biology: excretion

• What is excretion?

  The process whereby metabolic waste products and toxic substances are removed from the body
• Why must metabolic waste products be excreted?

  They can harm the body if they accumulate to high concentrations
• What is an example of excretion in unicellular organisms?

  Diffusion
• How do lungs contribute to excretion in humans?

  Lungs excrete CO2 during expiration
• What is urea and how is it excreted?

  Urea is a product of deamination filtered out by kidneys and excreted in urine
• What are the three ways excess water is excreted in humans?

  Through sweating, expiration, and in urine
• How is bile pigment excreted?

  Through feces
• What is the function of the renal artery?

  It brings blood of higher urea concentration away the heart to the kidney
• What is the function of the renal vein?

  It carries blood of lower urea concentration away from the kidney to the heart
• How do kidneys function in the excretion process?

  They act as filters, removing unwanted substances from the blood
• What carries urine to the bladder?

  The ureter
• What is the role of the urethra?

  It carries urine outside of the body
• What is a nephron?

  The basic functional unit of the kidney that filters and removes waste substances from the blood to form urine
• What processes are involved in the formation of urine?

  Ultrafiltration and selective reabsorption
• What is ultrafiltration?

  The process where most of the blood plasma and dissolved substances are forced out of the glomerulus into the Bowman's capsule by high blood pressure
• How does blood enter and leave the glomerulus?

  Blood enters through the afferent arteriole and leaves through the efferent arteriole
• Why does the efferent arteriole generate high blood pressure in the glomerulus?

  Because its lumen diameter is smaller than the afferent arteriole’s lumen diameter
• What substances can pass through the basement membrane of the glomerular capillaries?

  Small molecules like glucose, amino acids, mineral salts, urea, toxins, and medicine
• What is selective reabsorption?

  The process where certain substances are reabsorbed from the filtrate back into the blood as they pass through nephrons
• How is water reabsorbed in the nephron?

  Water is reabsorbed via osmosis
• What substances are reabsorbed via diffusion and active transport?

  Glucose, amino acids, and some mineral salts
• What is the procedure for dialysis treatment?

  • Blood is drawn from the vein in the patient's arm into a partially permeable tube
  • Tube does not allow large substances (blood cells, platelets) to pass through, but allows small substances (waste products) to diffuse out
  • Tube enters machine bathed in dialysis fluid/dialysate
  • Dialysate contains zero waste products, allowing waste products from blood to diffuse out
  • Dialysate has equal concentration of useful substances as healthy blood, preventing diffusion of useful substances
  • Tubing is long, narrow, and coiled, increasing surface area to volume ratio, enhancing diffusion rate
  • Dialysate flows opposite direction to blood, maintaining concentration gradient for more waste products to diffuse out
  • Cleaned blood is returned via the vein in the patient's forearm
• Why are veins preferred for drawing blood during dialysis treatment?

  Veins are safer as they are closer to the surface and have low pressure, making it easier to stop bleeding
• How does the design of the dialysis machine enhance its efficiency?

  It uses a dialysate with zero waste products and equal concentration of useful substances, along with long, narrow, coiled tubing
• What is the significance of the dialysate flowing in the opposite direction to blood?

  It maintains a steep concentration gradient along the entire length of the tubing for more waste products to diffuse out at a higher rate
• What happens to cleaned blood after dialysis treatment?

  It is returned via the vein in the patient's forearm
• The kidneys are the main organs involved in excretion.
• Kidney function is to remove waste products from blood, maintain water balance, regulate pH levels, and produce hormones such as renin and erythropoietin.
• Nephron - basic functional unit of the kidney
• Each kidney has over one million nephrons that filter blood plasma.
• Glomerulus - network of capillaries that filters blood
• Bowman’s capsule – surrounds glomerulus
• Proximal convoluted tubule (PCT) – absorbs most nutrients and water
• Bowman’s capsule surrounds glomerulus and collects filtered fluid (filtrate)
• Loop of Henle – allows reabsorption of more water than PCT
• The filtrate contains useful molecules such as glucose, amino acids, vitamins, hormones, urea, salts, and water
• why are kidneys important?

  they act as excretory organs: excrete metabolic waste products such as urea, excess water and mineral salts in the form of urine

  they act as osmoregulators: regulate the solute concentration and water potential in our blood to maintain a constant water potential in the blood
• What hormone is involved in water regulation in the body?

  Antidiuretic Hormone (ADH)
• What happens when the water potential in blood plasma decreases?

  The hypothalamus detects the drop, and the pituitary gland releases more ADH.
• What effect does increased ADH have on the collecting duct?

  It increases the permeability of the cell walls to water.