Histology of Urinary System

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Overall Function

• The urinary system regulates water & electrolyte balance.

• Kidneys form urine.

• Ureters convey urine.

• Urinary bladder stores urine.

• Urethrafinal conduit to outside.

Kidney Functions:

1. Regulates water & inorganic ion balance by selective resorption or secretion to remove excess water from blood.

A. Kidney tubules can extract material from the lumen & deliver it to the interstitium and vice versa.

B. Fluid passes through walls of glomerular capillaries.

2. Removes metabolic waste products from blood.

3. Removes foreign chemicals from the blood.

4. Secretes hormones

A. Eryrthropoietin - stimulates RBC production

B. Renin - a factor in the renin-angiotensin system that regulates blood pressure.

5. Maintains blood pH.

Parts of a Nephron:

Renal Capsule and Lobe

Renal capsule Convex border & concave border with renal hilum leading to a space within the kidney, the renal sinus, occupied by the renal pelvis that divides into 2-3 major calyces and then to 7-14 minor calyces that fit over a renal papilla.

• Renal lobe is the gross structural unit of the kidney.

Each lobe consists of:

• Renal pyramid - in medulla, base at cortico-medullary junction and apex at renal papilla.

• Renal columns are lateral and are cortical tissue that dips into the medulla.

Uriniferous tubule is a nephron plus collecting duct

This is the smallest functional unit of the kidney. The nephron is the basic functional unit of the kidney.

A nephron consists of:

1. A renal corpuscle which is made up of a glomerulus and Bowman's capsule

2. Renal tubule which is made up of proximal convoluted tubule, loop of Henle and distal convoluted tubule

Glomerulus - tuft or ball of capillaries

Supplied by large afferent arteriole and drained by efferent arteriole.

Types of Nephrons

Juxtacapsular (cortical) Nephrons:

• The majority are of this type. Their short loop stays in the cortex.

Juxtamedullary Nephrons:

• Located at corticomedullary junction and their loops extend deep into the medulla to the tip of the papilla. They are the most active in concentrating urine.

Intermediate Nephrons:

• Their loops extend into the peripheral medulla.

Collecting Ducts:

• Straight tubules that extend from the cortex to papilla. Transfer urine from distal convoluted tubule to papilla and final concentration of urine before it enters the ureter.

Parts close to Cortical Tissue

• Cortical labyrinth - containing proximal convoluted tubule, distal convoluted tubule and glomeruli.

• Medullary rays - finger-like extensions of medulla that projects into the cortex. A renal lobule is a portion of the renal cortex composed of a medullary ray with associated cortical tissue.

Bowman's Capsule

Contains Bowman's space and has a visceral and parietal component.

Podocytes

• Podocytes (visceral layer) cover the capillary endothelium & parietal layer (simple squamous) covers outer portion of renal capsule.

• Podocytes have primary and secondary processes. Secondary processes end as pedicles or podocyte end feet between which are filtration slits.

• Functional role of slits is debatable, glomerular basement membrane (GBM) is the prime filter.

Glomerular Basement Membrane

• GBM is located between podocyte & capillary endothelium.

• It is a thick fusion of the 2 basal laminae.

• It lacks a lamina fibroreticularis.

Layers of the GBM:

• Capillary endothelium.

• Inner lamina.

• Lucida.

• Thick lamina densa.

• Outer lamina.

• Lucida podocyte.

• GBM is the main filtering structure of the kidney.

• Filters out physically large molecules (physical barrier) & acts as an electrostatic barrier (allows only molecules that lack a high negative charge).

Mesangial Matrix

• Includes collagen, chondroitin sulfate proteoglycan and fibronectin.

• Supports capillary where podocytes are absent, in addition to GBM.

• GBM is always present.

• Mesangial cells are phagocytic and clears any material that occludes the GBM.

• Also degrades basal lamina material that is always being renewed.

Renal Corpuscle

Consists of Bowman's capsule plus glomerulus, therefore the following:

• Glomerulus Mesangial cells.

• Glomerular basement membrane (GBM).

• Podocytes.

• Bowman's capsule.

Filteration Action

• Kidneys produce 125 ml of filtrate/min but 124 ml is resorbed and only 1 ml is released as urine.

• Form about 1,500 ml urine/day.

• Kidneys receive 25% of the cardiac output.

• All circulating blood passes the kidneys every 4-5 minutes.

Proximal Convoluted Tubule

Proximal convoluted tubule has a convoluted and straight portion.

• Proximal convoluted tubule is longest segment of the nephron, therefore the most numerous in section.

• Acidophilic, irregular small lumen with striate border (microvilli).

• Lateral borders interdigitate so lateral margins are difficult to resolve with LM.

60-70% water and sodium is resorbed from tubule and delivered to peritubular capillary network.

Loop of Henle:

• Loop of Henle connects proximal convoluted tubule & distal convoluted tubule.

• Loop can occur at any point.

• Thick descending part.

• Thin descending & ascending parts - freely permeable to water.

• Thick ascending part - impermeable to water.

Distal Convoluted Tubule

Is an ion exchange site where acid-base balance of the blood is maintained.

When thick part of distal convoluted tubule returns to cortical labyrinth it comes into close contact to its own renal corpuscle where it rests between the afferent & efferent arterioles.

Macula Densa

• Macula densa is the distal convoluted tubule at this point.

• Macula densa monitors for sodium ions & influences juxtaglomerular apparatus cells.

• Has closely apposed nuclei.

• Macula densa monitors sodium ions in the lumen of proximal convoluted tubule& influences juxtaglomerular cells, which adjust blood pressure.

• Macula densa is sensitive to to the actions of aldosterone (a hormone produced by the zona glomerulosa of the adrenal cortex).

• Aldosterone stimulates resorption of sodium and water from distal convoluted tubule, therefore, increases blood pressure.

Juxtaglomerular Apparatus

• This is a specialized region of the afferent arteriole.

• Juxtaglomerular apparatus cells are modified smooth muscle that contain renin that is active in the angiotensin system to adjust blood pressure.

• Lacis cells are extraglomerular mesangial cells that (most likely) produce erythropoietin that stimulates formation of new RBC.

• Lacis cells produce erythropoietin and increases the number of RBC.

• Juxtaglomerular apparatus cells monitor chloride and blood pressure, secretes renin that cascades the angiotensin system (produced in the liver) to form angiotensin II, which constricts blood vessels, therefore, increasing blood pressure.

Renin Angiotensin Aldosterone System Diagram:

Renin Angiotensin Aldosterone System

Collecting ducts

• Collecting ducts pass to the renal papilla.

• Main function is to transport urine and also to serve as the final step in urine concentration.

• Vasopressin (antidiuretic hormone, ADH from pars nervosa of pituitary) acts on collecting ducts to increase permeability of collecting ducts to water.

• When ADH is not present, it results in diuresis, production of large amount of dilute, hypotonic urine.

Vasa Rects

Vasa recta form a countercurrent multiplier system that assists in maintaining the proper gradient between lumenal contents and interstitium.

Ureters & urinary bladder have dome cells (transitional).

Urinary bladder has thick membrane plaques that alternate with standard plasmalemma which forms pivot joints allowing the bladder to collapse then expand as the bladder empties and fills.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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