Urinary System Functions

• The urinary system maintains salt, water, and pH homeostasis in the blood
• It carries out excretion, removing metabolic wastes from the body

Function of the Urinary System

• Primary function involves excretion of metabolic wastes
• Wastes are mostly nitrogenous, including urea, creatinine, ammonium, and uric acid
• Urea is a waste product of amino acid metabolism
• Uremia occurs when there are elevated levels of urea in the blood
• Creatinine results from the breakdown of creatine phosphate in muscles
• Uric acid comes from the metabolic processing of nucleotides
• The kidneys maintain water-salt balance
• Salts, like NaCl, influence osmosis rate and direction
• Blood pressure is regulated by controlling Na+ & K+ concentrations in the blood
• The kidneys maintain levels of ions like bicarbonate (HCO3-) and calcium
• The kidneys maintain acid-base balance
• Healthy blood pH is about 7.4
• The kidneys monitor and control pH by excreting H+ and reabsorbing HCO3- as needed
• Urine pH is usually 6 or lower due to acidic foods in the diet
• Hormones secreted include renin, aldosterone, and erythropoietin (EPO)
• Kidneys reabsorb filtered nutrients and synthesize vitamin D

Organs of the Urinary System

• The urinary system consists of the kidneys, ureters, urinary bladder, and urethra

Kidneys

• Kidneys are on each side of the vertebral column, partially protected by the rib cage
• The right kidney is slightly lower than the left due to the liver
• They are bean-shaped and reddish-brown
• Kidneys are covered by the renal capsule
• The concave side of the kidney is where a renal artery enters, and the renal vein and ureter exit

Ureters

• Ureters conduct urine from the kidneys to the bladder
• Ureter walls have three layers: inner mucosa, smooth muscle, and outer fibrous coat
• Peristaltic contractions push urine into the bladder, even when lying down

Urinary Bladder

• The urinary bladder stores urine
• It has three openings: two for ureters and one for the urethra
• Rugae folds in the mucosa disappear as the bladder expands
• Transitional epithelium allows the bladder to stretch
• Mucosa folds act as valves to prevent urine backflow into the ureter
• The urethra exits the bladder through two sphincters, internal and external

Urination (Micturition)

• Stretch receptors activate when the urinary bladder fills with urine
• Receptors send sensory nerve signals to the spinal cord
• Motor nerve impulses cause the bladder to contract and sphincters to relax, enabling urination
• The brain controls micturition via the external urethral sphincter

Urethra

• Urethra extends from the urinary bladder to an external opening
• The short female urethra makes women more susceptible to infections
• In males, the urethra is encircled by the prostate gland as it leaves the bladder

Kidney Structure

• The three regions of a kidney are the renal cortex, renal medulla, and renal pelvis
• The renal cortex is the outer layer that dips between the inner layer (renal medulla)
• The renal medulla contains cone-shaped tissue masses called renal pyramids
• The renal pelvis is a central space continuous with the ureter
• Kidneys are composed of over 1 million nephrons
• Nephrons filter the blood and produce urine
• Several nephrons empty into one collecting duct
• Collecting ducts empty into the renal pelvis

The Nephron

• Blood supply goes from the renal artery to the afferent arteriole, which transports blood to the glomerulus inside the glomerular capsule
• The efferent arteriole carries blood away from the glomerulus
• The efferent arteriole empties into the peritubular capillary network surrounding the nephron
• Blood then enters a venule, carrying it to the renal vein

Anatomy of a Nephron

• The Glomerular capsule consists of an outer layer of squamous epithelial cells
• An inner layer of podocytes cover the glomerulus, leaving pores for molecules to exit
• Cuboidal epithelial cells with microvilli form a brush border, increasing surface area in the proximal convoluted tubule
• The Loop of Henle has a descending and ascending limb with different permeabilities to water and solutes
• The Distal convoluted tubule's primary function is ion exchange between blood and the renal tubule
• Distal convoluted tubules of nephrons enter a collecting duct, carrying urine to the renal pelvis
• The Glomerular capsule and convoluted tubules are in the renal cortex
• Loops of Henle dip down into the renal medulla
• Collecting ducts are located in the renal medulla and give the renal pyramids a striped look

Urine Formation

• Three stages of urine formation: glomerular filtration, tubular reabsorption, and tubular secretion

Glomerular Filtration

• Occurs as blood enters the glomerulus via the afferent arteriole
• The Afferent arteriole has a larger diameter than the efferent arteriole, increasing glomerular blood pressure
• Increased blood pressure forces more fluid and molecules out of the glomerulus, entering the glomerular capsule as glomerular filtrate
• Filtration occurs because large molecules and elements cannot pass through the capillary wall
• Blood components in the glomerulus are filterable and nonfilterable

Filterable Blood Components

• Water, nitrogenous waste, nutrients, and salts (ions)

Nonfilterable Blood Components

• Formed elements (blood cells and platelets) and plasma proteins
• Nonfilterable components exit through the efferent arteriole
• Filtrate has the same concentration of filterable components as plasma

Tubular Reabsorption

• Molecules are passively and actively reabsorbed from the nephron into the blood of the peritubular capillary network
• Two ways Na+ is actively transported are symport and antiport
• Symport involves coupling with larger solutes like amino acids or glucose
• Antiport involves moving into the cell while transporting H+ out

Diabetes Mellitus

• Blood glucose rises above normal, resulting in glucose appearing in the urine
• Glucose transporters are overwhelmed and can't reabsorb all glucose in the filtrate
• Excess glucose in the filtrate raises its osmolarity

Tubular Secretion

• Substances move from blood to tubular fluid in a second way after filtration
• H+, creatinine, and drugs like penicillin are secreted
• Secretion happens along the kidney tubules
• Urine contains filtered but unreabsorbed substances, plus secreted substances

Kidneys Excrete Waste Molecules

• The liver produces urea, muscles make creatinine, and all body cells make uric acid
• The cardiovascular system carries these wastes to the kidneys
• Sweat glands secrete water, salt, and some urea
• In kidney failure, urea is excreted by sweat glands, forming uremic frost on the skin

Water-Salt Balance

• Most water in the filtrate is reabsorbed into the blood to concentrate urine
• All nephron parts and the collecting duct participate in water reabsorption
• Salt reabsorption precedes water reabsorption, which occurs by osmosis
• Water passes through aquaporins, which are water channels in the plasma membrane during reabsorption
• More than 99% of Na+ filtered at the glomerulus is returned to the blood
• The kidneys excrete or reabsorb potassium (K+), bicarbonate (HCO3-), and magnesium (Mg2+) ions as needed

Reabsorption of Salt and Water from Cortical Portions of the Nephron

• 65% of water filtered into the glomerular capsule is reabsorbed into the blood at the proximal convoluted tubule

• First, Na+ is actively reabsorbed, then Cl- follows passively

• Water follows these ions because aquaporins are always open

• Hormones regulate sodium and water reabsorption in the distal convoluted tubule

• Aldosterone, secreted by the adrenal glands, promotes ion exchange

• Potassium ions are excreted, and sodium ions are reabsorbed into the blood by aldosterone

• The kidneys start the release of aldosterone.

• The juxtaglomerular apparatus is the contact region between the afferent arteriole and the distal convoluted tubule

• The juxtaglomerular apparatus secretes renin when blood volume (and pressure) are too low

• Renin leads to aldosterone secretion by the adrenal glands

• Without ADH, water cannot be reabsorbed in the distal convoluted tubule

• A hormone secreted by the heart atria due to increased blood volume is Atrial natriuretic hormone (ANH)

Reabsorption of Salt and Water from Medullary Portions of the Nephron

• The Loop of Henle comprises a descending and ascending limb
• NaCl passively diffuses out of the ascending limb's lower portion
• Salt concentration increases toward the inner medulla
• Water leaves the descending limb along its entire length due to an osmotic gradient
• The fluid encounters an increasing osmotic solute concentration in the descending limb
• The ascending limb doesn't reabsorb water (lacks aquaporins)
• The collecting duct's job is to maintain the solute concentration gradient
• Water is returned to the cardiovascular system when reabsorbed

The Collecting Duct

• Fluid encounters the same osmotic gradient established by the ascending limb in the collecting duct
• Water diffuses through the collecting duct into the blood if ADH is present
• Diuresis refers to “increased urine,” while antidiuresis refers to “decreased urine.”

Diuretics

• Alcohol causes diuresis by inhibiting ADH secretion
• Caffeine increases glomerular filtration rate and decreases Na+ reabsorption
• Diuretic drugs for high blood pressure also decrease Na+ reabsorption
• A decrease in water reabsorption causes a decrease in blood volume and pressure
• Normal body fluid pH is 7.35 to 7.45, which is needed for proper protein function

Acid-Base Balance of Body Fluids

• Acidosis is a blood pH below 7.35
• Alkalosis is a blood pH above 7.45
• Ingested food either adds basic or acidic substances to the blood
• Body fluid pH is maintained by acid-base buffer systems, the respiratory center, and the kidneys

Acid-Base Buffer Systems

• Buffers can absorb excess H+ or hydroxide ions
• Carbonic acid (H2CO3) and bicarbonate ions is one the key buffers in the blood
• H+ reacts with HCO3-, resulting in H2CO3 when in the blood
• OH- and H2CO3 result in HCO3- and H2O when OH- enters the blood

Respiratory Center

• The respiratory center is a blood buffer
• The next adjustment to keep blood pH constant happens in the lungs
• Rising breathing rate rids the body of H because the following reaction takes place in pulmonary capillaries
• The kidneys are slower but more powerful than the other two mechanisms.
• The kidneys reabsorb HCO3- and excrete H+ as needed
• The kidneys excrete H+ and reabsorb bicarbonate ions if the blood is acidic
• Ammonia (NH3) provides additional buffering and removes the hydrogen ions in urine

The Kidneys Assist Other Systems

• They secrete erythropoietin (EPO), stimulating RBC production in the red bone marrow
• They regulate blood Ca2+ by converting vitamin D to its active form
• Active vitamin D is required for Ca2+ absorption in the digestive tract
• They also regulate electrolyte excretion, including Ca2+
• The kidneys regulate Na+ and K+ content in the blood
• These ions are needed for nerve conduction and heart/muscle contraction

Urinary System Disorders

• Many illnesses, especially diabetes and hypertension, can cause progressive renal disease and renal failure
• Urethritis is an infection of the urethra
• Cystitis is a urinary bladder infection
• Pyelonephritis is an infection of the kidney
• Kidney stones may form with urinary tract infections, an enlarged prostate gland, pH imbalances, or too much calcium intake
• Kidney stones are hard granules containing calcium, phosphate, uric acid, and protein
• They are formed in the renal pelvis and usually pass unnoticed in the urine flow; however, they can block the renal pelvis or ureter if too large, and a reverse pressure builds up and destroys nephrons
• Albumin, white blood cells, or red blood cells in the urine is one of the first signs of nephron damage.
• A urinalysis can detect abnormalities in urine
• Uremia means an accumulation of urea and other wastes in the blood.
• Uremia can occur when more than two-thirds of nephrons are inoperative because of kidney damage
• Nephron damage to nitrogenous waste can cause serious damage, the retention of water and salt is of even greater concern
• The latter causes edema—fluid accumulation in the body tissues
• Abnormal ion balance can lead to loss of consciousness and heart failure.

Hemodialysis

• Hemodialysis can be carried out via either an artificial kidney machine or continuous ambulatory peritoneal dialysis (CAPD)
• Dialysis diffuses dissolved molecules through a membrane that allows only small molecules to pass through
• In an artificial kidney machine, blood is passed through a membranous tube in contact with a dialysis solution, or dialysate
• Substances more concentrated in the blood diffuse into the dialysate, and substances more concentrated in the dialysate diffuse into the blood
• The artificial kidney removes waste products, toxic chemicals, and drugs, adding substances to the blood
• Hemodialysis usually takes 3 to 6 hours, twice a week
• CAPD means the peritoneum is the dialysis membrane
• Dialysate is introduced directly into the abdominal cavity
• Waste and salt molecules pass from blood vessels in the abdominal wall into the dialysate before the fluid is collected 4 to 8 hours later.
• The individual can go about normal activities during CAPD
• Patients with renal failure sometimes undergo a kidney transplant
• There is the possibility of organ rejection
• The 1-year survival rate is 97% for a relative, and 90% if received from a nonrelative when one receives a kidney