The Excretory System Function Maintains homeostasis regulates composition

The Excretory System

Function • Maintains homeostasis – regulates composition of blood and extracellular fluid – Ex. water balance • Excretion - Process that eliminates metabolic wastes – Wastes are the result of break down of food molecules and cellular respiration – Ex. protein break-down produces ammonia

• Skin Organs of Excretion – Excretes sweat (salt and water) • Lungs – Excretes carbon dioxide and water vapor • Liver – Converts nitrogen wastes into urea • Kidneys – Excrete urine (urea, water and salts)

The Urinary System • Composed of Kidneys, ureters, urinary bladder and urethra – Regulates blood levels of ions (e. g. sodium, potassium) – Maintains proper p. H of blood – Regulates water content of blood – Retains important nutrients (e. g. glucose, amino acids) in blood – Eliminates cellular waste produces (e. g. urea) – (Secretes substances to help regulate blood pressure and blood oxygen levels)

Kidneys • Filter the blood • Blood enters the kidney through the renal artery and exits through the renal vein • Nephron – Filtering unit of the kidneys – 1 million per kidney – 2 parts: • Glomerulus – knot of capillaries • Tubule – long, twisted tube – Empties into collecting duct

Inside a Nephron 1. Filtration: blood enters glomerulus a) water and small, dissolves molecules and ions forced through capillary walls b) “filtrate” enters Bowman’s capsule of the nephron (water, urea, glucose, salts, amino acids, vitamins, etc. ) 2. Tubular reabsorption: in tubule, most water and nutrients reabsorbed into capillaries surrounding tubule

Inside a Nephron 3. Tubular secretion: additional wastes actively transported into tubule from blood (excess H+, ammonia, drugs, food additives, toxins, etc. ) 4. Concentration: Loop of Henle produces a salt concentration gradient in the extracellular fluid so more water can diffuse out of filtrate

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Then What? • Material that is not reabsorbed is urine (urea, water, salts and excess vitamins) • Urine is transported out of the kidney through the ureter to the urinary bladder • Urine stored here • Urine exits the bladder via the urethra

Kidneys and Homeostasis • Kidneys maintain the salt and water balance in the blood • Well hydrated kidneys filter out excess water and urine is dilute • Dehydrated kidneys conserve water urine has a higher concentration of urea and salts • Regulated by endocrine system (hormones)

Kidney Failure • Diabetes and high blood pressure can damage kidneys • Also damaged due to infection or overdoses of pain meds • Treated with dialysis – A machine removes wastes from blood – Blood pumped through semipermeable tubes suspended in special fluid – Only “bad” stuff diffuses out – Takes 4 -6 hours, 3 x a week

Kidney Transplant • Blood type as well as several “markers” on cells must match reduces chance of rejection • No adverse health effects for donor • Recipient must take immune-suppressing drugs for life increases risk of infections and cancer