Functional Anatomy of the

KidneyA.A.J. RAJARATNE

1. Regulation of Water Balance2. Regulation of Electrolyte Balance2. Excretion of Metabolic Waste Products3. Excretion of Foreign Chemicals &

Drugs4. Regulation of Arterial Blood

Pressure5. Regulation of Erythropoiesis6. Regulation of Vitamin D7. Gluconeogenesis

Functions:

Objectives

Recognize that the functional unit of the kidney is the nephron

List the two different types of nephrons and state the structural differences between them

Describe the process of urine formation

ANATOMY OF THE KIDNEY

The kidneys are paired organs lying in the retroperitoneal space.

Medial border = hilum renal pelvis and ureter.

The renal pelvis subdivides into major calyces, then minor calyces projecting to a renal pyramid.

Divided into outer cortex and inner medulla.

Structure of Kidney:

Parts of the nephron - glomerulus

The cortex is granular, due to ~ 1-1.2 million nephrons per kidney.

The glomerulus is composed of capillary loops invaginated in Bowman's capsule. Glomerulus also contains mesangial cells in a central glomerular tuft. These contain myofilaments and may contract altering GFR.

Bowman's capsule joins the proximal tubule, a single layer of epithelial cells lying on a basement membrane (BM)

Structure of Kidney - contd:

GLOMERULUS

Glomerulus - 200 m in diameter

Formed by invagination of capillary tuft into blind end of nephron - Bowman’s capsule

Afferent and efferent arterioles

Filtration barrier - two layers of cells separating blood from glomerular filtrate

The glomerular barrier:The filtration barrier of the glomerulus consists of

1. capillary endothelium,

2. basement membrane and

3. the epithelial layer of Bowmans capsule consisting of podocytes with foot processes.

Capillary endothelium

The holes or fenestrations of the endothelium have a radius of approximately 70-90 nm (covered by a thin diaphragm) and are permeable to peptides and small protein molecules.

An aggregation of three cell types,1. granular cellsDifferentiated smooth muscle cells of afferent arterioles. Secretory vesicles contain renin.2. extraglomerular mesangial cells - continuous with the mesangium.3. macula densa cellsIn terminal part of thick ascending limb.Probably control both renin secretion and GFRNB: JGA has a rich sympathetic nerve supply

Juxtaglomerular Apparatus:

1.Proximal tubule histologically 2 segments

i. pars convolutaii. pars recta

2. Loop of Henle 3 partsi. descending thin limbii. ascending thin limbiii. ascending thick limb

3. Macula densa - passing between arterioles of own glomerulus

Tubules:

4. Distal tubule - also termed the late distal tubulei. distal convoluted tubuleii. connecting segmentiii. initial collecting tubule

5. Collecting System:1. Cortical collecting tubule2. Medullary collecting tubule3. Papillary collecting ducts

These drain into a calyx of the renal pelvis, then to the ureter & bladder.

Size - 15 mm long and 55 m in diameter.

Wall - single layer of cells united by apical tight junctions.

Bases - lateral intercellular spaces

Luminal edges - have brush border with innumerable microvilli

Proximal convoluted tubule (pars convoluta):

PCT drains into straight portion (pars recta) - forms first part of loop of Henle.

Loop of Henle - epithelium of flat attenuated cells

Total length of thin segment varies from 2 - 14 mm.

Ends in thick segment 12 mm long. Cells are cuboidal , numerous mitochondria. Modified to form macula densa, which, together with its own afferent and efferent arterioles - forms Juxtaglomerular apparatus.

Regional differences are present.

Loop of Henle:

Nephrons are not homogeneous and vary according to the location of the glomerulus in the cortex.

a. Superficial cortical nephrons - all short looped

b. Midcortical nephrons - mixture

c. Juxtamedullary nephrons (15% in man) - all long looped.

Regional Differences:

Regional Differences:

Regional Differences:

The efferent arterioles of the juxtamedullary glomeruli are long and form vascular bundles (vasa recta) extending into the medulla.

The descending vasa recta extend to the inner medulla, networking and forming a countercurrent exchanger with the ascending vasa recta.

Regional Differences:

5 mm long.

Epithelium - flatter than PCT, few microvilli, no brush border.

Join together to form collecting ducts - 20 mm long, pass through cortex and medulla to empty into renal pyramids at medullary pyramids.

Distal convoluted tubule:

Epithelium made up of

a. Principal cells (P cells)

- Na+ reabsorption

- ADH induced H2O reabsorption

b. Intercalated cells (I cells)- more microvilli, vesicles,

mitochondria - Acid secretion, HCO3 transport

Collecting ducts:

Blood Supply:

1. Renal artery2. Interlobar arteries3. Arcuate arteries4. Interlobular arteries5. Afferent arterioles - parallel series from each interlobular artery6. Glomerular capillaries7. Efferent arterioles8. Peritubular capillaries - nonspecificNB: Interlobular, arcuate, interlobar and renal veins parallel arteries.

Blood Supply:

Renal nerves -

a. PG sympathetic to afferent and efferent arterioles, PCT and DCT, juxtaglomerular cells

b. Sensory nociceptive afferents

Innervation: