renal anatomy

• The renal system consists of the kidneys, ureters, and urinary bladder.
• General function involves filtration, reabsorption, and secretion in the kidneys.
• Transports urine through the ureter and then stores and releases it via the urinary bladder through micturition.

Kidney: Location and Structure

• Kidneys are retroperitoneal organs.
• Kidneys span vertebral levels T12 to L3.
• The right kidney is located lower than the left kidney.
• Both kidneys are anterior to the diaphragm, psoas major, quadratus lumborum, and the 12th rib.
• The left kidney is anterior to the 11th rib.
• The kidneys contain a renal capsule, cortex, medulla, and renal pelvis.
• At the hilum of the medial border, urine flows from the renal pelvis into the ureter.
• The ureter and renal pelvis are located behind the renal artery, which is posterior to the renal vein, at the hilum.
• Renal corpuscles (glomeruli) and convoluted tubules are primarily in the renal cortex.
• Loops of Henle, collecting ducts, and vasa recta are located in the renal medulla.
• The right renal vein is longer than the left and crosses anterior to the abdominal aorta.
• The left kidney is positioned higher than the right because of the larger size of the right lobe of the liver.

Nephron

• It represents the anatomical and functional unit of the kidney.
• Each nephron consists of a renal corpuscle and a renal tubule.
• The renal corpuscle includes the glomerulus surrounded by Bowman's capsule.
• Consists of a proximal convoluted tubule, loop of Henle, and distal convoluted tubule.

Renal Corpuscle

• It has an outer (parietal) and an inner (visceral) layer.
• Podocytes with filtration slits are found in the parietal layer.
• The visceral layer consists of simple squamous epithelium.
• Glomeruli are capillaries of afferent arterioles, forming a small cluster (tuft) within the visceral layer of Bowman's capsule.
• Capillaries have gaps in their endothelium to facilitate filtration.

Renal Tubule

• The proximal convoluted tubules, distal convoluted tubules, and collecting ducts are lined with simple cuboidal epithelium.
• Thin segments of the loops of Henle are lined with simple squamous epithelium.
• The simple cuboidal epithelium in the proximal tubule contains brush borders for reabsorption.
• The distal tubule is involved in ion exchange and its simple cuboidal epithelium lacks brush borders.

Histology

• Juxtaglomerular cells are modified smooth muscle cells in the afferent arteriole and secrete renin.
• Macula densa cells detect sodium concentration in tubular fluid.
• The juxtaglomerular apparatus includes the macula densa, juxtaglomerular cells, and extraglomerular mesangial cells (mesangium).
• The macula densa, a part of the distal convoluted tubule, senses the sodium concentration in the filtrate.

Nephron Types

• There are two main types of nephrons: cortical and juxtamedullary.
• Cortical nephrons make up 85% of nephrons.
• Juxtamedullary nephrons make up 15% of nephrons.
• Juxtamedullary nephrons should not be confused with the juxtaglomerular complex (apparatus) as they are different.

Ureter

• The collecting system includes the minor calyces, major calyces, and renal pelvis.
• Urine from the renal papillae drains into the minor calyces, which then join to form the major calyces.
• The ureter is a muscular passage conducting urine from the kidney to the urinary bladder.
• The ureter crosses the pelvic brim in front of the sacroiliac joint, where the common iliac artery divides into the external and internal iliac arteries.
• The ureter walls contain smooth muscle.
• Transitional epithelium (urothelium) lines the ureter.
• Renal calculi (stones) passing along the ureter can cause pain carried by sensory nerves to T10-L2 spinal cord segments.
• Renal pain is referred to the loin and groin, corresponding to dermatomes T10-L2.

Urinary Bladder

• It is lined by transitional epithelium that stretches as it fills.
• Detrusor muscle is the smooth muscle of the urinary bladder.

Nerves

• Pelvic splanchnic nerves carry parasympathetic nerve fibers originating from the sacral segments of the spinal cord (S2-S4).
• Pelvic splanchnic nerves stimulate the detrusor muscle to contract and the internal urethral sphincter to relax.
• The sacral plexus gives off the pudendal nerve, a mixed spinal nerve with sensory and motor fibers.
• The pudendal nerve stimulates the skeletal muscles in contracting the external urethral sphincter.

Renin-Angiotensin-Aldosterone System

• The kidney secretes renin.
• The liver produces angiotensinogen.
• Angiotensin I converts to angiotensin II in the lungs.
• The zona glomerulosa of the suprarenal gland produces aldosterone.

Suprarenal Gland

• The cortex of the suprarenal gland has three layers.
• Mineralocorticoids like aldosterone are secreted by the zona glomerulosa.
• Corticosteroids such as cortisol are secreted by the zona fasciculata.
• Androgens are secreted by the zona reticularis.
• The medulla of the suprarenal gland secretes catecholamines (like adrenaline) in response to stress, producing a 'flight-or-fight' response.

Imaging the Renal System

• IV Pyelography includes plain abdominal X-ray, injecting IV contrast medium and X-ray imaging after 5 minutes.
• Other scans include Intravenous Urogram-3D CT Reconstruction, and Ultrasound Real-time Imaging of Kidney.