NPNS

The video introduces Non-Protein Nitrogenous (NPN) compounds, which reflect kidney function. Key learning objectives include identifying kidney parts and functions, understanding different NPNs and their clinical implications, and differentiating types of azotemia. NPNs are characterized by low molecular weight and crystalline nature, unlike high molecular weight, colloidal proteins.

The urinary system consists of kidneys, ureters, urinary bladder, and urethra. Blood flows from the heart via the aorta to the kidneys through the renal artery, where it is filtered. Filtered blood returns to the bloodstream via the renal vein, while waste exits through the ureters. Kidneys, despite their small size, receive about 25% of cardiac output, indicating extensive vascularization and efficient filtration of about 1.1 liters of blood per minute, leading to complete blood filtration every five minutes.

The renal artery branches into arterioles, leading blood to the glomerulus for filtration. Oxygenated blood enters through afferent arterioles and leaves through efferent arterioles, which eventually form the renal vein. Key structures include the renal cortex (outer part), renal medulla (middle part), and nephrons—the functional units situated between these two layers, numbering millions per kidney. The renal calyx and renal pelvis collect urine, which then moves to the ureter. The renal hilum is the exit point for vessels and ureter.

Blood filtration occurs in the glomerulus, a tuft of capillaries. The afferent arteriole leads into the glomerulus, while the efferent arteriole leads out. This intricate network of vessels eventually forms the renal vein, returning filtered blood to the body. Small molecules like sodium and glucose pass into Bowman's space, while larger proteins are typically retained. This is facilitated by fenestrated endothelial cells and a semi-permeable basement membrane. Podocytes, leg-like projections from tubular cells, help maintain the filtration barrier.

The nephron consists of the glomerulus (main filtration site), Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT), and collecting duct. The PCT reabsorbs electrolytes, glucose, and water. The loop of Henle uses a countercurrent multiplication system where the descending limb reabsorbs water and the ascending limb reabsorbs ions, creating a salty medulla. The DCT further reabsorbs ions and plays a role in blood pressure regulation via the juxtaglomerular apparatus. The collecting duct gathers waste and reabsorbs water and urea, maintaining medullary osmolarity. Processed blood returns to circulation via peritubular capillaries and the renal vein.

Kidney functions include excretion (eliminating metabolic waste through glomerular filtration, tubular reabsorption, and secretion), synthesis (producing erythropoietin, renin, and prostaglandins), and metabolism (inactivating aldosterone, activating glucagon and insulin, converting vitamin D to its active form, and forming creatine). A mnemonic 'A WET BED' summarizes kidney functions: acid-base balance, water balance, electrolyte balance, toxin removal, blood pressure control, erythropoietin production, and vitamin D metabolism.

The main NPN in plasma is urea, followed by amino acids, uric acid, creatinine, creatine, and ammonia. Urea is the major product of protein catabolism, formed in the liver from carbon dioxide and ammonia. It is readily filtered by the glomerulus, with 90% excreted and 10% reabsorbed. Urea levels are a general indicator of renal function, though not highly sensitive, as 70-80% of glomerular destruction must occur for significant increases. Urea levels are also affected by protein intake and metabolism. Serum creatinine is a more sensitive indicator and is often considered together with urea for comprehensive assessment.

Azotemia refers to elevated urea in the blood and is categorized into pre-renal, renal, and post-renal types. Pre-renal azotemia, characterized by normal kidney function, results from reduced blood flow to the kidneys (e.g., in heart failure, shock, dehydration, high protein diet, or muscle wasting). Renal azotemia involves kidney dysfunction itself (e.g., acute/chronic renal failure, glomerulonephritis). Post-renal azotemia occurs due to obstruction of urine flow from the kidneys (e.g., kidney stones, UTIs, tumors), causing urea to build up in the blood.

Creatinine is an anhydride of creatine, a waste product that is not reused in metabolism. It is synthesized in the liver from arginine, glycine, and methionine. Creatinine is filtered and secreted, but not reabsorbed, making it a reliable indicator of kidney function, less affected by diet than urea. It is commonly used to assess GFR (glomerular filtration rate). Elevated creatinine can indicate skeletal muscle necrosis or decreased GFR. The creatinine clearance test, calculated from urine and serum creatinine levels, provides a measure of kidney filtration efficiency.

Uric acid (urate) is a breakdown product of nucleic acids and purine catabolism, found in foods like liver and beans. At normal blood pH, it exists as monosodium urate. Its formation requires xanthine oxidase, an enzyme primarily found in the liver. About 90% of uric acid is reabsorbed. Clinically, hyperuricemia (increased uric acid) can lead to gout, characterized by inflammatory arthritis and deposition of monosodium urate crystals in joints and tissues, and nephrolithiasis (kidney stones).

Ammonia, derived from bacterial action in the colon and amino acid deamination, is highly toxic and normally metabolized by the liver into urea for disposal. Elevated plasma ammonia is particularly harmful to the central nervous system. Impaired ammonia metabolism is seen in severe liver diseases. In children, Reye's syndrome, an acute encephalopathy with hepatic dysfunction but without hyperbilirubinemia, is associated with high ammonia levels and linked to aspirin consumption during viral illnesses, affecting both liver and kidneys.

Amino acids are readily absorbed in renal tubules by active transport, with less than 5% excreted in urine, making them the second most abundant NPN after urea. Increased urinary excretion of amino acids can be due to overflow aminoacidurias (increased plasma concentration of amino acids from acquired, secondary, or inborn errors of metabolism) or renal aminoacidurias (diminished tubular absorption due to functional disorders of renal tubular absorptive mechanisms).