🫁Honors Anatomy and Physiology Unit 14 – Urinary System

Key Structures and Functions

• The urinary system consists of the kidneys, ureters, bladder, and urethra which work together to filter blood, remove waste products, and maintain fluid and electrolyte balance
• Kidneys are bean-shaped organs located in the retroperitoneal space that filter blood to produce urine and regulate blood pressure, pH, and electrolyte levels
• Ureters are muscular tubes that transport urine from the kidneys to the bladder using peristaltic contractions
• The bladder is a hollow, muscular organ that stores urine until it is ready to be excreted from the body
• The urethra is a tube that carries urine from the bladder to the outside of the body during micturition (urination)
• The urinary system maintains homeostasis by regulating blood volume, blood pressure, pH, and electrolyte concentrations
• The kidneys produce hormones such as erythropoietin (stimulates red blood cell production) and calcitriol (active form of vitamin D)

Anatomy of the Kidneys

• Kidneys are located in the retroperitoneal space, with the right kidney slightly lower than the left due to the liver's position
• Each kidney is surrounded by a fibrous capsule and a layer of adipose tissue (perirenal fat) for protection
• The renal hilum is an indentation on the medial side of each kidney where the renal artery, renal vein, and ureter enter and exit
• The outer layer of the kidney is called the renal cortex, which contains the glomeruli and convoluted tubules of the nephrons
• The inner layer is the renal medulla, which consists of pyramid-shaped structures called renal pyramids containing the loops of Henle and collecting ducts
• The renal pelvis is a funnel-shaped structure that collects urine from the collecting ducts and leads to the ureter
• Blood supply to the kidneys is provided by the renal arteries, which branch directly from the abdominal aorta

Nephron Structure and Function

• Nephrons are the functional units of the kidney, responsible for filtering blood and producing urine
• Each kidney contains approximately 1 million nephrons, which consist of a renal corpuscle and a renal tubule
• The renal corpuscle contains the glomerulus (a network of capillaries) and the Bowman's capsule (a cup-shaped structure surrounding the glomerulus)
• The renal tubule is divided into the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct
  • The PCT reabsorbs nutrients, water, and electrolytes from the filtrate
  • The loop of Henle creates a concentration gradient in the medulla for water reabsorption
  • The DCT and collecting duct are sites of hormonal regulation for fine-tuning urine concentration
• Filtration occurs in the glomerulus, where blood pressure forces fluid and solutes out of the capillaries and into the Bowman's capsule
• Reabsorption and secretion occur along the renal tubule to modify the composition of the filtrate and produce urine

Urine Formation Process

• Urine formation involves three main processes: filtration, reabsorption, and secretion
• Filtration occurs in the glomerulus, where blood pressure forces fluid and solutes (except for proteins and blood cells) into the Bowman's capsule, forming the glomerular filtrate
• Reabsorption takes place along the renal tubule, where essential nutrients, water, and electrolytes are selectively transported back into the bloodstream
  • The PCT reabsorbs glucose, amino acids, and most of the water and electrolytes
  • The loop of Henle reabsorbs more water and creates a concentration gradient in the medulla
  • The DCT and collecting duct reabsorb sodium and water under hormonal control
• Secretion involves the active transport of substances (such as hydrogen ions, potassium, and certain drugs) from the peritubular capillaries into the renal tubule
• The remaining fluid, now called urine, flows through the collecting ducts, into the renal pelvis, and out of the kidney via the ureter
• The composition of urine can vary depending on factors such as hydration status, diet, and hormonal regulation

Regulation of Water and Electrolytes

• The kidneys play a crucial role in maintaining fluid and electrolyte balance in the body
• Water balance is regulated by antidiuretic hormone (ADH), also known as vasopressin, which is released by the posterior pituitary gland in response to dehydration or increased blood osmolarity
  • ADH increases water reabsorption in the collecting ducts, leading to the production of more concentrated urine
  • In the absence of ADH, the collecting ducts are less permeable to water, resulting in dilute urine
• Sodium balance is regulated by aldosterone, a hormone produced by the adrenal cortex in response to low blood pressure or high potassium levels
  • Aldosterone increases sodium reabsorption in the DCT and collecting ducts, which in turn increases water reabsorption and blood volume
  • Atrial natriuretic peptide (ANP), released by the heart in response to high blood pressure, counteracts the effects of aldosterone by promoting sodium excretion
• Potassium balance is maintained by the actions of aldosterone and the sodium-potassium pump in the DCT and collecting ducts
  • Aldosterone stimulates the secretion of potassium into the renal tubule in exchange for sodium reabsorption
  • The sodium-potassium pump actively transports sodium out of the tubule cells and potassium into the cells, maintaining the concentration gradient

Hormonal Control of the Urinary System

• Several hormones regulate the functions of the urinary system, including ADH, aldosterone, ANP, and parathyroid hormone (PTH)
• ADH (vasopressin) is released by the posterior pituitary gland in response to dehydration or increased blood osmolarity and acts on the collecting ducts to increase water reabsorption
• Aldosterone, produced by the adrenal cortex, increases sodium reabsorption and potassium secretion in the DCT and collecting ducts, leading to increased water retention and blood volume
• ANP, secreted by the heart in response to high blood pressure, promotes sodium excretion and reduces blood volume
• PTH, released by the parathyroid glands in response to low blood calcium levels, increases calcium reabsorption in the DCT and stimulates the activation of vitamin D in the kidneys
• The renin-angiotensin-aldosterone system (RAAS) is a hormonal cascade that regulates blood pressure and fluid balance
  • Renin, an enzyme produced by the juxtaglomerular cells in the kidneys, converts angiotensinogen to angiotensin I
  • Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II, a potent vasoconstrictor that stimulates aldosterone release
• Erythropoietin (EPO), produced by the kidneys in response to hypoxia, stimulates red blood cell production in the bone marrow

Common Urinary System Disorders

• Urinary tract infections (UTIs) are caused by bacterial growth in the urethra, bladder, or kidneys and can lead to symptoms such as frequent urination, burning sensation, and cloudy or bloody urine
• Kidney stones are hard deposits of minerals that can form in the kidneys or ureters, causing pain, nausea, and hematuria (blood in the urine)
• Glomerulonephritis is an inflammation of the glomeruli, often caused by an autoimmune response or infection, leading to impaired kidney function and proteinuria (protein in the urine)
• Polycystic kidney disease (PKD) is an inherited disorder characterized by the development of numerous fluid-filled cysts in the kidneys, which can lead to enlarged kidneys and progressive loss of kidney function
• Renal failure occurs when the kidneys are unable to filter waste products effectively, leading to the accumulation of toxins in the blood
  • Acute renal failure has a sudden onset and is often reversible with prompt treatment
  • Chronic renal failure develops gradually over time and may require dialysis or kidney transplantation
• Urinary incontinence is the involuntary leakage of urine, which can be caused by factors such as weak pelvic floor muscles, overactive bladder, or neurological disorders
• Benign prostatic hyperplasia (BPH) is a non-cancerous enlargement of the prostate gland that can compress the urethra and cause difficulty urinating in older men

Clinical Applications and Diagnostics

• Urinalysis is a common diagnostic test that involves examining the physical, chemical, and microscopic properties of urine to detect abnormalities or signs of disease
  • Physical examination assesses the color, clarity, and specific gravity of the urine
  • Chemical analysis uses dipstick tests to measure pH, glucose, protein, blood, and other substances
  • Microscopic examination looks for the presence of cells, crystals, or microorganisms in the urine
• Blood tests, such as serum creatinine and blood urea nitrogen (BUN), can be used to assess kidney function and detect signs of renal failure
• Imaging techniques, including ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI), can visualize the structure of the urinary system and identify abnormalities such as kidney stones, cysts, or tumors
• Renal biopsy involves removing a small sample of kidney tissue for microscopic examination to diagnose conditions such as glomerulonephritis or renal cancer
• Cystoscopy is a procedure that uses a thin, flexible tube with a camera (cystoscope) to examine the inside of the bladder and urethra for signs of inflammation, tumors, or other abnormalities
• Urodynamic testing assesses the function of the bladder and urethra by measuring pressure, flow rate, and volume during urination to diagnose conditions such as urinary incontinence or obstruction
• Treatment options for urinary system disorders vary depending on the specific condition and may include antibiotics for UTIs, lithotripsy for kidney stones, dialysis or transplantation for renal failure, and medications or surgery for BPH or urinary incontinence