium which makes up the lamina propria (no division into mucosa and submucosa here because there is no muscularis mucosae), followed by indistinctly arranged, jumbled smooth muscle bundles in the wall. Textbooks describe an inner longitudinal and an outer circular layer of smooth muscle in the upper ureter, with a third, outer longitudinal layer being gradually added as you pass down toward the bladder. Based on this, do you think this a is more proximal or distal part of the ureter?

Slide 212 University of Michigan, Urinary Bladder, H&E. There are 4 annotation boxes.

In this image, you can see the larger lumen in comparison to the ureter. You can also appreciate the thicker muscular layer. Focus on identifying the layers- mucosa, submucosa (not all sources agree this layer exists, some resources combine all connective tissue between the epithelium and muscularis externa as lamina propria), muscularis externa, and adventitia/serosa. This specimen has a clear submucosa layer of dense connective tissue compared to the lamina propria of loose connective tissue. The mucosa has transitional epithelium that ranges in thickness between about 5 and 15 cell layers with umbrella cells. The possible submucosa layer (depending on the source) is present through most of the bladder except where the urinary trigone is present due to a different embryologic origin. The urinary bladder has 3 layers of smooth muscle but can be difficult to distinguish. Most texts will describe an inner longitudinal, middle circular, and outer longitudinal layer of muscle. You will be able to find neurovasculature bundles throughout the specimen, especially between the muscle layers.

Slide 75, Urethra/Vagina, H&E. There are 2 annotation boxes.

In this slide, focus on identifying the urethra. We will cover the vagina portion of the specimen in the female reproductive system content. The urethra is located towards bottom of the slide just to the right of the center of the image. The female urethra is short and the distal portion is lined with stratified squamous unkeratinized epithelium, just like the vagina. The urethra does not contain the organized layers we’ve seen elsewhere in the urinary system, it is a simple, fibromuscular tube. It contains an epithelial lining surrounded by connective tissue and muscle fibers. Depending on the location, these muscle fibers can be smooth muscle or skeletal muscle. The urethra opens into the vestibule (space between the labia minora) just anterior to the opening of the vagina in anatomical position.

Slide 71, Penis from Monkey, H&E. There are 2 annotation boxes.

For this slide, focus on the urethra. The penile tissues are covered in the male reproductive content. This is a cross-section of the penis. The penile urethra (aka spongy urethra) travels through the corpus spongiosa, an erectile tissue. The more proximal part of the spongy urethra is lined by pseudostratified columnar epithelium while the most distal portion is lined by stratified squamous. Other parts of the male urethra include the membranous (mix of pseudostratified columnar and urothelium), prostatic (urothelium), and intramural (aka pre-prostatic) urethra (urothelium).

More Practice

Use the following slides for at home practice. Answers are below in the next section.

University of Michigan 286. There are 2 annotation boxes.

Use this slide to determine what structure of the urinary system you are observing.

Slide University of Michigan 209. There are 6 annotation boxes.

Slide 65, Toluidine Blue. There are 3 annotation boxes.

Use this slide to determine what part of the urinary system it is. Review the majors structures.