Study sheet: Human Excretion and Homeostasis

📋 Course Outline

1. Excretion and Waste Products
2. Urinary System Organization
3. Kidney and Nephron Structure
4. Glomerular Ultrafiltration
5. Tubular Reabsorption and Secretion
6. Urine Formation and Excretion
7. Renal pH Regulation
8. Osmoregulation and Salt Balance

📖 1. Excretion and Waste Products

★ Must-know

📌 Excretion is the removal of metabolic waste from an organism, whereas secretion is the release of useful substances such as enzymes or saliva from cells or glands.

• The main human excretory waste products are:

  • carbon dioxide
  • water
  • bile pigments
  • urea
  • mineral salts

• The lungs excrete carbon dioxide and water vapour produced by cellular respiration in exhaled air.

• The kidneys excrete urea from deamination of excess amino acids in the liver, together with excess mineral salts and water, in urine.

Further detail

• The skin excretes mineral salts, traces of urea, and water extracted from the blood as sweat, while the colon excretes bile pigments and excess mineral salts in faeces.

💡 Memory Hook

Excretion removes metabolic waste, whereas secretion releases useful substances.

📖 2. Urinary System Organization

🔑 Key Concepts & Definitions

• Urinary system : Consists of two kidneys, two ureters, the bladder, and the urethra.

★ Must-know

• The four main functions of the urinary system are:

  • osmoregulation
  • excretion of nitrogenous waste such as urea
  • regulation of body-fluid pH
  • regulation of body-fluid salt concentration

• The renal artery branches from the aorta and carries oxygenated, unfiltered blood containing waste products to the kidney, while the renal vein carries deoxygenated, filtered blood away.

Further detail

• Adipose tissue and a renal capsule protect each kidney from damage and infection.

💡 Memory Hook

Kidneys → ureters → bladder → urethra

📖 3. Kidney and Nephron Structure

🔑 Key Concepts & Definitions

• Nephron : The microscopic functional unit of the kidney that filters blood and regulates waste, water, and other substances.
• Malpighian body : Or renal corpuscle, consists of the Bowman's capsule and the glomerulus and lies in the kidney cortex.

★ Must-know

• The renal tubule consists of:
  • the proximal convoluted tubule
  • the loop of Henle
  • the distal convoluted tubule
  • collecting ducts

Further detail

• Each human kidney contains about 1 million nephrons, which are made of tubes, arterioles, capillaries, and ducts.

• Renal-tubule epithelial cells have microvilli that increase surface area and many mitochondria that provide energy for active transport.

💡 Memory Hook

A nephron is a microscopic tube wrapped in blood vessels, beginning with a cup-shaped capsule.

📖 4. Glomerular Ultrafiltration

📝 Essential Points

• 🔄 Glomerular ultrafiltration proceeds as follows: blood enters through the afferent arteriole, blood leaves through the narrower efferent arteriole, high pressure forces plasma into Bowman's capsule

• The glomerulus has thin, single-layer capillary walls and podocyte filtration slits, so only small dissolved substances pass into Bowman's capsule while large proteins remain in the blood.

• Useful substances filtered include:

  • glucose
  • amino acids
  • vitamins
  • minerals
  • water

💡 Memory Hook

Small dissolved substances cross the filtration slits, whereas large proteins remain in the blood.

📖 5. Tubular Reabsorption and Secretion

📝 Essential Points

• In the proximal convoluted tubule, glucose, amino acids, vitamins, and other important substances are actively reabsorbed, while about 65% of the water returns to the blood by osmosis.

• In the ascending loop of Henle, cells are impermeable to water and actively pump salt into the medulla, making the medulla hypertonic and establishing a steep water-potential gradient.

• Substances removed by tubular secretion include:

  • creatinine
  • hydrogen ions
  • ammonia
  • sodium ions
  • potassium ions
  • bicarbonate ions
  • drugs such as penicillin

💡 Memory Hook

Proximal reabsorption → loop concentration → distal regulation → collecting duct

📖 6. Urine Formation and Excretion

📝 Essential Points

• 🔄 Urine passes through the urinary tract in this order:

  1. collecting ducts
  2. renal pelvis
  3. ureter
  4. bladder
  5. urethra

• Urine consists of urea, excess water, and excess salts, while useful substances should not be excreted in urine.

💡 Memory Hook

Collecting ducts → renal pelvis → ureter → bladder → urethra

📖 7. Renal pH Regulation

📝 Essential Points

📌 When blood pH is low because hydrogen-ion concentration is high, hydrogen ions move from peritubular capillaries into the distal convoluted tubule and blood pH returns toward 7.4.

💡 Memory Hook

Excess hydrogen or bicarbonate ions enter the tubule, causing blood pH to return toward 7.4.

📖 8. Osmoregulation and Salt Balance

🔑 Key Concepts & Definitions

• Osmoregulation : The homeostatic control of water levels in the blood and tissue fluid.

★ Must-know

• ADH is produced by the hypothalamus and secreted by the pituitary gland; it increases the water permeability of the distal convoluted tubule and collecting ducts, reducing water loss in urine.

• During dehydration, increased ADH causes more water to be reabsorbed and a smaller volume of concentrated urine to be excreted, whereas during overhydration, reduced ADH causes less water to be reabsorbed and more dilute urine to be excreted.

• When blood sodium levels are low, the adrenal gland releases aldosterone, which stimulates sodium reabsorption from the filtrate into the blood and reduces sodium excretion.

Further detail

• Alcoholic and caffeine-containing drinks act as diuretics that increase urination and water loss, opposite to the water-retaining effect of ADH.

💡 Memory Hook

ADH conserves water, whereas reduced ADH promotes dilute urine; aldosterone conserves sodium.

📊 Synthesis Tables

Hormonal control of water and salts

Hormone	Control centre	Kidney effect	Urine or salt outcome
ADH	Hypothalamus and pituitary gland	Increases water permeability of distal convoluted tubules and collecting ducts	More water reabsorbed and more concentrated urine
Aldosterone	Adrenal gland	Stimulates sodium reabsorption from filtrate	Less sodium excreted in urine