(i) Excretion
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
2.70Metabolic waste gasesUnderstand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf.2.71Excretory products of lungs, kidneys and skinKnow the excretory products of the lungs, kidneys and skin (organs of excretion).272B Kidney excretion and osmoregulationUnderstand how the kidney carries out its roles of excretion and osmoregulation.273B Urinary system structureDescribe the structure of the urinary system, including the kidneys, ureters, bladder and urethra.274B Nephron structureDescribe the structure of a nephron, including the Bowman’s capsule and glomerulus, convoluted tubules, loop of Henle and collecting duct.275B UltrafiltrationDescribe ultrafiltration in the Bowman’s capsule and the composition of the glomerular filtrate.276B Water reabsorption from the collecting ductUnderstand how water is reabsorbed into the blood from the collecting duct.277B Selective reabsorption of glucoseUnderstand why selective reabsorption of glucose occurs at the proximal convoluted tubule.278B ADH and blood water contentDescribe the role of ADH in regulating the water content of the blood.279B Urine compositionUnderstand that urine contains water, urea and ions.
Release metabolic waste gases from leaves
Leaf metabolism can produce carbon dioxide and oxygen faster than the plant uses them; the excess gases diffuse out through stomata.
| Metabolic process | Gas produced | When it becomes a waste product |
|---|---|---|
| respiration | carbon dioxide | production exceeds its use in photosynthesis, especially in darkness |
| photosynthesis | oxygen | production exceeds its use in respiration, commonly in sufficient light |
Each gas moves from a higher concentration in the leaf air spaces to a lower concentration outside. Stomata provide the pore through which this diffusion occurs.
Plants respire in both light and darkness. Calling oxygen or carbon dioxide a waste product depends on net production at that time; neither gas is always waste to the plant.
Match excretory organs to their products
Excretion removes waste products of metabolism and substances present in excess; the lungs, kidneys and skin remove different mixtures.
| Organ | Excretory products | Route out |
|---|---|---|
| lungs | carbon dioxide and water vapour | exhaled air |
| kidneys | urea, excess water and excess mineral ions | urine |
| skin | water, mineral ions and a small amount of urea | sweat |
Excretion is not egestion: faeces contain mainly unabsorbed food leaving the gut, whereas excretory products were made by metabolism or are excess internal substances.
Use kidneys for excretion and osmoregulation
Kidneys clean the blood by excreting urea and regulate blood water and ion content by adjusting what the nephrons return to the blood.
| Nephron process | Contribution |
|---|---|
| ultrafiltration | forces water and small dissolved substances out of glomerular blood |
| selective reabsorption | returns all useful glucose and required ions from filtrate to blood |
| variable water reabsorption | returns more or less water according to the body's water balance |
| urine formation | leaves urea plus excess water and ions for removal |
Osmoregulation maintains a suitable blood water concentration. Dehydration causes more water reabsorption and a smaller volume of concentrated urine; excess body water causes less reabsorption and a larger volume of dilute urine.
Excretion removes metabolic waste such as urea; osmoregulation controls water and ion balance. They occur in the same organ but are not identical functions.
Trace urine through the urinary system
The urinary system makes urine in two kidneys, carries it through two ureters, stores it in the bladder and releases it through the urethra.
| Structure | Direction and function |
|---|---|
| renal artery | brings unfiltered blood to each kidney |
| kidney | filters blood and forms urine in nephrons |
| renal vein | returns adjusted blood from each kidney |
| ureter | carries urine from one kidney to the bladder |
| bladder | stores urine temporarily |
| urethra | carries urine from the bladder out of the body |
A ureter connects a kidney to the bladder; the urethra leaves the bladder. Blood enters by the renal artery, but urine never travels through a blood vessel.
Follow filtrate through a nephron
A nephron is the kidney's microscopic processing unit; its connected regions filter blood, recover useful substances and form urine.
| Route through nephron | Main structural relationship |
|---|---|
| 1. glomerulus inside Bowman's capsule | capillary knot where filtrate is forced into the capsule |
| 2. proximal convoluted tubule | first coiled region leaving the capsule |
| 3. loop of Henle | long U-shaped section extending into the kidney medulla |
| 4. distal convoluted tubule | second coiled region returning toward the cortex |
| 5. collecting duct | receives fluid from nephrons and carries it toward the renal pelvis and ureter |
The glomerulus is a blood-capillary network; Bowman's capsule surrounds it and receives filtrate. The collecting duct is downstream of the convoluted tubules and loop, not part of the blood supply.
Filter small molecules at Bowman's capsule
Ultrafiltration occurs when high pressure in the glomerulus forces water and small dissolved substances through the filtration barrier into Bowman's capsule.
| Substance in blood | Enters glomerular filtrate? | Reason |
|---|---|---|
| water, glucose, urea and mineral ions | yes | small enough to pass the filtration barrier |
| blood cells and large plasma proteins | no | too large to cross and remain in the blood |
The glomerulus has high hydrostatic pressure, helped by blood entering through a wider arteriole than the one leaving. The filtrate collected in Bowman's capsule then enters the proximal convoluted tubule.
Ultrafiltration is non-selective among small molecules: useful glucose enters the filtrate alongside urea. Selective recovery happens later, so filtrate and urine do not have the same composition.
Reabsorb water from the collecting duct
Water moves from fluid in the collecting duct into the surrounding kidney tissue and then the blood by osmosis.
The medulla around the collecting duct has a lower water potential because it contains a high concentration of dissolved ions. When the duct wall is permeable, water moves down this water-potential gradient and is carried away by capillaries.
| Water reabsorbed | Urine outcome |
|---|---|
| more | smaller volume and higher concentration |
| less | larger volume and lower concentration |
Water is reabsorbed by osmosis, not active transport. The amount depends on collecting-duct permeability, which ADH regulates in the next control step.
Recover glucose in the proximal tubule
Glucose enters glomerular filtrate because it is small, but a healthy kidney selectively reabsorbs it from the proximal convoluted tubule into the blood.
Glucose is a useful respiratory substrate, so losing it in urine would waste chemical energy. Proximal-tubule cells use active transport, requiring energy from ATP, to move glucose from filtrate even against its concentration gradient; it then returns to nearby capillaries.
Selective reabsorption is different from ultrafiltration: filtration lets small molecules leave blood without deciding whether they are useful, while reabsorption retrieves specific useful substances. Normally, no glucose remains in urine.
Regulate blood water content with ADH
ADH regulates blood water content by changing the permeability of collecting ducts and therefore how much water is reabsorbed.
| Blood state | Control response | Kidney effect | Urine |
|---|---|---|---|
| too little water; blood too concentrated | hypothalamic osmoreceptors stimulate more ADH release from the pituitary | collecting ducts become more permeable; more water returns to blood | small volume, concentrated |
| too much water; blood too dilute | less ADH is released | collecting ducts become less permeable; less water returns | large volume, dilute |
As blood concentration returns toward normal, the original stimulus falls and ADH release adjusts. This negative feedback prevents uncontrolled correction.
ADH is released from the pituitary and acts on collecting ducts; it does not add water to urine or filter urea. More ADH produces less urine, not more.
Identify what urine contains
Urine contains water, urea and mineral ions that remain after filtration and selective reabsorption.
| Component | Why it remains |
|---|---|
| water | the amount not required to maintain blood water balance |
| urea | nitrogen-containing metabolic waste made in the liver and excreted by kidneys |
| mineral ions | quantities present in excess of the body's needs |
Urine volume and concentration vary with water balance: reabsorbing more water concentrates the urea and ions left in the tubule.
Healthy urine normally lacks blood cells, large proteins and glucose. Urine is not simply filtered blood: useful substances and much water have been reabsorbed before it reaches the bladder.