Vertebrate Excretory System: Kidney, Nephron, and Urine Production
Excretory System in Vertebrates
The excretory system in vertebrates is crucial for eliminating waste products and maintaining internal balance. Among the various excretory organs, the kidneys are the most important, composed of numerous nephrons where urine (a liquid composed of water and waste substances) is formed.
Mammalian Kidney and Nephron Anatomy
In mammals, there are two kidneys located in the abdominal cavity, each approximately 10 cm long. These are highly vascular organs; blood enters through the renal artery and exits through the renal vein.
In a longitudinal section of a kidney, from outside to inside, one can distinguish:
- Renal Capsule: The outermost layer of connective tissue.
- Renal Cortex: Has a granular appearance due to the presence of Malpighian corpuscles.
- Renal Medulla: Appears striated and is divided into sections called renal pyramids.
- Renal Pelvis: The area where urine is collected.
The nephron is the physiological unit of the kidney, located in both the cortex and the medulla. Each kidney contains over a million nephrons, all surrounded by a vast vascular system.
Nephron Structure: Key Components
- Bowman’s Capsule: Located in the renal cortex, it consists of a double wall of epithelial cells encircling a mass of capillaries called the glomerulus. The Bowman’s capsule and glomerulus together form a single structure known as the Malpighian corpuscle.
- Proximal Tubule: Located in the renal cortex, it follows the Bowman’s capsule.
- Loop of Henle: A continuation of the proximal tubule, it penetrates into the renal medulla and has two parts: a descending limb and an ascending limb.
- Distal Convoluted Tubule: Following the Loop of Henle, it is located in the renal cortex. Along with others from different nephrons, it flows into a collecting tubule.
Other Excretory Organs in Vertebrates
While kidneys are primary, other organs also play a role in excretion:
- Sweat Glands: Their function is to form sweat, which not only eliminates waste substances but also regulates ionic concentration and body temperature.
- Liver: Bile pigments, including bilirubin from hemoglobin degradation, are metabolized in the liver. The resulting waste products are removed through the intestine and urine.
Urine Formation in Vertebrates: Physiology
The formation of urine within the nephron is a complex process consisting of three main stages:
- Glomerular Filtration: This occurs in the Bowman’s capsule due to pressure differences between the capillaries of the glomerulus and within the capsule. The higher pressure in the glomerular capillaries results from the afferent arteriole (blood inlet) being larger than the efferent arteriole (blood outlet). As a result of this pressure, a liquid called the filtrate leaks into the Bowman’s capsule. This filtrate contains both waste and usable substances.
- Tubular Reabsorption: In each section of the nephron, various substances are reabsorbed back into the blood through the numerous capillaries that surround it. Reabsorption mechanisms can occur by diffusion or, for certain substances, by active transport.
- Tubular Secretion: This is a process by which some substances pass by diffusion or active transport from the capillaries surrounding the nephron into the tubule, especially within the distal tubule. This secretion is important in maintaining the concentration of certain ions.
At the end of this process, urine is formed. It passes from the nephron’s collecting tubules, which merge and dump the urine into the renal pelvis. From there, it travels through the ureter to the bladder, where it is stored until it is removed from the body via the urethra.