Urinary System and Kidney Functions

Urinary System

• The urinary system consists of two kidneys, two ureters, the urinary bladder, and the urethra.
• Kidneys are located in the upper abdominal cavity, situated on either side of the vertebral column, behind the peritoneum.
• Ureters extend from the kidneys, descending and uniting at the urinary bladder.
• The urethra is the pathway for urine from the bladder to the exterior.
• Kidneys filter waste, including urea, creatinine, and ammonia, to form urine. The rest of the system acts as an excretory pathway.

Kidney Structure and Function

• Body cells produce waste products requiring removal for proper function.
• Kidneys regulate blood volume by conserving or excreting water.
• Kidneys control electrolyte balance by retaining or eliminating electrolytes like sodium and potassium.
• Acid-base balance is regulated by kidneys' retention or excretion of hydrogen ions (H+) and bicarbonate ions (HCO3-). Hydrogen ions are acidic while bicarbonate ions are alkaline.
• Kidneys regulate tissue fluid, the fluid surrounding cells.

Kidney Anatomy

• Ribs, adipose tissue, and renal fascia (fibrous connective tissue) protect the kidneys.
• The kidneys are located in the upper abdominal cavity, with the upper part bordering the diaphragm and the lower part reaching the lower diaphragm surface.
• The kidney's medial side has an indentation called the hilus
• The renal artery delivers oxygenated blood to the kidneys.
• The renal vein carries deoxygenated blood away from the kidneys to the inferior vena cava.
• Ureters carry urine from the kidneys to the urinary bladder.
• The kidney has distinct cortex and medulla layers.

Kidney Function

• The renal cortex contains the renal corpuscles and convoluted tubules.
• The renal medulla consists of loops of Henle and collecting tubules.
• The renal medulla is organized into wedge-shaped renal pyramids.
• The tip of each pyramid is called the apex.
• The renal pelvis is the expanded ureter portion within the kidney and contains funnel-shaped calyces.
• Urine flows from the renal pyramids to the calyces, then the renal pelvis, and finally to the ureter.

Nephrons - The Functional Units of the kidneys

• Nephrons are the structural and functional units of the kidneys.
• Approximately one million nephrons exist per kidney.
• Nephrons are associated with blood vessels and crucial for urine formation.

Renal Corpuscle

• The renal corpuscle is a glomerulus surrounded by Bowman's capsule.
• The glomerulus is a capillary network that branches from an afferent arteriole and drains into an efferent arteriole.
• The efferent arteriole diameter is smaller than the afferent arteriole, maintaining high pressure within the glomerulus.
• Bowman's capsule surrounds the glomerulus and is the expanded portion of the renal tubule.

Renal Tubules

• Renal tubules, originating from Bowman's capsule, comprise proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
• Distal convoluted tubules merge to form collecting tubules.
• Multiple collecting tubules unite to form a papillary duct, draining into the calyx.
• Microvilli in the proximal convoluted tubule increase surface area for material exchange.

Blood Vessels in the Kidneys

• The renal artery, branching from the abdominal aorta, supplies blood to the kidneys.
• The renal artery divides into afferent arterioles leading to the glomerulus.
• Blood exits the glomerulus via efferent arterioles, which lead to peritubular capillaries.
• Peritubular capillaries combine to form the renal vein, connecting with the inferior vena cava.
• Kidneys have two capillary beds: glomerulus (filtration) and peritubular capillaries (reabsorption).

Formation of Urine

• Urine formation includes glomerular filtration, tubular reabsorption, and tubular secretion:
  • Glomerular filtration: High glomerular pressure forces plasma and dissolved substances into Bowman’s capsule, creating renal filtrate (similar to blood plasma without cells or large proteins).
  • Tubular Reabsorption: Needed substances are selectively reabsorbed from renal filtrate back into the bloodstream, mainly in the proximal convoluted tubule.
  • Tubular Secretion: Active movement of substances from peritubular capillaries into renal tubules for elimination in urine; removes excess waste/substances.

Glomerular Filtration Rate (GFR)

• GFR is the filtrate produced by the kidneys per minute.
• Normal GFR is 100 to 125 mL/minute.
• GFR is directly affected by blood flow (increased flow = increased GFR).

Urine Composition

• Urine is composed of water, electrolytes, waste products, and other substances.
• Urine composition varies due to factors like fluid input, diet, and health.
• Urine removes waste and excess water, maintaining internal balance.

Kidney Function

• Kidneys filter 150–180 liters of filtrate daily; 1–2 liters are excreted as urine, the rest is reabsorbed.
• Reabsorption occurs in the proximal convoluted tubules, distal convoluted tubules, and collecting tubules.
• Micro villi enhance reabsorption surface area.
• Peritubular capillaries surround tubules for reabsorption of useful substances into the blood.

Glomerular Filtration

• High glomerular blood pressure forces water, glucose, amino acids, small proteins, minerals, and waste into Bowman’s capsule.
• Large proteins and blood cells generally stay in the glomerulus except in specific conditions.

Tubular Reabsorption

• Useful substances are reabsorbed into the bloodstream.
• Water, glucose, amino acids, small proteins, and minerals are reabsorbed.

Tubular Reabsorption Mechanisms

• Active Transport: Requires ATP to move substances against their concentration gradient. Transports glucose, amino acids, vitamins, and positive ions.
  • Threshold Level: Limit for reabsorption. If levels exceed the threshold, the excess is excreted in urine.
• Parathyroid Hormone: Drives calcium reabsorption.
• Aldosterone: Influences sodium reabsorption and potassium excretion.
• Passive Transport:
  • Osmosis: Water reabsorption driven by osmotic pressure.
  • Pinocytosis: Reabsorption of small proteins too large for active transport.

Tubular Secretion

• Waste products are removed from blood and enter urine.
• Secreted substances include ammonia (protein metabolism), creatinine (muscle metabolism), medications (partially metabolized), and hydrogen ions (blood pH regulation).

Hormones Influencing Kidney Function

• Aldosterone: Promotes sodium and water retention, increasing blood volume and blood pressure; potassium is excreted.
• ANP (Atrial Natriuretic Peptide): Released by heart atria when blood volume increases. Decreases sodium and water reabsorption, increasing urination, and lowering blood volume.
• ADH (Antidiuretic Hormone): Released by the posterior pituitary when blood volume decreases. It promotes water reabsorption, increasing blood volume.

Urine Formation Summary

• Blood is filtered in the glomerulus.
• Filtered fluid (glomerular filtrate) enters the renal tubules.
• Essential components are reabsorbed into the blood.
• Tubular secretion removes waste products into the urine.
• Hormones regulate urine formation and blood volume.

Other Key Information

• Renal Arteries: Supply blood to the kidneys.
• Renal Veins: Return filtered blood from the kidneys to the inferior vena cava.
• Ureters: Transport urine from kidneys to bladder.
• Bladder: Stores urine.
  • Detrusor Muscle: Smooth muscle contracting for urine expulsion.
  • Internal Urethral Sphincter: Involuntary control over bladder-to-urethra urine flow.
  • External Urethral Sphincter: Voluntary control over urine flow.
• Urethra: Carries urine from bladder to exterior.
• Micturition (Urination): Urine expulsion from bladder.
  • Urinary Reflex: Involuntary urination reflex regulated by the nervous system.
• Urine Characteristics:
  • Volume: Normal output: 1–2 liters/day
  • Color: Yellow; intensity reflects hydration (dark = concentrated, light = dilute).
  • Specific Gravity: 1.010–1.025; measures urine dissolved substances.
  • pH: 4.6–8; affected by diet.
  • Composition: Primarily water, nitrogenous wastes (urea, creatinine, uric acid).

Aging & Kidney Function

• Nephron number decreases with age, influencing urine concentration.
• Glomerular filtration rate declines due to arteriosclerosis.
• Bladder size decreases, increasing urination frequency.
• Urinary incontinence is not a normal aging process.

Renin-Angiotensin-Aldosterone System (RAAS)

• Activated when blood pressure drops.
• Renin: Kidney-released enzyme.
• Angiotensin II: Powerful vasoconstrictor, increases blood pressure, triggers aldosterone release.
• Aldosterone: Increases sodium and water reabsorption, increasing blood volume and pressure.

Erythropoietin

• Erythropoietin: Kidney-produced hormone stimulating red blood cell production in bone marrow.
• Stimulus: Low blood oxygen levels.

Vitamin D Activation

• Vitamin D: Kidney converts to active form.
• Function: Increases intestinal calcium and phosphorus absorption.

This quiz explores the anatomy and physiology of the urinary system, including the structure and functions of the kidneys. It covers the components of the urinary system, their locations, and their roles in waste elimination and regulation within the body. Test your knowledge about how kidneys filter waste and maintain homeostasis.