Lecture Wednesday September 5: Histology of the urinary System

*A good video on the basic function of the kidney and nephron appropriate for this block
www.youtube.com/watch?v=cc8sUv2SuaY

A. Basic structure of the nephron.

Vascular components:
- afferent arteriole
- glomerular capillaries
- efferent arterioles
- peritubular capillaries

Tubular components:
- Bowman's capsule
- Proximal tubule (distinguished by the presence of a brush border)
- Loop of Henle
- Distal tubule
- Collecting tubule





B. Structure and Function of the Glomerulus

-Glomerular capillaries, basal lamina and Podocytes (visceral layer of Bowman's capsule)
- Bowman's capsule (parietal layer).

- A portion of the plasma containing ions, glucose, amino acids and other small molecules is filtered through the fenestrated capillary endothelial cells, basal lamina (charged filter) and Podocytes (filtration slits) into Bowman's Space.



*Be able to recognize the components of the glomerulus in an electron micrograph.


1. Urinary space
2. Glomerular Capillary
3. Podocytes
4. Mesangial cells
5. Basal lamina


C. The 3 basic functions of the nephron: Filtration, Secretion, Reabsorption
*note the directions of flow*


- Proximal convoluted tubule: reabsorption of most of the primary filtrate including glucose, amino acids and some ions

- Descending loop of Henle: reabsorption of water due to hypertonic medullary space

- Ascending loop of Henle: reabsorption of Na and Cl and establishment of a hypertonic medullary space.

- Distal convoluted tubule: further modification/reabsorption of ion concentration in the filtrate

- Collecting duct: reabsorption of water in response to ADH. Concentration of the urine.

*Secretion occurs at multiple sites along the nephron*


D. The Juxtaglomerular Apparatus



- Primary Function: Senses tonicity of tubule fluid and degree of stretch of afferent arteriole – secretes renin.

The three cellular components of the apparatus are
1. Macula Densa cells of the distal convoluted tubule (next to the afferent arteriole)
2. Juxtaglomerular cells (JG cells) specialized smooth muscle cells of the afferent arteriole.
3. Extraglomerular mesangial cells (unknown function).

JG cells sense pressure in afferent arteriole and secret renin to regulate blood pressure. (low blood pressure > renin secretion by JG cells > Angiotensin causes systemic vasoconstriction; release of ADH causes increased water reabsorption in collecting ducts which increases blood volume > increased systemic blood pressure).

Macula Densa cells sense NaCl concentration in distal tubule. If NaCl concentration is high these cells secrete a locally acting molecule to cause constriction of afferent arteriole to decrease glomerular filtration rate and decrease filtrate formation.

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