Study guide: Urinary System

📋 Course Outline

1. Urinary Organs and Kidney Anatomy
2. Nephron Structure and Urine Formation
3. Hormonal Regulation of Kidney Function
4. Urine Characteristics and Renal Failure
5. Urinary Tract and Micturition

📖 1. Urinary Organs and Kidney Anatomy

★ Must-know

• The urinary system consists of:
  • two kidneys
  • two ureters
  • one urinary bladder
  • one urethra

• The kidney has three regions: the renal cortex, which contains renal columns; the renal medulla, which contains renal pyramids; and the renal pelvis, which collects urine and funnels it into the ureter.

• The kidneys:

  • regulate blood volume and electrolytes
  • regulate acid-base balance
  • excrete nitrogenous waste
  • regulate blood pressure
  • regulate red blood cell production

Further detail

• The organs of excretion are:

  • kidneys
  • skin (sweat glands)
  • lungs
  • intestines

• The kidney is enclosed by a renal capsule and receives blood through the renal artery, which drains through the renal vein.

📖 2. Nephron Structure and Urine Formation

🔑 Key Concepts & Definitions

• Nephron: The functional unit of the kidney, consisting of renal tubules and renal blood vessels.

★ Must-know

• The renal tubules comprise:
  • the glomerular (Bowman's) capsule
  • the proximal convoluted tubule
  • the loop of Henle
  • the distal convoluted tubule
  • the collecting duct

• Urine formation consists of:

  • glomerular filtration
  • tubular reabsorption
  • tubular secretion

• During glomerular filtration, water and dissolved solutes pass from the glomeruli into Bowman's capsule because pressure is greater on one side of a membrane than the other; water, sodium, potassium, chloride, glucose, and uric acid pass easily, while large blood cells and proteins cannot fit through the pores.

• Glomerular filtration rate (GFR), the rate at which water and solutes are filtered, is determined by blood pressure and is normally 180 L of water per 24 hours or 125 mL per minute.

• Tubular reabsorption returns filtrate from the renal tubules to the blood in the peritubular capillaries, occurs mostly in the proximal convoluted tubule, and is selective in the type and quantity of substances reclaimed.

• The kidneys reabsorb more than 99% of filtered sodium and water, 100% of glucose, 50% of urea, and 0% of creatinine.

Further detail

• The nephron blood-vessel pathway runs through:

  • the renal artery
  • the afferent arteriole
  • the glomerulus
  • the efferent arteriole
  • the peritubular capillaries
  • the renal vein

• The proximal convoluted tubule reabsorbs most of the filtrate's:

  • glucose
  • amino acids
  • water
  • sodium

📌 Tubular reabsorption can be active, as when sodium is pumped from the tubule into peritubular capillaries, or passive, as when water and chloride follow sodium into those capillaries.

• Substances secreted into the tubules include:
  • potassium ions
  • hydrogen ions
  • uric acid
  • ammonium ions
  • drugs

💡 Memory Hook

Filter, reclaim, then secrete: the three steps of urine formation.

📖 3. Hormonal Regulation of Kidney Function

★ Must-know

• Aldosterone acts on the distal tubule and upper collecting duct, increasing sodium and water reabsorption, blood volume, and blood pressure.

• 🔄 The renin-angiotensin-aldosterone sequence is:

  1. Low blood pressure prompts the kidney to release renin.
  2. Renin activates the system acting on circulating angiotensinogen.
  3. Angiotensinogen becomes angiotensin I.
  4. A converting enzyme released by the lungs produces angiotensin II.
  5. The adrenal cortex releases aldosterone.
  6. The distal tubule reabsorbs sodium and water and excretes potassium.

• Low blood volume or high concentration triggers release of antidiuretic hormone (ADH) from the posterior pituitary; ADH moves water from the collecting duct into tissue spaces and then peritubular capillaries, decreasing water excretion and producing concentrated urine.

Further detail

• Atrial natriuretic peptide (ANP) is secreted by the atrial walls in response to increased blood volume and causes sodium excretion.

• B-type natriuretic peptide (BNP) is secreted by the ventricular walls in response to elevated ventricular volume.

• Parathyroid hormone and calcitonin regulate calcium and phosphate electrolytes and have a phosphaturic effect; their primary stimulus is a low plasma calcium level.

💡 Memory Hook

Aldosterone retains sodium and water; natriuretic peptides promote sodium excretion.

📖 4. Urine Characteristics and Renal Failure

🔑 Key Concepts & Definitions

• Uremia: The presence of urine in the blood resulting from renal failure.

★ Must-know

• Average urine volume is 1500 mL per 24 hours; oliguria is less than 400 mL per 24 hours, and polyuria is greater than 1500 mL per 24 hours.

📌 Urinary retention occurs when the kidneys produce urine but the bladder does not empty; treatment may include catheterization or drugs.

📌 Renal suppression occurs when the kidneys do not produce urine, leading to uremia; treatment is dialysis.

Further detail

• Urine has an average pH of 6.0, with a normal range of 5.0–8.0.

• Urine specific gravity is 1.001–1.035, making urine slightly heavier than water.

• Normal urine is amber or straw-colored, deep yellow with dehydration, and pale yellow with overhydration.

• Dialysis removes waste from the blood and may use an artificial kidney or peritoneal dialysis.

💡 Memory Hook

Retention means the kidneys make urine but the bladder cannot empty; suppression means the kidneys do not make urine.

📖 5. Urinary Tract and Micturition

🔑 Key Concepts & Definitions

• Urethra: The tube that carries urine from the bladder to the outside of the body.

★ Must-know

• 🔄 The micturition reflex proceeds as follows:
  1. Bladder filling stimulates stretch receptors.
  2. Sensory signals travel to the spinal cord.
  3. Motor signals travel to the bladder.
  4. The bladder contracts and the internal sphincter relaxes.

Further detail

• The ureters are 10 to 13 inches long.

• The urinary bladder has four layers.

• The muscular layer of the urethra contracts to help expel urine during micturition.

📌 The female urethra is 1.5 inches or 3–4 cm long, whereas the male urethra is 7–8 inches or 18–20 cm long.

💡 Memory Hook

Bladder stretch → spinal signal → bladder contraction and sphincter relaxation.

📊 Synthesis Tables

Hormonal Effects on Urine and Blood Volume

Hormone	Trigger or site	Kidney effect
Aldosterone	Adrenal cortex; renin-angiotensin system	Increases sodium and water reabsorption
ADH	Low blood volume or high concentration	Decreases water excretion
ANP	Increased blood volume; atrial walls	Causes sodium excretion
BNP	Elevated ventricular volume; ventricular walls	Secreted in response to volume elevation