Excretory system
HOMEOSTASIS: Includes the processes by which organisms maintain their internal environment within a narrow range of conditions necessary for cells to function properly under changing environments.
These changes include:
•Temperature
•Water and salt concentrations
•Glucose concentrations
•pH
•Oxygen and carbon dioxide concentration.
The skin, the lungs and the kidneys are the main excretory systems in mammals. The skin and the kidneys eliminate wastes, ions and salts dissolved in water through sweat and urine, the lungs excrete carbon dioxide and water vapor.
Excretion is the process by which living things maintain their internal balance. All cellular processes produce wastes that can be harmful for the organism. In order to keep homeostasis, living things developed excretory systems to get rid of wastes and excess of substances.
Introductory Questions
1. How do animals maintain homeostasis?
2. Is it possible for a human to live without kidneys? Why?
3. How is blood filtered in our body?
4. Where do wastes such as urea come from?
5. Which are the stages involved in the process of excretion by the urinary system?
Structure and functions of the urinary system.
Animals have different systems to maintain equilibrium. Some Invertebrates such as sponges and cnidarians have speciallized cells called choanocytes or nematocysts that have flagella to produce a current of fluids to facilitate osmosis and active transport to keep osmolarity (water balance)
Flat worms arranged specialized cells called flame cells into a thin tissue forming tubes called protonephridia.
Arthropodes developed tube-like structures called Malphigian tubules that are in direct touch with the circulatory system, so they help filtrate plasma
Round worms developed structures called nephridia that are tubes surrounded by capillaries that filter plasma.
Vertebrates developed kidneys. Kideys have important functions in our body such as:
•Regulate blood levels of ions such as Na, K, Cl and Ca.
•Maintain pH of blood by regulating H and H2CO3
•Regulate water contents.
•Retain important nutrients such as glucose and aminoacids.
•Eliminate cellular waste such as urea.
•Secreting hormones.
Kidney Dissection
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Kidneys have structures similar to nephridia called nephrons. Nephrons are closely related to capillaries in the circulatory system to facilitate the process of filtering blood. This process is divided into three stages:
FILTRATION: Blood enters the Bowman’s capsule under relatively high pressure. 1/5 of plasma is filtered (water, glucose, vitamins, salts and urea).
REABSORPTION: Many substances are reabsorbed selectively in the walls of the proximal tubule and the distal tubule by osmosis and active transport.
The Loop of Henle keeps the high concentration of Cl and Na ions to ensure reabsorption of water in the distant tubule.
ELIMINATION: All collecting tubes from the pyramids meet at the renal pelvis where they transport urine. Then, it goes through a narrow tube called ureter that goes downwards to a sac-like structure called urinary bladder.
Contractions of ring-like muscles called sphincters force urine out of the body through a tube called urethra.
