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Urinary System Overview

• The urinary system's function is to filter waste from blood and maintain body fluid balance.
• Kidneys are the primary organs of the urinary system.
• Kidney anatomy includes a location retroperitoneal (behind the peritoneum) near the spine.
• The kidney capsule is easily separated (in most animals), with additional connective tissue.
• In cats, only connective tissue is present; in other animals, smooth muscle is also found.

Kidney Types

• Unilobar kidneys are found in carnivores and some small ruminants.
• Multilobar kidneys are found in cattle, humans, and pigs. Cattle's lobes are externally distinct.

Kidney Functions

• Filter waste products from blood.
• Reabsorb water, electrolytes, and glucose.
• Secrete drugs and toxins.
• Regulate acid-base balance and electrolytes.
• Produce erythropoietin and renin.

Kidney Structures

• Renal cortex: The outer layer containing glomeruli and tubules. The cortex-to-medulla ratio is typically 1:2 or 1:3 (1:5 in desert camels).
• Renal medulla: Contains renal pyramids. Renal pyramids have renal papillae (small structures resembling tiny projections). Papillae carry urine to the minor and major calyces, and eventually to the renal pelvis.
• Papillae: Tip of the renal pyramid; drain urine into the calyces. The area at the tip of the pyramid is called the area cribrosa.
• Calyces: Cup-like structures that collect urine from papillae
• Renal pelvis: Collects urine from the calyces, which leads to the ureter.
• Renal columns: Extensions of the cortex that separate the renal pyramids. The medulla contains renal pyramids, composed of renal columns and renal pyramids.
• Renal lobules: These are functional units within the kidney
• Renal artery & vein: Bring blood to and from the kidneys, delivering and removing materials related to filtration and regulation

Renal Corpuscle and Tubules

• Glomerulus: A network of capillaries within Bowman's capsule.
• Bowman's capsule: Surrounds the glomerulus; forms the urinary space.
• Tubules (Proximal, Distal, and Loop of Henle): Specialized tubules that further process urine.

Glomerular Filtration Barrier

• Endothelium: Allows passage of small molecules, water, sodium, glucose, and urea.
• Basement membrane: Contains type IV collagen, acting as a major filtration barrier.
• Podocytes: Epithelial cells with extensions forming filtration slits.
• Proteinuria: Presence of protein in urine.
• Edema: Reduction in plasma osmotic pressure; a symptom.

Renal Blood Circulation

• Renal arteries: Branch into smaller vessels that eventually feed into the capillaries of glomeruli.
• Afferent arterioles: Feed into the glomerular capillaries.
• Efferent arterioles: Exit the glomerular capillaries.
• Peritubular capillaries: Surround the tubules for reabsorption.
• Vasa recta: Specialized capillaries in the medulla that support the kidney's fluid balance system

Juxtaglomerular Complex

• Function: Regulates blood pressure via the renin-angiotensin-aldosterone system.
• Components: Special cells called juxtaglomerular cells, macula densa, and extraglomerular mesangial cells.

Renin-Angiotensin-Aldosterone System

• Regulates blood pressure and fluid balance.

Kidney Diseases

• Acute Renal Failure (ARF): Classified as pre-renal, renal, or post-renal based on the source of the issue, pre-renal meaning problems before the kidney, renal the kidney itself is affected, and finally, post-renal meaning there are issues after the kidney.

• Chronic Renal Failure (CRF): Characterized by various symptoms and complications.

• Acute Tubular Necrosis (ATN): Damage to the renal tubules, caused by toxins or ischemia (lack of blood flow).

• Scarring / Fibrosis: Persistent damage leading to irreversible tissue replacement by fibrous and connective tissue.

• Infarction: Death of tissue due to lack of blood supply.

• Congenital Anomalies: Developmental disorders affecting kidney structure and function.

• Glomerulonephritis: Inflammation of the glomeruli, often immune-mediated.

• Amyloidosis: Accumulation of abnormal protein in the kidneys.

• Pyelonephritis: Bacterial infection of the renal pelvis and kidney parenchyma.

• Viral Glomerulopathy/ Interstitial Nephritis/ Vesicoureteral Valve dysfunction: Infectious illnesses impacting the kidney through different mechanisms.

• Hydronephrosis: Swelling of the renal pelvis due to obstruction.

Kidney Tissue Examination

• Macroscopic: Visual examination of the kidney's appearance.
• Microscopic: Examination of kidney tissue at the cellular level using a microscope.
• Dye staining: Differentiation of renal structures. Some examples include Congo red, H&E, and others depending on the disease being examined.