Biology Chapter: Excretion and Urinary System

Excretion

• Excretion is the removal of metabolic waste, a crucial process for all organisms.
• In humans (vertebrates), the kidneys, lungs, and skin are the primary organs for excretion.

Urinary System Structure

• The urinary system consists of two main parts:
  • The upper urinary tract: Composed of a pair of kidneys, renal pelvis, and ureters. These structures are responsible for urine formation.
  • The lower urinary tract: Includes the urinary bladder, urethra, and supporting structures. These structures are involved in urine storage and elimination.

Kidney Structure

• The kidney has an outer cortex and inner medulla.
• The renal capsule, along with fat and adjacent structures (diaphragm, ribs), protects and cushions the kidney.

Nephron Structure

• The nephron is the functional unit of the kidney.
• Each kidney contains approximately 1-1.5 million nephrons, each measuring 3-5 cm.
• A nephron consists of:
  • Renal corpuscle: Including Bowman's capsule and the glomerulus (a network of blood vessels).
  • Renal tubules: Including Proximal convoluted tubule, loop of Henle, Distal convoluted tubule, and collecting duct.

Kidney Function

• Maintaining the body's pH level.
• Reabsorbing nutrients.
• Regulating blood pressure.
• Excreting waste products from the body.
• Removing excess fluid from the body.
• Secreting hormones that assist in red blood cell production and acid regulation.

Nephron Function

• Glomerular filtration: The initial process of filtering blood in the glomerulus.
• Tubular reabsorption: The selective reabsorption of essential substances (water, glucose, amino acids) back into the bloodstream.
• Tubular excretion: The active secretion of excess ions and waste into the filtrate.

Blood Supply to the Nephron

• The nephron's blood supply originates from the afferent arteriole.
• Blood exits the glomerulus via the efferent arteriole.

Blood Supply to the Kidney

• Arterial supply comes from the renal arteries.
• Venous drainage is through the renal veins.

Ureters

• The ureters are long, tubular organs, roughly 24-30 cm in length.
• The left ureter is slightly longer than the right.
• They transport urine from the kidneys to the bladder.
• The ureters terminate at the posterior base of the bladder.

Urinary Bladder

• The urinary bladder is a hollow, muscular organ.
• It stores urine (approximately 300-600 mL) in healthy adults.

Urethra

• The urethra is the final pathway for urine excretion.
• Urine passes through the urethra to exit the body.

Urine Formation Physiology

• Human urine formation occurs via three phases: glomerular filtration, reabsorption, and secretion.

Glomerular Filtration

• Filtration happens at the glomerulus where pressure forces water, electrolytes and small molecules into Bowman's capsule forming filtrate.
• Nonfilterable substances remain in the blood, e.g., blood cells, proteins.

Tubular Reabsorption

• The filtrate travels through renal tubules, selectively absorbing water, glucose, and useful substances back to the blood.

Tubular Secretion

• Excess substances, or waste products are actively secreted into the collecting duct, adding to the urine.

Urine Concentration

• The loop of Henle concentrates urine by adjusting water and solute reabsorption.
• Antidiuretic hormone (ADH) plays a crucial role in controlling water reabsorption.

Urine Composition

• Urine is primarily water (approximately 95%).
• It contains waste products (5%): nitrogenous waste (urea, creatinine, ammonia, uric acid).
• Also contains ions (sodium, potassium, hydrogen, calcium)

Urologic Disease

• Urologic conditions include: urinary tract infection, kidney stones, bladder control issues, and prostate problems.