Anatomy of the Kidney and Nephron Functions

Physiology of the Urinary System

• The kidneys are vital organs for homeostasis, regulating water and electrolyte balance, excreting metabolic waste products, foreign chemicals, and regulating blood pressure
• The kidneys perform several essential functions, including regulating water and electrolyte balance, excreting metabolic waste products such as urea, creatinine, uric acid, bilirubin, and metabolites of hormones, excreting foreign chemicals like drugs, pesticides, and food additives, and regulating arterial blood pressure
• The kidneys are also involved in the regulation of erythrocyte production (EPO), Vitamin D activity (producing 1, 25-dihydroxyvitamin D3), gluconeogenesis (synthesizing glucose from amino acids) and acid-base balance.

Physiologic Anatomy of Kidneys

• General Organization: The two adult human kidneys are bean-shaped, approximately 150 grams each and located outside the peritoneal cavity on the posterior abdominal wall. They consist of an outer cortex and inner medulla. Renal pyramids in the medulla project into the renal pelvis, which divides into major and minor calyces. The walls of the calyces, pelvis, and ureters contain contractile elements that move urine towards the bladder.

• Nephron: The functional unit of the kidney is the nephron. The nephron is composed of a glomerulus (filtering component) and a tubule that extends from the glomerulus. The glomerulus is a tuft of capillaries which filters a protein-free filtrate from plasma into Bowman's capsule. The tubular portion extends into the medulla (for the loop of Henle) and then back into the cortex before draining further into collecting ducts that eventually form the ureter through the pelvis to the bladder.

• Blood Supply: The renal artery branches into interlobar, arcuate, and interlobular arteries. These eventually lead to afferent arterioles, which supply the glomerular capillaries. The glomerular capillaries drain into efferent arterioles that form the peritubular capillaries. This arrangement allows for exchange of substances and water between the tubular lumen and capillaries. The renal circulation features two capillary beds (glomerular and peritubular) arranged in series, which are crucial to the nephron's function and fluid movement between the tubules and blood circulation.

• Regional Differences (Nephron Structure): Approximately 80% of nephrons are outer cortical nephrons, with relatively short loops of Henle. The remaining 20% are juxtamedullary nephrons that have long loops of Henle extending deep into the medulla; this specialization is critical for concentrating urine.

The Juxtaglomerular Apparatus (JGA)

• Interstitial Cells: Interstitial cells located between adjacent tubules and capillaries produce and release prostaglandins in response to appropriate stimuli.

• Structure: The JGA includes the macula densa and juxtaglomerular cells. The macula densa, a short segment of the distal convoluted tubule, is adjacent to the afferent and efferent arterioles at the vascular pole of the glomerulus. Juxtaglomerular cells are in the walls of these arterioles.

• Function: Macula densa cells monitor sodium chloride concentration in the filtrate, influencing renin release by juxtaglomerular cells. This release of renin starts the renin-angiotensin-aldosterone system (RAAS), which is a crucial mechanism for regulating blood pressure.

Urine Formation

• Glomerular Filtration: The first step in urine formation, glomerular filtration, is the process of filtering plasma into Bowman's capsule. This fluid is essentially protein-free and lacks cellular elements. The filtration membrane consists of capillary endothelium, basement membrane, and epithelial cells (podocytes). The high filtration rate and selectivity of the glomerular membrane is critical in controlling which substances pass into the filtrate. The filtration process depends on size and charge of molecules.

  • Ultrafiltration: The process is called ultrafiltration because even small particulates are filtered; however, the plasma protein's size prevents their filtration.
  • Forces influencing Filtration: Forces involved in filtration include Glomerular capillary hydrostatic pressure (PG), Bowman's capsule hydrostatic pressure (PB), and Glomerular capillary colloid osmotic pressure (πG).
  • Net Filtration Pressure (NFP): The final pressure determines the amount of fluid filtration.

• Urinary Excretion Rate: The overall excretion rate is determined by glomerular filtration rate minus the rate of tubular reabsorption and plus the rate of tubular secretions.

Regulation of GFR

• Factors Regulating GFR: GFR depends on glomerular capillary filtration coefficient (Kf), Net filtration pressure (NFP), total surface area available for filtration. Factors including changes in afferent or efferent arteriolar diameter, vasoconstriction, and resistance play a significant role in regulating GFR.

  • Changes in Afferent/Efferent Arterioles: Vasodilation in the afferent arteriole increases RBF, PG, and GFR; vasoconstriction in the afferent arteriole reduces RBF, PG, and GFR. Vasodilation in the efferent arteriole decreases RBF, increases πG, and decreases GFR; vasoconstriction in the efferent arteriole increases RBF, decreases πG, and increases GFR.

• Other Controls: Kidney stones can influence GFR. Changes in plasma protein concentration also affect GFR.

Renal Handling of Substances

• Waste products (e.g., creatinine)
• Electrolytes
• Nutritional substances (e.g., glucose, amino acids)
• Organic acids/bases and foreign compounds/drugs are handled by different mechanisms in their movement through each part of the nephron and in the tubular system in the kidney.