The Kidney (11.3) Flashcards
Define excretion.
The removal of waste products of metabolic pathways from the body.
What are the functions of an excretory system?
- To remove nitrogenous waste that may be toxic to the body in large concentrations.
- To remove excess water to maintain a suitable osmolarity within the tissues and cells.
Define osmoregulation.
The control of the water balance of the blood, tissue or cytoplasm of a living organism.
How are waste products lost in plants?
Gases are eliminated through the stomata, pores or surface of the leaves.
How are waste products lost in aquatic animals?
Their nitrogenous waste is excreted as ammonia directly into the external environment. Ammonia is toxic but also highly soluble and there is ample water available for dilution.
How are waste products lost in terrestrial animals? (Birds, insects and mammals).
Terrestrial animals have less access to water so their nitrogenous waste must be packaged in less toxic forms.
Birds excrete uric acid (a white paste). This is metabolically more expensive but is relatively non-toxic and takes less water to flush.
Insects use malpighian tubules to excrete metabolic waste as uric acid.
Mammals excrete urea, which is combined with water and other ions to form urine.
What are the main excretory processes in humans?
The formation of urine (urea, water, excess salts and minerals) in the kidneys. Expelled in urination.
The formation of carbon dioxide as a result of respiration. Exhaled by the lungs (exhalation).
Perspiration is another excretory process which removes salts and water from the body but its primary purpose is cooling.
Why is defecation not considered an excretory process?
Because faeces are undigested food remnants and not metabolic waste products.
What is the order of toxicity for nitrogenous compounds?
Ammonia > Uric acid > Urea
Define osmoconformers and osmoregulators
Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment. This minimises water movement into and out of cells.
Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions. This is a more energy intensive process.
What is the function of the kidney?
To remove waste products and excess fluid from the body by producing urine.
Define the nephron.
The functional unit of the kidney. Filters the blood and eliminates waste as urine.
What are the three processes by which blood is filtered in the kidneys and urine is formed?
1) Ultrafiltration - nonspecific filtration of the blood.
2) Selective reabsorption - reuptake of useful substances from the filtrate.
3) Osmoregulation - the control of the water balance in the blood, tissues and cytoplasm.
Describe ultrafiltration.
Ultrafiltration occurs in the glomerulus and the Bowmans Capsule. The capillaries in the glomerulus are fenestrated. Fluid can pass through these gaps. On the outside of the capillary wall there is a basement membrane. This is a mesh-like structure which acts as a sieve, allowing small molecules to pass through. Large molecules like protein remain in the blood. Blood in the glomerulus is at a high pressure. This forces small molecules out of the blood into the Bowmans Capsule. The filtrate produced flows on to the proximal convoluted tubule.
Describe reabsorption.
As the filtrate passes along the nephron many substances (around 90% of the filtrate) are reabsorped back into the blood. This occurs mainly in the proximal convoluted tubule but also in the loop of Henle and the distal convoluted tubule.
