Urine Formation

• The urinary system, specifically urine formation, involves several key processes.

Overview of Urine Formation

• The kidneys convert blood plasma to urine through glomerular filtration, tubular reabsorption and secretion, and water conservation
• Glomerular filtration creates a plasmalike filtrate of the blood.
• Tubular reabsorption removes useful solutes from the filtrate and returns them to the blood.
• Tubular secretion removes additional wastes from the blood and adds them to the filtrate.
• Water conservation removes water from the urine and returns it to the blood, thus concentrating wastes.
• Glomerular filtrate is similar to blood plasma but lacks protein.
• Tubular fluid is the fluid in the renal tubule after tubular cells have altered it through removal and addition of substances.
• Urine is the fluid that enters the collecting duct, with water content being the primary remaining change.

Glomerular Filtration

• Glomerular filtration is a special case of capillary fluid exchange where water and some solutes from blood plasma pass from the glomerulus capillaries into the nephron's capsular space.
• Almost any molecule smaller than 3 nm can pass freely through the filtration membrane.
• The filtration membrane includes the fenestrated endothelium of glomerular capillaries.
• The fenestrated endothelium excludes blood cells but are highly permeable.
• The filtration membrane includes the basement membrane, which consists of a proteoglycan gel with negative charge, and excludes molecules greater than 8nm.
• Albumin is repelled by the negative charge of the basement membrane.
• The filtration membrane includes the filtration slits of podocytes.
• Podocytes have pedicels that wrap around the capillaries forming a barrier layer with 30 nm filtration slits, and negative charge acts as an additional obstacle for large anions.
• Water, electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes, and vitamins can pass the filtration membrane.
• Kidney infections and trauma can damage the filtration membrane and allow albumin or blood cells to filter.
• Proteinuria (albuminuria) is the presence of protein in the urine.
• Hematuria is the presence of blood in the urine.

Filtration Pressure

• Three forces determine the net filtration pressure: blood hydrostatic pressure (BHP), hydrostatic pressure in capsular space (CP), and colloid osmotic pressure (COP) of blood.
• Blood hydrostatic pressure (BHP) is 60 mm Hg, considered higher than in other capillaries because the afferent arteriole is larger than the efferent arteriole.
• Hydrostatic pressure in capsular space (CP) is 18 mm Hg, caused by high filtration rate and continual accumulation of fluid in the capsule.
• Colloid osmotic pressure (COP) of blood is 32 mm Hg.
• Net filtration pressure is 10 mm Hg.
• High blood pressure in the glomerulus can make kidneys vulnerable to hypertension and renal failure.

Tubular Reabsorption and Secretion

• Conversion of glomerular filtrate to tubular fluid involves the removal and addition of chemicals.
• Tubular reabsorption and secretion occur through the proximal convoluted tubule to the distal convoluted tubule.
• The PCT reabsorbs about 65% of glomerular filtrate, removes some substances from the blood, and secretes them into the tubular fluid for disposal in urine.
• There are two routes of reabsorption, transcellular and paracellular.
• The transcellular route involves substances passing through the cytoplasm of the PCT epithelial cells.
• The paracellular route involves substances passing between PCT cells, where junctions are leaky and allow significant water passage.
• Secretion allows the renal tubule to extract chemicals from the capillary blood and secrete them into tubular fluid.
• Secretion serves two purposes in the proximal convoluted tubule and nephron loop.
• Secretion is responsible for waste removal.
• Secretion is responsible for acid-base balance.
• Waste removal includes excreting urea, uric acid, bile acids, ammonia, catecholamines, prostaglandins, creatinine, blood pollutants, morphine, penicillin, aspirin, and other drugs.
• Acid-base balance includes secretion of hydrogen or bicarbonate ions.

Nephron Loop

• The primary function of the nephron loop is to generate a salinity gradient that enables the collecting duct to concentrate the urine and conserve water.
• The thick segment reabsorbs 25% of Na+, K+, and Cl-.
• Water can not follow since the thick segment is impermeable.
• The tubular fluid is very dilute as it enters the distal convoluted tubule.

Water Conservation

• The kidney eliminates metabolic wastes and prevents excessive water loss.
• As the kidney returns water to the tissue fluid and bloodstream, the fluid remaining in the renal tubules passes as urine and becomes more concentrated.

Distal Convoluted Tubule and Collecting Duct

• The fluid arriving in the DCT contains about 20% of the water and 7% of the salts from glomerular filtrate.
• The DCT and collecting duct reabsorb variable amounts of water and salt, regulated by aldosterone, atrial natriuretic peptide, antidiuretic hormone, and parathyroid hormone.

Antidiuretic Hormone

• Antidiuretic hormone (ADH) is secreted by the posterior lobe of the pituitary gland in response to dehydration and rising blood osmolarity.
• ADH makes the collecting duct more permeable to water.
• ADH causes the appearance of special water channels (aquaporins) in the membranes of the DCT and collecting ducts aiding water reabsorption.

Hormones and Their Effects

• Aldosterone is secreted by the adrenal cortex and stimulates the reabsorption of more Na+, which retains NaCl and water and raises blood pressure.
• Atrial natriuretic peptide (ANP) is secreted by atrial myocardium in response to high blood pressure, resulting in excretion of more salt and water in the urine.
• Parathyroid hormone (PTH) is secreted from parathyroid glands in response to calcium deficiency and increases phosphate excretion in the PCT and increases calcium reabsorption in the DCT.

Summary of Urine Formation

• Glomerular Filtration produces glomerular filtrate and is regulated by renal autoregulation, sympathetic control, and hormonal control.
• Tubular Reabsorption and Secretion produces tubular fluid.
• The proximal convoluted tubule reabsorbs about 65% of glomerular filtrate and secretes substances into the tubular fluid for disposal in urine.
• The nephron loop generates a salinity gradient that enables the collecting duct to concentrate the urine and conserve water.
• The distal convoluted tubule duct reabsorbs variable amounts of water and salt, and are regulated by several hormones.
• Water Conservation produces urine, as the collecting duct passes through the medulla, reabsorbing water and concentrates urine up to four times, regulated by antidiuretic hormone (ADH).