what is excretion
process of removing metabolic waste products, toxic substances and excess substances from the body
why is excretion necessary
so that harmful substances will not build up in the body to cause harm
what are the 2 functions of kidney
1) excrete metabolic waste products such as urea, excess water and mineral salts
2) regulates water potential and solute concentration in the blood to maintain a constant water potential in the body
what are ureters
narrow tubes connecting the kidneys to the urinary bladder
what is the urinary bladder
elastic muscular bag that stores urine
what is the urethra
duct which urine passes from the bladder to the outside of the body
what are the 4 parts that make up a kidney nephron
1) Bowman’s capsule
2) proximal convoluted tubule
3) loop on Henlé
4) distal convoluted tubule
describe the process of ultrafiltration
- afferent arteriole being wider than the efferent arteriole creates a high blood pressure in the glomerulus to ensure a pressure difference
- blood plasma is forced of the glomerular blood capillaries into the Bowman’s capsule
- filtered blood plasma (glomerular filtrate) contains small, soluble substances forced out into the Bowman’s capsule
what are 3 adaptations of the glomerulus blood capillaries?
1) network of blood capillaries
- provides large surface area for ultrafiltration
2) one-cell thick walls with tiny pores
- shorter diffusion distance, allows small soluble substances to be filtered into the Bowman’s capsule
3) covered by a thin, partially permeable membrane (basement membrane)
- only allows very small, soluble molecules/ions to pass through
- impermeable to blood cells, platelets and large molecules (e.g. proteins)
what happens at the proximal convoluted tubule during selective reabsorption
- most water reabsorbed by osmosis
- most mineral salts reabsorbed by diffusion and active transport
- all glucose and amino acids reabsorbed by active transport
what happens at the loop of Henlé during selective reabsorption
- some water reabsorbed by osmosis
- some mineral salts reabsorbed by active transport
what happens at the distal convoluted tubule during selective reabsorption
- some water reabsorbed by osmosis
- some mineral salts reabsorbed by active transport
what happens at the collecting duct during selective reabsorption
- some water reabsorbed by osmosis
what is osmoregulation
maintenance of constant body water potential by controlling the water potential and solute concentration in the blood
*controlled by antidiuretic hormone (ADH)
*secreted/produced by hypothalamus, released into bloodstream by pituitary gland
what could happen if water potential in the blood is too high
water could enter the RBCs via osmosis and the RBCs would swell and may lyse
what could happen if water potential in the blood is too low
water could leave the RBCs via osmosis and the RBCs would shrink and crenate
what happens if the water potential in the blood is too high
- pituitary gland releases less antidiuretic hormone (ADH)
- cells in the walls of the collecting ducts become less permeable to water; less water reabsorbed from collecting ducts into blood capillaries
- larger volume, lower concentration of urine produced
- water potential of blood returns to normal
what happens if the water potential in the blood is too low
- pituitary gland releases more antidiuretic hormone (ADH)
- cells in the walls of the collecting ducts become more permeable to water; more water reabsorbed from collecting ducts into blood capillaries
- smaller volume, higher concentration of urine produced
- water potential of blood returns to normal
what are 3 causes for kidney failure
1) high blood pressure
2) diabetes
3) alcohol abuse
what is the process of kidney dialysis
- blood drawn from vein in patient’s arm is pumped through tubing in dialysis machine
- tubing bathed in a specially-controlled dialysis machine, walls of tubing partially permeable
- small molecules diffuse out of the tubing into the dialysis fluid; blood cells, platelets and large molecules (e.g. proteins) remain in the blood
what are 3 features of dialysis machines
1) dialysis tubing is narrow, long and coiled
- increases surface-area-to-volume ratio to speed up rate of exchange of substances between blood and dialysis fluid
2) blood flows in the opposite direction of dialysis fluid
- maintains concentration gradient for complete removal of urea from blood into dialysis fluid
3) dialysis fluid (37°C) has the same composition as blood but lacks nitrogenous waste
- prevents essential molecules (e.g. glucose and amino acids) from diffusing out of the blood into the dialysis fluid due to the lack of a concentration gradient
- absence of nitrogenous waste maximises concentration gradient for fast removal of such waste from blood via diffusion
- set at 37°C to prevent loss/gain of body heat
