Urinary System and Nephrons

• The urinary system includes the kidneys, ureters, urinary bladder, and urethra.
• Kidneys account for 0.4% of the body weight, but receive 21% of the blood.

Kidney Functions

• Kidneys filter the blood and excrete metabolic wastes.
• They also excrete toxins, drugs, hormones, salts, and hydrogen ions.
• Kidneys regulate blood pressure and volume through water output.
• They regulate blood osmolarity, electrolyte balance, and acid-base balance of body fluids.

Nitrogenous Wastes

• A metabolic waste product is a waste substance produced by the body.
• Nitrogenous wastes are particularly toxic.
• About 50% of nitrogenous waste comes from urea (proteins).
• Uric acid (from nucleic acids) and creatinine (from creatine phosphate) are also nitrogenous wastes.

The Nephron

• The nephron is the functional unit of filtration.
• Each kidney contains roughly 1.2 million nephrons.
• Nephrons consist of a renal corpuscle and a long, coiled renal tubule.

Renal Corpuscle

• The renal corpuscle is composed of the glomerulus and Bowman's capsule, encircling the glomerulus.

Renal Tubule

• The renal tubule's order includes the proximal convoluted tubule, the Loop of Henle (with descending and ascending limbs), and the distal convoluted tubule.
• The distal convoluted tubule drains into a collecting duct.
• The collecting duct receives input from multiple nephrons, and drains into the space that leads to the ureter.

Renal Circulation

• Blood flows through the afferent arteriole and then to the glomerulus.
• Blood exits via the efferent arteriole, which leads to the peritubular capillaries.

Formation of Urine

• Urine formation from blood plasma involves glomerular filtration, tubular reabsorption, tubular secretion, and water conservation.
• Fluid in the capsular space is called glomerular filtrate, lacking protein, and is similar to blood plasma.
• Fluid in the rest of the tubule is tubular fluid.
• The fluid from the collecting duct onward can be called urine.

Glomerular Filtration

• Water and materials like electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes, and vitamins pass from the glomerulus capillaries into the nephron's capsular space.
• Filtration is driven by high pressures in the glomerular capillaries due to the afferent arteriole being larger than the efferent arteriole.
• High pressures make the kidneys vulnerable to hypertension.
• The glomerular filtration rate (GFR) is the amount of filtrate formed per minute by both kidneys.
• GFR in young adult males is about 125 ml/min, while in young adult females, it is about 105 ml/min.
• Around 99% of the filtrate is reabsorbed.
• GFR regulation comes from regulating glomerular blood pressure.

Tubular Reabsorption

• Tubular reabsorption reclaims water and other filtrates from the tubular fluid, returning them to the blood.
• About 99% of filtered materials are reabsorbed.
• The proximal convoluted tubule reabsorbs about 65% of glomerular filtrate.
• Further reabsorption, especially of water, sodium, potassium, and chloride, happens in the loop of Henle, distal convoluted tubule, and collecting duct.

Tubular Secretion

• The renal tubule extracts materials from the peritubular capillaries and secretes them into the tubular fluid.
• Nitrogenous wastes, drugs, and other contaminants are moved during tubular secretion.

Water Conservation

• The collecting duct's main role is to complete water reabsorption.
• Urine can become more concentrated by a factor of four along its length in the collecting duct.
• Water moves out as urine moves through the collecting duct because the medullary portion of the collecting duct is more permeable to water than to salts.
• The osmolarity of the extracellular fluid is four times higher in the lower medulla than in the cortex.
• ADH increases water reabsorption in the collecting duct.

The Ureters

• The ureters are tubes that carry urine from the kidneys to the bladder.
• Peristaltic waves are used to push the urine toward the bladder.

Urinary Bladder

• The urinary bladder stores urine until it is periodically released in larger volumes.

The Urethra

• The urethra carries urine from the bladder out of the body.
• All individuals have an external urethral sphincter, which consists of skeletal muscle and is under voluntary control.

Urination

• During urination, stretch receptors in the bladder send signals to the spinal cord, then send signals to cause contraction of the smooth muscle in the bladder.
• The external urethral sphincter remains contracted if it is not a good time for urination.
• Urine will be retained until that sphincter is relaxed.