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The Excretory System - Get the Knowledge
1. What is excretion?
Excretion in Physiology is the process of elimination of metabolic wastes and other toxic substances from the body.
2. What are nitrogen wastes?
Nitrogen wastes are residuals derived from the degradation of proteins. They are made from chemical transformation of the amine group of amino acid molecules.
3. What are the three main types of nitrogen wastes excreted by living beings?
The main nitrogen wastes excreted by living beings are ammonia, uric acid and urea. Living beings that secrete ammonia are known as ammoniotelic. Creatures that secrete uric acid are known as uricotelic. Organisms that secrete urea are called ureotelic.
The Excretory System - Image Diversity: ammonia molecule uric acid molecule urea molecule
4. Why are most ammoniotelic beings aquatic animals?
Aquatic animals, like crustaceans, bony fishes and amphibian larvae, generally are ammoniotelic since ammonia diffuses more easily through membranes and it is more water-soluble than the other nitrogen wastes. Ammonia is still the most energetically economical nitrogen waste to be synthesized.
5. Why after the passage of animals from the aquatic to the terrestrial habitat does the abandonment of the ammoniotelic excretion occur?
Ammonia is a highly toxic molecule if not diluted and quickly excreted out of the body. For this reason the ammoniotelic excretion was abandoned in terrestrial habitats because the availability of water for dilution is reduced in this medium and wastes cannot be excreted so promptly to the exterior.
6. Comparing toxicity and the need for dilution in water how different are the ureotelic and the uricotelic excretions? What are some examples of animals that present these respective types of excretion?
Urea is more water-soluble than uric acid (an almost insoluble substance). Urea is also more toxic. Both however are less toxic than ammonia.
Some invertebrates, chondrichthian fishes, adult amphibians and mammals are ureotelic. Reptiles, birds and most arthropods are uricotelic.
7. What is the nitrogen waste in amphibian larvae and in the adult animal?
Since amphibian larvae are aquatic they excrete ammonia. The terrestrial adult excretes urea.
8. Why is the uricotelic excretion essential for avian and reptile embryos?
In reptiles and birds the excretory system is uricotelic since uric acid is insoluble, less toxic and suitable to be stored within the eggs where their embryos develop.
9. How do embryos of placental mammals excrete nitrogen wastes?
Placental animals, including embryos, excrete urea. In the adult placental mammal urea is excreted through the urine. In embryos the molecule passes to the mother’s blood through the placenta and it is excreted in the mother’s urine.
10. What is the main nitrogen waste of humans?
Human beings excrete mainly urea eliminated with the urine.
11. How is urea formed in the human body?
Urea is a product of the degradation of amino acids. In the process amino acids lose their amine group which is then transformed into ammonia. In the liver ammonia reacts with carbon dioxide to form urea and water, a process called ureogenesis.
In the intermediary reactions of the ureogenesis a molecule of ornithine is consumed and another is produced. For this reason ureogenesis is also known as the ornithine cycle.
The Excretory System - Image Diversity: ureogenesis
12. Which are the organs of the excretory system?
The excretory system is formed of kidneys (two), ureters (two), bladder and urethra.
The Excretory System - Image Diversity: human excretory system
13. What are the vessels that carry blood to the kidneys? Is this blood arterial or venous?
The arterial vessels that carry blood to be filtrated by the kidneys are the renal arteries. The renal arteries are ramifications of the aorta and so the blood filtered by the kidneys is arterial (oxygen-rich) blood.
The Excretory System - Image Diversity: kidneys
14. Which are the vessels that drain filtered blood from the kidneys?
The venous vessels that collect the blood filtered by the kidneys are the renal veins. The renal veins carry the blood that has been reabsorbed in the nephron tubules.
15. What is the functional unity of the kidneys?
The functional (filtering) unity of the kidneys is the nephron. A nephron is made of afferent arteriole, efferent arteriole, glomerulus, Bowman's capsule, proximal tubule, loop of Henle, distal tubule and collecting duct.
In each kidney there are about one million nephrons.
The Excretory System - Image Diversity: nephron
16. What are the three main renal processes that combined produce urine?
Urine is made by the occurrence of three processes in the nephron: glomerular filtration, tubular resorption and tubular secretion.
In the nephron the blood carried by the afferent arteriole enters the glomerular capillary network where it is filtered. The filtration implies that part of the blood returns to the circulation through the efferent arteriole and the other part, known as the glomerular filtrate, enters the proximal tubule of the nephron. In the nephron tubules (also known as convoluted tubules) substances of the glomerular filtrate like water, ions and small organic molecules are resorbed by the cells of the tubule wall and gain again the circulation. These cells also secrete other substances inside the tubules. The urine is formed of not resorbed filtered substances and of secreted (by the tubules) substances. Urine is drained by the collecting ducts to the ureter of each kidney, then it enters the bladder and later it is discharged through the urethra.
The nephron tubules are surrounded by an extensive capillary network that collects resorbed substances and provides others to be secreted.
17. What is the main transformation presented by the glomerular filtrate in comparison to the blood?
Glomerular filtrate is the name given to the plasma after it has passed the glomerulus and entered the Bowman’s capsule. The glomerular filtrate has a different composition compared to urine since the fluid has not yet undergone tubular resorption and secretion.
The main difference between the blood and the glomerular filtrate is that in the latter the amount of proteins is at a minimum and there are no cells or blood platelets.
18. What is proteinuria? Why is proteinuria a sign of glomerular renal injury?
Proteinuria means losing of proteins through urine. Under normal conditions proteins are too big to be filtered by the glomerulus and they are practically absent in the urine (the few filtered proteins may also be resorbed in the nephron tubules). Proteinuria is an indication that a more than expected amount of proteins is passing the glomerulus suggesting glomerular disease, e.g., in diabetic nephropathy.
The glomerulus also blocks the passage of blood cells and platelets (hematuria is often a sign of urinary disease although less specific of kidneys since the blood may come from the lower parts of the excretory tract).
The Excretory System - Image Diversity: glomerulus
19. Where does most of the water resorbed after glomerular filtration go? What are the other substances resorbed by the nephron tubules?
Only 0.5 to 1% of the glomerular filtrate is eliminated as urine. The remaining volume, containing mainly metabolic ions, glucose, amino acids and water, is resorbed through the nephron tubules (by means of active or passive transport) and gains the blood circulation again.
The convolute tubules of the nephron are responsible for the resorption of substances.
The Excretory System - Image Diversity: nephron tubules
20. Why do cells of the nephron tubules present a great amount of mitochondria?
The cells of the tubule wall have high number of mitochondria because many substances are resorbed or secreted through them by means of active transport (a process that spends energy). Therefore many mitochondria are necessary for the energetic supply (ATP supply) of this type of transport.
21. What is tubular secretion? What are some examples of substances secreted through the renal tubules?
Tubular secretion is the passage of substances from the blood capillaries that surround the nephron tubules to the tubular lumen for these substances to be excreted with urine. Ammonia, uric acid, potassium, bicarbonate and hydrogen ions, metabolic acids and bases, various ingested drugs (medicines) and other substances are secreted by the nephron tubules.
22. In which nephron portion does the regulation of acidity and alkalinity of the plasma occur?
The regulation of the acid-basic equilibrium of the body is done by the kidneys and depends upon the tubular resorption and secretion.
23. How do kidneys participate in the regulation of the acid-basic equilibrium of the body? How are alkalosis and acidosis respectively corrected by the kidneys?
Kidneys can regulate the acidity or alkalinity of the plasma varying the excretion of hydrogen and bicarbonate ions.
In alkalosis (abnormally high level of the plasma pH) the kidneys excrete more bicarbonate and the equilibrium of formation of bicarbonate from water and carbon dioxide shifts towards formation of more hydrogen ions and bicarbonate and then the plasma pH is lowered. When the body undergoes acidosis (abnormal low level of the plasma pH) the kidneys excrete more hydrogen ions and retain more bicarbonate thus the equilibrium of formation of bicarbonate from water and carbon dioxide shifts towards more hydrogen consumption and the plasma pH is increased.
24. How do kidneys participate in the blood volume control? How is the blood volume of the body related to the arterial pressure?
The kidneys and the hormones that act upon them are the main physiological regulators of the total blood volume of the body. As more water is resorbed in the nephron tubules the more the blood volume increases; as more water is excreted in urine the more the blood volume lowers.
The blood volume in its turn has a direct relation to blood pressure. The blood pressure increases when the blood volume increases and it lowers when the blood volume lowers. That is the reason why one of the main groups of antihypertensive drugs is the diuretics. Doctors often prescribe diuretics for the hypertensive patients to excrete more water and thus lower their blood pressure.
25. Which are the three hormones that participate in the regulation of the renal function?
Antidiuretic hormone (or ADH, or vasopressin), aldosterone and atrial natriuretic factor (or ANF) are hormones that participate in the regulation of the excretory system.
26. What is the function of the antidiuretic hormone? Where is it made and which are the stimuli that increase or reduce its secretion?
The antidiuretic hormone is secreted by the hypophysis (also known as pituitary) and it acts in the nephron tubules increasing the resorption of water. When the body needs to retain water, for example, in cases of blood loss and abrupt blood pressure lowering or in cases of abnormally high blood osmolarity, there is stimulus for ADH secretion.
When the body has an excess of water, as in cases of excessive ingestion or in abnormally low blood osmolarity, the secretion of ADH is blocked and the diuresis increases. ADH is also known as vasopressin since it increases the blood volume and thus heightens the blood pressure.
27. Why does the ingestion of alcohol increase diuresis?
Alcohol inhibits the secretion of ADH (antidiuretic hormone) by the pituitary. That is why when it is drunk to excess the person urinates too much.
28. How does aldosterone act and where is it produced?
Aldosterone is a hormone that acts upon the nephron tubules stimulating the resorption of sodium. Therefore it contributes to the increase of the blood osmolarity and consequently to the increase of the blood pressure.
Aldosterone is made by the adrenals, glands located over the superior portion of the kidneys.
29. What is an evolutionary explanatory hypothesis for the secretion by the heart of a hormone that regulates the renal function? Which is that hormone?
The renal regulator hormone secreted by the heart is the atrial natriuretic factor (or ANF). The ANF increases the excretion of sodium in the nephron tubules causing less resorption of water, more urinary volume, and thus lowering the blood pressure. The atrial natriuretic factor is secreted when there is an increase of the length of the heart muscle fibers in response to high blood pressure. The ANF is a natural antihypertensive substance. Since the health of the heart depends largely upon the stability of the normal blood pressure the evolution should have preserved the atrial natriuretic factor to allow information from the heart to be an additional mechanism for the renal control of the blood pressure.
30. What is hemodialysis?
Hemodialysis is the artificial blood filtration made by specific machines in substitution of the kidneys. Hemodialysis may be necessary in patients suffering from diseases that cause renal failure, like diabetic renal complications, lupic renal complications and others. During hemodialysis the blood of the patient is deviated to the filtering machine and after the filtration it returns to the body.
Hemodialysis is generally done two, three or more times a week in a process that takes several hours. Sometimes kidney transplantation is an alternative to hemodialysis.
The Excretory System - Image Diversity: hemodialysis
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