chap 7 excretion in humans

• what are some examples of metabolic waste products ?

  Carbon dioxide, urea, mineral salts or ions, water
• what are some of the harms of having abnormally high levels of carbon dioxide in the body ?

  it is a condition called hypercabia and the patient would suffer from headache, confusion, rapid breathing and premature heartbeats
• what are some of the harms of having abnormally high levels of urea in the body ?

  it may cause abdominal pain, nausea or vomiting. it may even result in an irregular heartbeat and muscle cramps
• what are some of the harms of having abnormally high levels of mineral salts or ions in the body ?

  it may lower the water potential in blood plasma, causing water to be passed out of the tissue cells into the blood by osmosis, causing cells to become dehydrated and plasmolysed
• what are some of the harms of having abnormally high levels of water in the body ?

  it will increase the water potential in the blood plasma. water will enter the tissue cells by osmosis, causing the cells to swell and burst
• what is excretion ?

  it is the removal of metabolic waste products, toxic substances and substances in excess of the body's requirements
• where is carbon dioxide excreted and as what ?

  lungs as gas in expired air
• where is excess mineral salts excreted and as what ?

  kidney as a constituent of urine and the skin as constituent of sweat
• where is excess water excreted and as what ?

  kidney as a constituent of urine and the skin as constituent of sweat
• where is excess urea excreted and as what ?

  kidney as a constituent of urine and the skin as constituent (traces) in sweat
• the left kidney is slightly higher than the right kidney
• what is the main function of the kidneys ?

  to excrete urea and excess salt and water as urine
• what are the four main parts of a nephron ?

  bowman's capsule, proximal convoluted tubule, loop of henle and distal convoluted tubule
• what blood capillaries are around the nephron ?


  • blood enters the kidneys by the renal artery and leaves the kidney by the renal vein
  • a small branch of the renal artery called the afferent arteriole carries the blood into the glomerulus
  • glomerulus is a knot of blood capillaries in the bowman's capsule
  • an efferent arteriole transports blood away from the glomerulus
  • blood continues into the blood capillaries surrounding the nephron and eventually is lead into a branch of the renal vein
• what are the stages of urine formation ?

  ultrafiltration and selective reabsorption
• describe the process of ultrafiltration

  • the afferent arteriole is wider than the efferent arteriole, creating a high blood pressure in the glomerulus
  • blood plasma is forced out of the glomerular blood capillaries into the bowman's capsule
  • the glomerular filtrate contains small, soluble molecules like glucose, amino acids and urea that are forced out into the bowman's capsule
• what are some adaptations of the glomerulus ?


  • is a knot or network of capillaries
  • provides a large surface area to volume ratio for the filtration process
  • blood capillary walls are one- cell thick
  • provides a short diffusion distance for more efficient filtration
  • blood capillaries are covered by a thin partially permeable membrane and there are tiny pores in the capillary walls
  • allows very small soluble molecules or ions to pass through and is impermeable to blood cells, platelets and large molecules such as proteins
• describe the process of selective reabsorption

  • as the glomerular filtrate passes along the nephron, useful substances are reabsorbed into the surrounding blood capillaries
  • most of the water is reabsorbed by osmosis
  • some salts are reabsorbed by active transport
  • all the glucose and amino acids are reabsorbed by active transport
  • waste products such as urea and excess water and mineral salts are passed out of the nephron as urine
• what is selectively reabsorbed at the proximal convoluted tubule ?

  • most of the water is reabsorbed by osmosis
  • all of the glucose and amino acids are reabsorbed by active transport
  • most of the mineral salts are reabsorbed by diffusion and active transport
• what is selectively reabsorbed at the loop of henle ?


  • some water is reabsorbed by osmosis
  • some mineral salts are reabsorbed by active transport
• what is selectively reabsorbed at the distal convoluted tubule ?

  • some water is reabsorbed by osmosis
  • some mineral salts are reabsorbed by active transport
• what is selectively reabsorbed at the collecting duct ?

  • some water is reabsorbed by osmosis
• what is osmoregulation ?

  it is the control of water potential and solute concentration in the blood to maintain a constant water potential in the body
• what hormone is secreted for osmoregulation and where ?

  antidiuretic hormone (ADH) is produced in the hypothalamus and is released by the pituitary gland
• describe the process of osmoregulation when the water potential of blood plasma decreases

  • the decrease in water potential of blood plasma stimulates the hypothalamus to send impulses to the pituitary gland to release more ADH into the bloodstream
  • the ADH causes the cells in the walls of the collecting ducts to become more permeable to water
  • more water is selectively reabsorbed from the collecting duct into the blood capillaries
  • this cause the urine excreted to be of a smaller volume and more concentrated
  • water potential of blood returns to normal
• describe the process of osmoregulation when the water potential of blood plasma increases

  • the increase in water potential of blood plasma stimulates the hypothalamus to send impulses to the pituitary gland to release less ADH into the bloodstream
  • the decrease in ADH causes the cells in the walls of the collecting ducts to become less permeable to water
  • less water is selectively reabsorbed from the collecting duct into the blood capillaries
  • this cause the urine excreted to be of a larger volume and more diluted
  • water potential of blood returns to normal
• what are some causes of kidney failure ?

  • high blood pressure
  • diabetes
  • alcohol abuse
  • severe accidents that physically damage the kidney
  • complications from undergoing major surgery
• how does a dialysis machine work ?

  • blood is drawn from the vein in the patient's arm and is allowed to be pumped throughout the tubing in the dialysis machine
  • the tubing is bathed in a specially controlled dialysis fluid
  • small molecules such as urea and other metabolic waste products diffuse out of the tubing, through the partially permeable membrane, into the dialysis fluid
  • blood cells, platelets and large molecules such as proteins remain in the tubing
  • the filtered blood is then returned to a vein in the patient's arm
• what are some features of dialysis fluid ?

  • contains the same concentration of essential substances as healthy blood
  • ensures essential substances like glucose, amino acids and mineral salts, do not diffuse out of the blood and into the dialysis fluid
  • if the patient's blood lacks these essential substances, they will diffuse from the dialysis fluid into the blood
  • does not contain metabolic waste products
  • sets up a concentration gradient that allows waste products, such as urea, and excess water and mineral salts to diffuse out of the tubing into the dialysis fluid, for removal
• what are some other features of a dialysis machine ?

  • tubing is narrow, long and coiled
  • increases the surface area to volume ratio which helps to speed up the rate of exchange of substances between the patient's blood and the dialysis fluid
  • direction of the blood flow is opposite to the flow of the dialysis fluid
  • maintains the concentration gradient for the removal of waste products