Urinary System Lab

Learning Objectives

  • Distinguish the key microscopic components of the renal cortex and medulla
  • Identify the structural components of the nephron
  • Describe the structure of the surface across which filtration occurs
  • Identify and distinguish the proximal tubule, distal tubule, and collecting duct
  • Identify the component cells of the juxtaglomerular apparatus
  • Name the important histological characteristics of the ureter, bladder, and urethra
  • Describe some key pathological conditions associated with the kidney

Pre-Lab Reading

Introduction

The urinary system is comprised of the kidney, ureter, urinary bladder, and urethra. The kidney produces urine, which contains excess water, electrolytes and waste products of the body. It then flows down the ureter into the bladder where it is temporarily stored. The bladder is then emptied via the urethra.

Kidney

The kidney has several important homeostatic, hormonal, and metabolic functions that include:

  • The maintenance of water and electrolyte homeostasis
  • Regulation of acid-base balance in conjunction with the respiratory system
  • Excretion of metabolic waste products, especially the toxic nitrogenous compounds
  • Production of renin for blood pressure control and erythropoietin, which stimulates red blood cell production in the bone marrow
  • Conversion of vitamin D into active form for the regulation of calcium balance

The kidney is composed of an outer cortex and inner medulla. Portions of the medulla extend into the cortex as the medullary rays, collections of straight renal tubules. The medulla contains multiple cone-shaped lobes, known as medullary pyramids. These urinary lobes are fused in the cortex. The urine drains into the renal pelvis, which is the initial part of the ureter. The hilum of the kidney is the site of entry and exit for renal artery, renal vein, and ureter.

Nephron

The nephron is the structural and functional unit of the kidney. There are about two million nephrons in each kidney. Nephrons begin in the cortex; the tubules dip down to the medulla, then return to the cortex before draining into the collecting duct. The collecting ducts then descend towards the renal pelvis and empty urine into the ureter.

The components of a single nephron include:

  • renal corpuscle
  • proximal convoluted tubule
  • loop of Henle
  • distal convoluted tubule

Different sections of nephrons are located in different parts of the kidney:

  • The cortex contains the renal corpuscle, proximal, and distal convoluted tubules.
  • The medulla and medullary rays contain the loops of Henle and collecting ducts.

Throughout the length of the nephron, capillaries called peritubular capillaries lie adjacent to all segments of the tubule. They originate from the efferent arteriole and are important for solute transport throughout the tubule.

Renal Corpuscle

The renal corpuscle is responsible for the filtration of the plasma. It contains two structures: the glormerulus and Bowman's capsule. The glomerulus is a cluster of capillary loops enclosed by Bowman's capsule, which is part of the renal tubule.

Bowman's capsule has two layers:

  • The visceral layer is in contact with the glormerulus, and is composed of specialized epithelial cells known as podocytes.
  • The parietal layer is the outer layer, and is composed of simple squamous epithelial cells. This layer is continuous with the epithelium of the proximal convoluted tubule.

The space between the two layers is named Bowman's space, and this space contains the ultrafiltrate of plasma. The plasma has to pass through a filtration barrier of three layers to enter Bowman's space: the capillary endothelium, the podocyte layer, and their fused basement membrane. Bowman's space is continuous with the proximal convoluted tubule.

Blood enters the renal corpuscle via afferent arterioles and then leaves via efferent arterioles. The part of renal corpuscle where afferent and efferent arterioles are located is known as the vascular pole. On the opposite end of the vascular pole is where the renal tubule begins and is known as the urinary pole.

Mesangial cells can also be found within the glomerulus. These cells secrete a matrix of basement membrane-like material to support the structure of the glomerulus.

Promixal Convoluted Tubule

The proximal convoluted tubule is the first segment of renal tubule. It begins at the urinary pole of the glomerulus. This is where the majority (65%) of the glomerular filtrate is reabsorbed. The convoluted portion of the tubule leads into a straight segment that descends into the medulla within a medullary ray and becomes the loop of Henle.

Loop of Henle

The loop of Henle forms a hair-pin structure that dips down into the medulla. It contains four segments: the pars recta (the straight descending limb of proximal tubule), the thin descending limb, the thin ascending limb, and the thick ascending limb. The turn of the loop of Henle usually occurs in the thin segment within the medulla, and the tubule then ascends toward the cortex parallel to the descending limb. The end of the loop of Henle becomes the distal convoluted tubule near its original glomerulus. The loops of Henle run in parallel to capillary loops known as the vasa recta. Recall from Physiology that the loop of Henle serves to create high osmotic pressure in the renal medulla via the counter-current multiplier system. Such high osmotic pressure is important for the reabsorption of water in the later segments of the renal tubule.

Distal Convoluted Tubule

The distal convoluted tubule is shorter and less convoluted than the proximal convoluted tubule. Further reabsorption and secretion of ions occur in this segment. The initial segment of the distal convoluted tubule lies right next to the glomerulus and forms the juxtaglomerular apparatus.

Juxtaglomerular Apparatus

The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole. It is located near the vascular pole of the glomerulus. The main function of the apparatus is the secretion of renin, which regulates systemic blood pressure via the renin-angiotensin-alodosterone system. The juxtaglomerular apparatus is composed of:

  • The macula densa, a collection of specialized epithelial cells of the distal convoluted tubule. These cells are enlarged as compared to surrounding tubular cells. The cells of the macula densa sense sodium chloride concentration in the tubule, which in turn reflects the systemic blood pressure.
  • The juxtaglomerular cells of the afferent arterioles, which are responsible for secreting renin. These cells are derived from smooth muscles cells of afferent arterioles.
  • The extraglomerular mesangial cells, which are flat and elongated cells located near the macula densa. Their function is currently unclear.

Collecting Ducts

The terminal portion of the distal tubule empties through collecting tubules into a straight collecting duct in the medullary ray. The collecting duct system is under the control of antidiuretic hormone (ADH). When ADH is present, the collecting duct becomes permeable to water. The high osmotic pressure in the medulla (generated by the counter-current multiplier system/loop of Henle) then draws out water from the renal tubule, back to vasa recta.

Renal Pelvis and Ureter

Numerous collecting ducts merge into the renal pelvis, which then becomes the ureter. The ureter is a muscular tube, composed of an inner longitudinal layer and an outer circular layer. The lumen of the ureter is covered by transitional epithelium (also called urothelium). Recall from the Laboratory on Epithelia that the transitional epithelium is unique to the conducting passages of the urinary system. Its ability to stretch allows the dilation of the conducting passages when necessary. The ureter connects the kidney and the urinary bladder.

Urinary Bladder

The ureter empties the urine into the bladder. The transitional epithelium continues over the surface of this organ. The thickened muscular layers become interwoven and cannot be clearly identified at this point.

Urethra

The urethra carries the urine away from the bladder to the outside of the body. In the male, it is joined by the genital system. The epithelium changes from transitional to stratified or pseudostratified columnar in the urethra, and to stratified squamous in the distal end of the urethra.

Pre-Lab Quiz

  1. Match each section of the renal tubule with its function:
  2. Proximal Tubule A. filters the plasma
    Distal Tubule B. most reabsorption occurs here
    Loop of Henle C. generates countercurrent gradient
    Collecting Duct D. site of action of ADH
    Renal Corpuscle E. cells from the juxtaglomerular apparatus
    Answer:
  3. Describe the function of each of the following cell types:
    • Podocyte
    • Mesangial Cell
    • Macula Densa
    • Juxtaglomerular Complex
    Answer:
  4. Describe the changes in the epithelium as urine moves from the ureter through the urethra.
  5. Answer:

Slides

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  1. Kidney
  2. Renal Corpuscle
  3. Renal Corpuscle 2
  4. Podocyte EM
  5. Podocyte Scanning EM
  6. Filtration Barrier
  7. Proximal Convoluted Tubule
  8. Proximal Convoluted Tubule EM
  9. Loop of Henle
  10. Distal Convoluted Tubule
  11. Juxtaglomerular Apparatus
  12. Collecting Ducts
  13. Renal Pelvis
  14. Ureter
  15. Urinary Bladder

Virtual Microscope Slides

  1. Kidney
  2. Begin by identifying roughly where the renal cortex and renal medulla are located.
  3. Ureter
  4. Ureters are tubes that propel urine from the kidneys to the urinary bladder. Examine the epithelium and two layers of smooth muscle.
  5. Urinary Bladder
  6. The urinary bladder collects and stores urine. Examine the epithelium and smooth muscle.

Pathology

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  1. Diabetic Nephropathy
  2. Glomerulonephritis
  3. Minimal Change Disease

Quiz

  1. Why might one want a transitional epithelium in this region?
  2. Answer: Stretching capability
  3. Name the region of the urinary system.
  4. Answer: Proximal tubule
  5. What is the purpose of this brush border epithelium?
  6. Answer: Increase surface area for uptake of materials
  7. How do the cells in this region detect low blood pressure?
  8. Answer: Osmoreceptors detect low sodium concentration in the distal tubule.
  9. What cells secrete the supporting matrix of this structure?
  10. Answer: Mesangial cells
  11. Name the layers of the filtration apparatus.
  12. Answer: Endothelium, basement membrane, foot processes of podocytes, filtration slits
  13. Describe the changes that you would see under the light microscope and the electron microscope when you compare a healthy kidney with one exhibiting the signs of minimal change nephrosis. Explain why these patients have edema.
  14. Answer: The kidneys would appear the same under the light microscope, but the foot processes of the podocytes would be missing in the EM of the minimal change kidney. Patients with this disease have edema because they can no longer repel proteins from entering the urine, and there is a loss of albumin from the blood into the urine, which is excreted. Albumin normally generates the osmotic pressure necessary to draw fluid back from the tissues into the bloodstream, and with hypoalbuminemia, the lymphatic system becomes overwhelmed and fluid accumulates in the tissues.