Excretion in Animals and Humans

• Excretion: The removal of metabolic waste produced by metabolic activity in cells

Excretion in Animals

• Unicellular organisms, invertebrates, and vertebrates (including mammals) are types of animals that excrete waste
• Endocytosis: General process by which cells take substances in
• Pinocytosis (“cell drinking”): Uptake of extracellular fluid and small dissolved molecules
• Phagocytosis (“cell eating”): Engulfing of large particles or whole cells

Main Excretory Products in Animals

• Water is excreted in sweat, carrying salts and urea
• Urea is a nitrogenous waste found in sweat and urine
• Carbon dioxide is a waste product from respiration, excreted by lungs (in mammals)

Nitrogenous Wastes

• Uric acid is excreted by insects, birds, and reptiles
• Urea is excreted by amphibians and mammals

Excretion in Humans

Waste Products of Metabolism

• Carbon dioxide comes from respiration
• Urea comes from protein deamination
• Bile pigments come from haemoglobin of red blood cells
• Water comes from cellular respiration
• Mineral salts come from ingestion and metabolism
• Heat is a product of respiration

Excretory Organs

• Lungs excrete carbon dioxide, water vapour, and heat
• Skin excretes water, heat, urea, and salts
• Kidneys excrete urea, water, salts, and bile pigments
• Liver detoxifies harmful substances
• The liver converts harmful chemicals from metabolism into harmless or less harmful compounds
• Ammonia (toxic to cells) is reacted with carbon compounds to form urea (less toxic), which is excreted by the kidneys
• The liver makes bile from haemoglobin of worn-out red blood cells; bile is stored in the gallbladder

Bile

• Bile is an excretory product made in the liver and stored in the gallbladder
• Bile enters the duodenum via the bile duct
• In the duodenum, bile emulsifies fats, which is a type of mechanical (physical) digestion, breaking large lipid droplets into smaller droplets
• Smaller lipid droplets provide a larger surface area for pancreatic lipase to work more efficiently.
• Emulsification facilitates digestion by creating a larger surface area for enzyme activity, leading to faster and more efficient breakdown of lipids

The Kidney

• Located below the rib cage on either side of the spine
• Excretes urea, a metabolic waste product
• Osmoregulation: Removes excess water
• Controls pH by neutralizing acids and incorporating them into urine via ultrafiltration and selective reabsorption
• Nephron is the functional unit of the kidney

The Nephron

• Functional unit of the kidney, responsible for filtering blood, reabsorbing essential substances, and excreting waste products, ultimately forming urine
• Blood enters the afferent blood vessel, from an arteriole, from the renal artery and into the Bowman’s capsule
• The afferent blood vessel is larger than the efferent blood vessel which leaves the Bowman’s capsule, resulting in high pressure in the capillaries of the glomerulus

Ultrafiltration in the Bowman’s Capsule

• Substances small enough to pass through the small pores in the glomerular capillaries pass into the Bowman’s capsule (water, urea, salts, glucose), forming the ultrafiltrate
• Proteins and red blood cells are too large to pass into the Bowman’s capsule

Selective Reabsorption in the Proximal Convoluted Tubule

• The efferent blood vessel forms a capillary network around the proximal convoluted tubule
• The cells of the proximal tubule reabsorb substances from the ultrafiltrate and pass them back to the bloodstream by diffusion and active transport.
• 70% of all the soluble substances in the ultrafiltrate (water, glucose, salts, vitamins, amino acids) are reabsorbed in the proximal convoluted tubule
• Urea is not reabsorbed

Loop of Henle

• Increases salt content in kidney medulla, which helps water reabsorption by osmosis in the second tubule and collecting ducts
• Principle function: Recover water and sodium chloride from urine
• Allows production of urine that is far more concentrated than blood, limiting the amount of water needed as intake for survival

Second/Distal Convoluted Tubule

• Principle Function: Reabsorb water and play a key role in osmoregulation
• Main function is to reabsorb water and depends on how much water is in the blood

Collecting Ducts and Urine

• Urine goes from the second convoluted tubule to the collecting ducts, then to the ureter, and is stored in the bladder
• The collecting ducts of many nephrons join in the pelvis of the kidney and then enter the ureter to be transported to the bladder

Adaptations of the Kidney for Reabsorption

• Two kidneys: Humans have two kidneys, each performing the vital function of filtering blood and removing waste
• 1 million nephrons: Each kidney contains approximately one million nephrons, which are the functional units of the kidney responsible for filtration and reabsorption
• Long and coiled tubules increase surface area for reabsorption
• Microvilli in the tubules, especially in the proximal convoluted tubule, increase surface area for absorption
• Mitochondria in tubule cells provide the energy (ATP) needed for active transport during reabsorption

Parts of the Kidney

• Bowman’s capsule: A structure surrounding the glomerulus that collects the filtrate from blood
• Glomerulus: A network of capillaries where blood is filtered, and small molecules like water, glucose, and salts pass into the filtrate
• Proximal convoluted tubule (PCT): The first part of the tubule where most reabsorption of water, glucose, and salts occurs
• Loop of Henle: A U-shaped section that helps concentrate urine by reabsorbing water and salts
• Distal convoluted tubule (DCT): The second part of the tubule where additional reabsorption occurs, regulated by hormones
• Collecting duct: The final part of the nephron where water reabsorption is adjusted, contributing to the regulation of urine volume
• Ureter: A tube that carries urine from the kidney to the bladder
• Bladder: A storage organ for urine
• Urethra: The duct through which urine is expelled from the body

Urination (Micturition)

• Process of discharging urine from the body
• The final step in excretion, where waste in the form of urine leaves the body through the urethra

Stimuli for Urination

• When the bladder fills with urine, stretch receptors in the bladder wall are activated
• These receptors send signals to the brain (specifically, the pons in the brainstem)
• The brain responds by triggering the micturition reflex (urination reflex)

Mechanism of Urination

• The brain sends signals to relax the sphincter muscles (muscles that control the opening of the bladder)
• Simultaneously, it signals the detrusor muscle (muscle in the bladder wall) to contract
• This pushes urine out of the bladder, through the urethra, and out of the body

Osmoregulation

• Osmoregulation: The process of maintaining the balance of water and salts in the body
• Ensures that blood doesn’t become too concentrated or too diluted, keeping conditions right for cells to function properly

Key Players in Osmoregulation

• Hypothalamus (brain): Detects the water level in blood by monitoring the concentration of solutes (like salts)
• If blood is too concentrated (e.g., after sweating a lot or not drinking enough), the hypothalamus sends a signal to release more ADH
• ADH (Antidiuretic Hormone): Produced by the pituitary gland (which is controlled by the hypothalamus)
• ADH tells the kidneys to retain more water instead of letting it go as urine
• More ADH = less urine, more water reabsorbed and Less ADH = more urine, less water reabsorbed
• Second Convoluted Tubule of the Kidney: Responds to ADH by reabsorbing water into the bloodstream

Osmoregulation in Action

• These three players work together to control the amount of water in your blood: Hypothalamus detects water imbalance, Pituitary gland releases ADH and Second convoluted tubule reabsorbs water under ADH’s command
• Water levels in the blood return to normal, and homeostasis is restored

Diabetes Insipidus

• Definition: A disease in which the secretion of or response to the pituitary hormone vasopressin is impaired, resulting in the production of very large quantities of dilute urine, often with dehydration and insatiable thirst

Homeostasis

• Homeostasis: The process by which the body maintains a stable internal environment despite changes in the external environment
• Keeps conditions like body temperature, water levels, pH, and blood glucose within a narrow, healthy range

Key Features of Homeostasis

• Involves monitoring and responding to changes
• Uses negative feedback mechanisms to restore balance
• Helps keep cells working efficiently

Examples of Homeostasis

• Temperature control (e.g., sweating or shivering)
• Osmoregulation (water balance using ADH)
• Blood glucose regulation (using insulin and glucagon)