IBDP BIO - AHL D3.3 Homeostasis

excretion

• process of metabolic waste removal from an organism

• performed by the excretory system (composed of kidneys, lung, skin, large intestine, liver)

• mammals excrete nitrogen waste in the form of urea

describe the composition of urine

water + urea

osmoregulation

• the process of regulating osmotic regulation

  • water is constantly being gained or loss and needs to be regulated in the body

osmotic concentration

concentration of solute particles per unit volume (measurement unit: osmoles per litre)

describe the process of excretion in the kidneys

• blood enters via renal artery

• blood is filtered (removal of metabolic waste) and exits via renal vein

• urine exits renal pelvis via ureter towards bladder

state the following structures of this kidney

cortex, medulla, renal artery, renal vein, renal pelvis, ureter

nephron

specialised functional units that perform filtration, excretion, osmoregulation

label the structure of a nephron

bowman’s capsule, proximal convolute tubule (PCT), collecting duct, vasa recta, loop of henle, renal vein, glomerulus

describe the functions of the nephron

• ultrafiltration (filters out good useful materials, e.g. minerals and vitamin)

• selective reabsorption (reuptake of useful materials)

• osmoregulation (balance solute concentration)

describe the movement of blood in the bowman’s capsule

• filtration of blood occurs under high pressures in the glomerulus (capillary network)

• blood enters via afferent arterioles and exits via efferent arterioles

• bowman’s capsule collects the filtrate (solutes filtered out from blood plasma)

distinguish between an afferent and an efferent arteriole.

afferent arteriole has a large and wide lumen, facilitating entry of blood, and efferent arterioles has a narrow and small lumen, creates high hydrostatic pressure within the glomerulus

• difference in diameter creates high pressure for ultrafiltration

podocytes

cells of the epithelial lining (lining of a surface) of bowman’s capsules

• wraps around capillaries and form filtration slits

• high pressure within capillaries (porous) - size selective

• prevents red and white blood cells and other large proteins from entering filtrate

• allows solutes such as urea, toxins, amino acids, salts, glucose and water into filtrate

epithelium

outer lining of a surface or inner lining of a cavity

endothelium

inner lining of blood vessels

ultrafiltration

• blood enters glomerulus via the afferent arteriole and exits via the efferent arteriole

• the difference in diameter creates high pressure within glomerulus - ultrafiltration of solutes from blood into filtrate

• capillaries are fenestrated

• podocytes wrap around capillaries and form filtration slits to prevent blood cells from entering filtrate

• filtrate collected in bowman’s capsule and continues to proximal convoluted tubule

reabsorption of selective solutes

• indirect active transport, and requires membrane proteins in the proximal convoluted tubule wall

• sodium potassium pumps actively transport Na+ out of the PCT into blood, helps maintain low concentration of Na+ in PCT cell

• sodium diffuses passively from filtrate into PCT cell through the cotransporter proteins, also transports glucose and amino acids from filtrate

• passive diffusion of glucose and amino acids from PCT cell back into blood

• water moves passively via osmosis

adaptations of the PCT structure

contains many mitochondria to power active transport, PCT wall is only one cell thick to decrease diffusion distance from filtrate to blood, PCT cells are connected by tight junctions to prevent leaking of larger materials, tight junctions adhere with cells and decreases the chances of solutes leaking into the blood stream, microvilli to increase surface area to maximise diffusion and reabsorption

effects of filtrate during osmoregulation

filtrate from the PCT travels down descending limb of loop of henle then back up ascending limb, active transport of sodium ions out of filtrate in ascending limb, active transport of sodium ions out of filtrate in ascending limb into fluid filled medulla region, creates high osmotic concentrations in medulla, filtrates continues to collecting duct

collection of urine during osmoregulation

final reabsorption of water from filtrate in collecting ducts, water passively exits filtrate into medulla through aquaporin channels, high Na+ concentration in medulla maintained by ascending limb, concentrated filtrate collects in renal pelvis and exits kidney via ureter into bladder

osmoreceptors

located in the hypothalamus, detects changes in solute concentration in blood

functions of the osmoreceptors

dehydration or high sodium levels result in high solute concentration in blood detected by osmoreceptors to stimulate release of antidiuretic hormone from pituitary gland, increases the number of aquaporins present in collecting duct membrane, higher water reabsorption from filtrate

increase in ADH levels

vesicles fuse with collecting duct cell membrane, aquaporins insert into cell membrane, water reabsorbed from filtrate

decrease in ADH levels

cell membrane invaginates, aquaporins stored back into vesicles