Urinary System Overview and Anatomy

Urinary System Overview

• The urinary system's main function is excretion, regulating blood volume and pressure, solute concentration in the blood, and vitamin D synthesis.

Kidney Anatomy

• The kidneys are bean-shaped organs about the size of a clenched fist.
• A renal capsule surrounds each kidney.
• The hilum is where blood vessels and nerves enter.
• The renal sinus houses blood vessels.
• The cortex is the outer layer and the medulla is the inner layer, surrounding the renal sinus.
• Renal pyramids are located between the cortex and medulla.
• Calyces funnel the urine from the pyramids.
• The renal pelvis is a larger funnel formed by the calyces.
• Ureters connect the kidneys to the urinary bladder.
• Nephrons are the functional units of the kidney.
• The renal corpuscle is the enlarged end of the nephron.
• The proximal convoluted tubule is part of the nephron.
• The loop of Henle is part of the nephron.
• The distal convoluted tubule is part of the nephron.
• Collecting ducts carry fluid through the medulla to the papillary ducts.
• Juxtamedullary nephrons have Henle loops that extend deep into the medulla.
• Cortical nephrons have Henle loops that don't extend deeply into the medulla.
• Bowman's capsule is part of the enlarged end of the nephron.

Glomerulus and Filtration Membrane

• A glomerulus is a tuft of capillaries in the Bowman's capsule.
• Podocytes form the inner layer of Bowman's capsule.
• The filtration membrane is formed by glomerular capillaries, basement membrane, and podocytes.

Urine Formation

• Filtration is the movement of water, ions, and small molecules through the filtration membrane into Bowman's capsule to form filtrate.
• Filtration pressure forces fluid from the glomerular capillaries into Bowman's capsule.
• Glomerular capillary pressure is blood pressure in the glomerular capillaries.
• Capsular pressure is pressure of filtrate in Bowman's capsule.
• Colloid osmotic pressure pushes fluid from Bowman's capsule into the blood.
• Filtration pressure is the difference between glomerular capillary pressure, capsular pressure, and colloid osmotic pressure.
• Tubular reabsorption returns needed substances to the blood.
• Tubular secretion moves substances from the blood into the filtrate as needed.

Arteries and Veins

• Renal arteries branch off the abdominal aorta.
• Interlobar arteries are between renal pyramids.
• Arcuate arteries arch between the cortex and medulla.
• Interlobular arteries branch from arcuate arteries and extend into the cortex.
• Afferent arterioles are branches from interlobular arteries, supplying the glomerular capillaries.
• Efferent arterioles are extensions from the glomerular capillaries.
• Peritubular capillaries surround the proximal and distal convoluted tubules and loops of Henle.
• Vasa recta are specialized parts of the peritubular capillaries extending into the medulla.
• Juxtaglomerular apparatus is where the distal convoluted tubule contacts the afferent arteriole.

Hormonal Regulation of Urine

• Renin-angiotensin-aldosterone mechanism regulates blood pressure by adjusting sodium and water reabsorption.
• Renin is secreted when blood pressure decreases, leading to aldosterone secretion and increasing sodium reabsorption.
• Antidiuretic hormone (ADH) increases water permeability in the distal convoluted tubules and collecting ducts, increasing water reabsorption.
• Atrial natriuretic hormone (ANH) increases sodium and water loss when blood pressure increases.

Body Fluid Compartments

• Intracellular fluid is inside cells.
• Extracellular fluid is outside cells, includes plasma and interstitial fluid.
• Regulation of extracellular fluid composition is important for homeostasis. Thirst regulates extracellular fluid volume through water intake.

Regulation of Acid-Base Balance

• Buffers resist changes in pH (e.g., proteins, phosphate, bicarbonate).
• The respiratory system adjusts pH by regulating CO2 elimination.
• The kidneys excrete hydrogen ions (H+) to regulate pH.

Kidney Diseases and Disorders

• Glomerulonephritis is inflammation of the filtration membrane.
• Renal failure is a condition of impaired kidney function.

Urine Volume and Composition

• Table 18.2 provides approximate volumes of body fluid compartments.