Excretory Products and Their Elimination Chapter 16

Excretory Products and Their Elimination

• Animals eliminate substances like ammonia, urea, uric acid, carbon dioxide, water, and various ions to maintain homeostasis.
• Major nitrogenous wastes include ammonia (most toxic), urea, and uric acid (least toxic).
• Ammonotelism: organisms such as bony fishes excrete ammonia, typically through diffusion due to its high solubility.
• Ureotelic animals (mammals, terrestrial amphibians) convert ammonia to urea for excretion.
• Uricotelic animals (reptiles, birds, insects) excrete uric acid, minimizing water loss.

Excretory Structures

• Invertebrates typically possess simple tubular excretory structures; vertebrates have complex kidneys.
• Protonephridia in flatworms and rotifers help with osmoregulation.
• Nephridia in earthworms assist in the removal of nitrogenous wastes.
• Malpighian tubules in insects and antennal (green) glands in crustaceans function in waste elimination.

Human Excretory System

• Composed of kidneys, ureters, bladder, and urethra.
• Each kidney measures 10-12 cm in length, with a weight of 120-170 g.
• Hilum: notch on the kidney's inner concave surface for entry of ureters, blood vessels, and nerves.
• Kidney structure includes:
  • Renal pelvis: funnel-shaped space leading to ureters.
  • Cortex: outer layer containing nephrons (nearly 1 million per kidney).
  • Medulla: inner layer with conical masses called medullary pyramids.

Nephrons and Urine Formation

• Nephrons consist of:
  • Glomerulus: tuft of capillaries for blood filtration.
  • Bowman’s capsule: surrounds glomerulus, forming the renal corpuscle.
  • Proximal convoluted tubule (PCT): reabsorbs 70-80% of water, nutrients, and electrolytes.
  • Loop of Henle: concentrates filtrate via a counter-current mechanism.
  • Distal convoluted tubule (DCT): reabsorbs Na+ and water, secreting H+ and K+ for pH balance.
  • Collecting duct: further concentrates urine by reabsorbing water and small amounts of urea.

Glomerular Filtration and Urine Output

• Average glomerular filtration rate (GFR): 125 ml/min, totaling about 180 liters per day.
• Renal tubules reabsorb nearly 99% of filtrate to produce approximately 1.5 liters of urine daily.
• Tubular secretion maintains ionic and acid-base balance.

Regulation of Kidney Function

• Hormonal feedback regulates kidney activity via the hypothalamus and juxtaglomerular apparatus (JGA).
• Antidiuretic hormone (ADH) facilitates water reabsorption, helping prevent diuresis.
• Renin-Angiotensin mechanism regulates blood pressure:
  • Renin releases angiotensin II, a vasoconstrictor that increases GFR.
  • Aldosterone promotes Na+ and water reabsorption, also raising blood pressure.
• Atrial Natriuretic Factor (ANF) lowers blood pressure, counteracting the renin-angiotensin mechanism.

Micturition Process

• Urine stored in the bladder triggers stretch receptors as it fills.
• Central nervous system activates micturition reflex causing bladder contractions and sphincter relaxation.
• Average urine output: 1-1.5 liters per day, typically light yellow and slightly acidic (pH 6.0).
• Urine analysis is valuable for diagnosing metabolic disorders and kidney dysfunctions.### Glucose and Ketone Bodies
• Presence of glucose (glycosuria) and ketone bodies (ketonuria) in urine are key indicators of diabetes mellitus.

Role of Organs in Excretion

• Other than kidneys, lungs, liver, and skin contribute to waste elimination.
• Lungs expel approximately 200 mL of CO2 per minute and significant water as vapor.
• The liver secretes bile with components such as bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs which exit via digestive wastes.
• Sweat glands produce sweat that contains NaCl, urea, and lactic acid, aiding in cooling and waste removal.
• Sebaceous glands secrete sebum, which consists of sterols, hydrocarbons, and waxes, providing skin protection and a barrier.
• Small amounts of nitrogenous wastes can also be found in saliva.

Disorders of the Excretory System

• Kidney malfunction leads to uremia (urea accumulation in blood), which is harmful and can result in kidney failure.
• Hemodialysis offers a treatment method where blood is filtered in an artificial kidney unit to remove urea.
• Blood is treated with anticoagulants (like heparin) to prevent clotting during dialysis; cleared blood is then returned to the body.
• Kidney transplantation is the ultimate solution for acute renal failures, ideally using a donor kidney from a close relative to reduce rejection risks.

Excretory Processes in Animals

• Animals eliminate nitrogenous wastes (ammonia, urea, uric acid) based on habitat and water availability.
• Common excretory organs include protonephridia, nephridia, malpighian tubules, green glands, and kidneys, maintaining ionic and acid-base balance.
• Human excretory system comprises two kidneys, ureters, a urinary bladder, and a urethra.

Kidney Structure and Function

• Each kidney contains over a million nephrons, the functional unit comprising a glomerulus and a renal tubule.
• Glomerulus consists of a tuft of capillaries, while Bowman’s capsule collects filtrate.
• Urine formation involves filtration, reabsorption, and secretion:
  • Filtration rate is approximately 125 mL/min (GFR), filtering around 1200 mL of blood per minute.
  • JGA regulates GFR; PCT is the primary site for reabsorption.
  • Loop of Henle maintains osmolar gradient (300 to 1200 mOsmol/L).
  • DCT and collecting duct facilitate water and ion reabsorption for osmoregulation.

Filtration Concentration Mechanisms

• A countercurrent mechanism operates in the loop of Henle and vasa recta for efficient filtrate concentration.
• Urine is stored in the bladder, released through urethra via voluntary micturition.

Additional Notes on Micturition and Hormonal Regulation

• Micturition is controlled through central nervous signals.
• ADH (antidiuretic hormone) plays a vital role in water reabsorption, affecting urine concentration.

Key Points about Excretory Functions

• Ammonotelism is predominant in aquatic animals, while terrestrial animals are often ureotelic or uricotelic due to water conservation.
• JGA (juxtaglomerular apparatus) is essential for regulating kidney function and GFR stability.

This quiz explores the various excretory products in humans and animals, such as ammonia, urea, uric acid, and more. It also covers how these substances are eliminated from the body through different systems and metabolic activities. Test your knowledge on the human excretory system and its functions.