Kidney Function: Excretion, Nephron Structure, and Urine Formation
Both secretion and reabsorption are performed depending on the concentration in the blood. Substances are deleted, acting as the excretory organs of expulsion and regulating the concentration of these substances. Examination of urine allows observation of the excretory agencies’ operation, but also controls the states of chemical equilibrium in the body.
Excretory Organ Types:
- Protonephridia: Typical of flatworms. Consist of a series of branching tubes, one end closed, located in flagellated cells called flame cells. Water and waste products enter the tubes from the animal’s internal environment and are pushed along by the flagella system. Reabsorbable useful substances and waste are eliminated through pores.
- Metanephridia: Present in annelids and molluscs. Tubular structures are open at both ends. One end communicates with the coelomic cavity funnel-shaped and ciliated (nephrostomes), and the other opens outward through a pore (nephridiopore).
- Antennal or Green Glands: Organs of crustaceans, located in the anterior cephalothorax. Consist of a blind sac, excretory tube, and a storage bladder, connected to the outside through a pore at the base of the antenna.
- Malpighian Tubes: Characteristic of insects. Thin tubes, closed at one end and connected to the digestive tract at the other. Waste products are dumped into the exterior with the remains of undigested foods.
- Kidneys: Vertebrates. Formed by a set of units called nephrons. Consist of a renal corpuscle, which filters blood plasma pressure, and a tubule of variable length, where reabsorption and secretion occur.
Nephron Types:
- Pronephros: Located in the vertebrate embryo and in primitive fish. Consists of a renal tubule ended in nephrostomes connected to the coelom and is close to the glomerulus.
- Mesonephros: Appears in fish and amphibians. The nephrostomes are stunted and do not communicate with the coelom, and the renal tubule ends in a cup-shaped outgrowth surrounding the glomerulus.
- Metanephros: Belongs to reptiles, birds, and mammals. The glomerulo nephrostomes disappears and is within a structure called Bowman’s capsule. The renal tubules and collecting ducts empty into the ureter.
Kidney Structure:
Parts:
- Renal Capsule: The outer layer, a membrane composed of fibrous connective tissue.
- Cortical Area: Has a granular appearance due to Malpighian corpuscles. Forms a continuous cover under the kidney capsule with extensions inland called renal columns.
- Core Area: Has a striated appearance, and renal columns divide it into sectors called renal pyramids.
- Renal Pelvis: The area that collects the urine.
Physiology of the Mammalian Nephron:
The kidneys of mammals are composed of over a million nephrons of the metanephros type, located in the cortex and renal medulla. Each nephron is made up of a Malpighian corpuscle, the proximal convoluted tubule, loop of Henle, and distal convoluted tubule, which is again in the cortical and empties into a collecting duct. Collecting ducts terminate in the renal pelvis. Each kidney has a ureter, which carries urine to the urinary bladder, where it is stored to be expelled to the outside.
Urine Formation:
The blood enters the kidney through the renal artery, which branches into arterioles, which, in turn, branch into capillaries that lead to the glomeruli. Capillaries are united to form venules, eventually leaving the kidney through the renal vein.
The formation of urine involves 3 processes:
- Filtration: Upon reaching the glomeruli, blood circulates through capillaries and is filtered inside the capsule due to blood pressure. This filtering is not selective and depends primarily on the size or molecular mass of substances. Proteins or cellular elements do not cross the walls of the capsule.
- Reabsorption: The initial urine contains a large amount of water. Only 1% of this initial urine is eliminated, and the rest is reabsorbed through tubules and returned to the blood capillary network around them. Besides a large quantity of water, glucose, amino acids, etc., are reabsorbed in the proximal tubule.
- Tubular Secretion: The passage of substances, mainly ions, from the blood into the glomerular filtrate. Serves to regulate the ionic content of the internal environment.