Urinary System: Organs and Functions

• The urinary system regulates blood pressure and produces red blood cells and glucose.
• Kidneys, ureters, urinary bladder and urethra are the main organs of the urinary system

Organs and functions

• Kidneys regulate homeostasis, blood pressure via renin, and red blood cell production. They also activate vitamin D and excrete wastes.
• Ureters transport urine from kidneys to the bladder.
• The urinary bladder stores and expels urine via the urethra.
• The urethra discharges urine from the body.

Kidneys

• Kidneys are located retroperitoneally in the superior lumbar region, against the dorsal body wall.
• Kidneys are positioned at the level of T12 to L3, with the right kidney slightly lower.
• Ureters, renal blood vessels, and nerves connect to the kidneys at the renal hilum.
• Adrenal glands sit atop each kidney.
• The kidneys in adults are about 12 cm long, 6 cm wide and 3 cm thick.

Coverings of the Kidneys

• Fibrous capsule: a transparent connective tissue that protects and maintains the kidney's shape
• Perianal fat capsule: a fatty tissue that protects the kidney and holds it in place
• Renal fascia: a connective tissue that anchors the kidney to surrounding structures and the abdominal wall

Kidney Regions

• Renal cortex: the outer region.
• Renal medulla: located inside the cortex, contains triangular renal/medullary pyramids, which have a striped appearance
• Renal columns: portions of the renal cortex located between renal pyramids
• Renal pelvis: the inner collecting tube, which contains calyces to funnel urine

Blood flow in the kidneys

• The flow: Aorta, Renal artery, Segmental artery, Interlobar artery, Arcuate artery, Cortical radiate artery, Afferent arteriole, Glomerulus (capillaries), Efferent arteriole, Peritubular capillaries, Cortical radiate vein, Arcuate vein, Interlobar vein, Renal vein, Interior vena cava.

Nephrons

• Nephrons are the structural and functional units of the kidneys responsible for forming urine.

Main Structures of the Nephrons

• Renal corpuscle; filters blood plasma
• glomerulus: a capillary network
• glomerular capsule: surrounds the glomerulus and consists of podocytes
• Renal tubule: a 3 cm long tube where filtered fluid passes before collecting ducts
• proximal convoluted tubule (PCT)

Urine Formation

• Glomerular filtration: water and solutes filter from the glomerulus into the renal tubule
• The resulting fluid is called filtrate
• Tubular reabsorption: reabsorption of water, glucose and amino acids from the filtrate in the PCT
• Nitrogenous wastes and excess water are not reabsorbed
• Tubular secretion: reverse of tubular reabsorption
• Some materials move from the capillaries into the renal tubules

Characteristics of Urine

• Volume: 1 to 2 liters in 24 hours, varies considerably.
• Color: yellow or amber due to urochrome and urobilin, varies with concentration and diet.
• Turbidity: transparent when freshly voided, becomes turbid on standing.
• Odor: mildly aromatic, becomes ammonia-like.
• pH: ranges from 4.6 to 8.0, averaging 6.0.
• Specific gravity (density): 1.001-1.035, increases with solute concentration.

Ureters

• Slender tubes attaching the kidney to the bladder, 25-30 cm long and 6mm in diameter.
• Peristalsis, aided by gravity, transports urine.
• Mucosal folds act as valves to prevent backflow to the ureters.

Urinary Bladder

• The urinary bladder is a muscular sac that stores urine temporarily.
• The trigone features two ureteral orifices and one internal urethral orifice.
• In males, the prostate gland surrounds the neck of the bladder.
• The bladder wall consists of three smooth muscle layers (detrusor muscle) and a transitional epithelium.
• Bladder walls are thick and folded when empty for expansion without much increase in pressure.

Urethra

• Transports urine from the bladder to the outside.
• Urine release is controlled by the involuntary internal urethral sphincter and the voluntary external urethral sphincter.

Micturition

• Micturition, or voiding, is the act of emptying the bladder.
• Typically, voiding occurs when urine volume exceeds 200-400 mL.
• The internal urethral sphincter relaxes, and the external urethral sphincter must be voluntarily relaxed.

Body Fluids & Fluid Compartments

• The amount of water in the human body must be maintained at sustainable levels to uphold body functions
• Young adult females: 50% water
• Young adult males: 60% water
• Babies: 75% water
• Old age: 45% water

Regulation of Water & Electrolyte Reabsorption

• Regulation is primarily by hormones, as well as through the active detection of cells in the kidneys and hypothalamus
• ADH prevents excessive water loss in urine
• Aldosterone regulates Na+ content of ECF
• These process are triggered by renin-angiotensin activity
• Changes in electrolyte balance such as Na+, K+ and Ca+, affect water distribution to different body parts.
• Can effect blood pressure and volume

Maintaining Water Balance

• Water intake should equal water output
• The thirst mechanism is a driving force for water intake
• The body increases or decreases blood pressure and volume to regulate blood volumes

Blood Buffers

• They act to prevent dramatic changes in H+ concentrations when acids/bases are added

Respiratory System Controls of Acid-Base Balance

• CO2 in the blood is converted to bicarbonate ion and transported in the plasma
• Increasing in H+ concentration produces more carbonic acid
• Excess H+ can be blown off with the release of CO2 from the lungs

Renal Mechanisms of Acid-Base Balance

• The kidneys can regulate blood levels of alkaline substances
• pH of urine varies between 4.0 and 8.0
• Excrete bicarbonate ions if needed
• Conserve/generate new bicarbonate ions if needed
• Fall in blood pH: bicarbonate is reabsorbed & H+ are secreted Rise in blood pH: bicarbonate ions are excreted & H+ are retained by the tubule cells

Nephroptosis

• Falling/floating kidney, is an inferior displacement/dropping of the kidney
• Due to lack of support from organs
• This can can be dangerous because the ureter may kink and block urine flow, can create pressure on kidney tissue

Renal Caliculi

• More commonly known as kidney stones
• They are the precipitation and solidification of salts present in urine
• Formed by the occasional precipitation & solidification of the crystals of salts present in urine

Acute Renal Failure

• Decrease/cessation of glomerular filtration
• Kidneys abruptly stop working entirely
• Main feature: suppression of urine flow (oliguria/anuria)

Adult Polycystic Kidney Disease

• Degenerative condition, 1 or both kidneys enlarge and have many blister-like sacs (cysts) containing urine