Excretion

Excretion and Homeostasis

Egestion vs Excretion

• Egestion: The process of eliminating undigested waste from the body in the form of feces.

• Excretion: The removal of metabolic wastes like urine, sweat, and carbon dioxide from the body.

Plant Waste

• Plants produce minimal waste, primarily losing gases like carbon dioxide and water vapor through stomata (on leaves) and lenticels (on stems).

Animal Excretion

• Excretory Organs:

  • Kidneys: Responsible for urine production.

  • Skin: Excretes sweat.

  • Lungs: Remove carbon dioxide.

  • Liver: Produces urea from excess amino acids.

Urine Composition

• Urine mainly consists of:

  • 96% water

  • 2.5% nitrogenous waste (urea)

  • 1.5% salts.

Homeostasis

• Homeostasis refers to the maintenance of a stable internal environment, playing a crucial role through excretion by:

  1. Regulating osmoregulation (water and salt levels) via the kidneys.

  2. Controlling body temperature through sweating.

  3. Maintaining pH of body fluids by excreting carbon dioxide through the lungs.

• This regulation allows for efficient metabolic processes, ensuring that enzymes function optimally under suitable conditions (pH and temperature).

The Urinary System

• Functions of Kidneys:

  1. Excretion: Produce urine.

  2. Osmoregulation: Control of salt and water balance in the body.

  3. pH Regulation: Monitoring the acidity/alkalinity of body fluids.

• Structure of Kidneys:

  • Bean-shaped organs, approximately the size of a fist, located against the back wall of the abdominal cavity, above the pelvis, and surrounded by protective fat.

• Blood Supply:

  • Renal Artery: Delivers oxygenated blood from the aorta.

  • Renal Vein: Transports deoxygenated blood to the inferior vena cava.

• Blood in the renal vein has lower levels of oxygen, salt, and urea compared to the renal artery.

Nephron: The Functional Unit of the Kidney

• Each kidney contains about 1 million nephrons, the functional units that create urine, located in the cortex and medulla of the kidney.

Urine Formation Steps

1. Filtration:

   • High blood pressure in the glomerulus pushes substances into Bowman’s capsule.

   • Large components such as red and white blood cells and platelets do not enter.

2. Reabsorption:

   • Useful materials (99%) are reabsorbed back into the blood from the nephron, with urea being kept for excretion.

   • Proximal Convoluted Tubule (PCT): Majority of water, glucose, amino acids, and vitamins are reabsorbed.

   • Loop of Henle: Water is absorbed in the descending loop; salts are absorbed in the ascending loop.

   • Distal Convoluted Tubule (DCT): Further water and salts are reabsorbed.

3. Secretion:

   • H+ ions are secreted into the filtrate from the blood in the distal tubule.

Osmoregulation

• High Blood Plasma Concentration:

  • Results in the excretion of small amounts of concentrated urine.

• Low Blood Plasma Concentration:

  • Leads to the excretion of large volumes of dilute urine.

Role of ADH (Anti-Diuretic Hormone)

• When experiencing low water intake, ADH is released, increasing water reabsorption and resulting in concentrated urine production.

• Conversely, high water intake leads to decreased ADH production, leading to dilute urine.

Skin Functions

1. Temperature Regulation:

   • Cold Conditions: Piloerection (hair stands up) and vasoconstriction (narrowing of blood vessels) minimize heat loss.

   • Warm Conditions: Vasodilation (widening of blood vessels) and sweat release enhance heat loss.

2. Protection:

   • The epidermis serves as a waterproof barrier; the dermis offers protection to internal organs.

   • Melanin helps shield the skin from UV radiation.

3. Excretion:

   • Sweat glands expel water and salts.

4. Sense Organ:

   • The skin detects environmental stimuli.

5. Vitamin Production:

   • Produces vitamin D in response to sunlight exposure.

6. Food Storage:

   • Adipose tissue stores fat.

Labelled Diagram of a Nephron

Diagram shows labeled parts of the nephron including the glomerulus, Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.