Kidney Structure and Function
Renal Structure
The kidneys are bean-shaped organs located in the back of the abdomen, behind the peritoneum, on each side of the spinal column. They measure about 12 x 6 x 3 cm and weigh between 130 and 150 g. The final product is urine, excreted through the urinary system.
Urine produced by the kidneys is collected in the renal pelvis, flows through the ureters into the bladder, and exits through the urethra.
The functional unit is the nephron. Each kidney contains 600,000 to >1,000,000 nephrons.
A nephron consists of:
Renal Corpuscle
- Glomerulus
- Bowman’s capsule
Renal Tubule
- Proximal
- Loop of Henle
- Distal
- Collecting duct
The Glomerulus
The glomerulus is a cluster of interconnected capillaries surrounded by a double-walled epithelial structure, the Bowman’s capsule. The space between the two walls, reflecting the glomerular filtration rate, is called Bowman’s space.
The glomerulus is responsible for the first step in urine formation, filtering to produce a solution virtually free of plasma proteins.
The filtration barrier consists of three layers:
- Capillary endothelium
- Basement membrane
- Visceral epithelium of Bowman’s capsule (podocytes)
In the glomerulus, there is a third cell type, the mesangial cells, located in the center of the capillary loops, providing support (mesangium).
Renal Tubule
- Proximal convoluted tubule
- Proximal straight tubule
- Thin descending limb
- Ascending thin limb
- Thick ascending limb
- Macula densa
- Distal convoluted tubule
- Connecting tubule
- Cortical collecting duct
- Outer medullary collecting duct
- Inner medullary collecting duct
Along the tubule, reabsorption and secretion processes occur.
Renal Circulation
The renal blood flow is about 1200 ml/min. The renal artery divides into 4 to 5 interlobar arteries, giving rise to the arcuate and interlobular (radial) arteries, which ascend to the capsule and branch, giving rise to afferent arterioles.
Afferent arterioles, glomerulus, efferent arterioles, peritubular capillaries, vasa recta, renal veins.
Juxtamedullary efferent arterioles extend deep into the medulla to form the vasa recta, surrounding the collecting ducts.
Vascular and tubular organization varies according to the nephron category:
- Superficial nephrons: Short loops
- Intermediate nephrons: Loops of variable length
- Juxtamedullary nephrons: Very long loops
Juxtaglomerular Apparatus
This area corresponds to the segment of the thick ascending limb of Henle’s loop in contact with arterioles in the hilum of its own renal corpuscle. In this segment is the macula densa.
Three cell types:
- Renin-secreting granular cells: Smooth muscle cells of arterioles.
- Extraglomerular mesangial cells: Similar to glomerular mesangial cells.
- Macula densa cells: Help control the filtration rate and renin secretion.
Abstract
The macro and microscopic structure of the kidney serves its functions.
Macroscopically, the pyramids and columns converge toward the renal pelvis where urine collects.
Each segment of the nephron plays a particular role, and these segments interact closely.
The renal corpuscle consists of the glomerulus and Bowman’s capsule.
The renal tubule consists of several segments: proximal convoluted tubule, loop of Henle (descending, thin ascending, thick ascending branches), distal convoluted tubule, and collecting duct.
The glomerular barrier is formed by the fenestrated endothelium, glomerular basement membrane, and podocytes.
Kidney Function
- Adjustment of fluid osmolality and volume
- Regulation of electrolyte balance
- Regulation of acid-base balance
- Excretion of metabolic products and foreign substances
- Production and secretion of hormones
The kidney performs three basic processes:
- Glomerular filtration
- Tubular secretion
- Tubular reabsorption
Renal clearance modifies glomerular filtration through reabsorption and secretion.
Reabsorption: Transport of molecules from the tubular lumen into the peritubular capillaries.
Secretion: Transport of molecules from the peritubular capillaries into the tubular lumen.
The combination of filtration, reabsorption, and secretion allows selective excretion of substances.
Transport Mechanisms
Diffusion: Free movement of molecules along a concentration or electrochemical gradient.
Facilitated diffusion: Molecular motion along the gradient, requiring a carrier.
Primary active transport: Movement against the gradient, requiring a transporter and energy directly from ATP.
Secondary active transport: Two molecules interact simultaneously with the same carrier, one moving along the gradient and the other against. Energy comes from the passive diffusion of the first molecule.
Cotransport and countertransport: Depending on whether molecules move in the same or opposite directions.
Endocytosis: Passage of macromolecules, requiring energy and ATP.
Solvent drag: Water passes through pores in the membrane along a concentration gradient, dragging small molecules dissolved in it.
Reabsorption
Two routes for transport from the lumen into the interstitium:
Paracellular: Molecules pass through junctions between cells by diffusion or solvent drag.
Transcellular: Molecules traverse through cells from the tubular lumen into the interstitium, crossing the luminal (apical) and basolateral (contraluminal) membranes and the cytosol.
Transport maximum (Tm): The maximum amount of material transported per unit time, limited by saturation of membrane proteins involved in active processes.
Renal plasma threshold: The plasma concentration of a substance at which it begins to appear in the urine, always lower than Tm due to nephron heterogeneity.
Luminal sodium enters the interstitium. Interstitial potassium enters the lumen.