1. Nephrons: These are the functional units of the kidneys. They are tiny tubes that filter blood and produce urine.
* Glomerulus: A network of capillaries within the nephron where filtration occurs. The high pressure forces water, small solutes, and waste products out of the blood and into the nephron.
* Bowman's capsule: A cup-shaped structure surrounding the glomerulus that collects the filtered fluid (filtrate).
* Proximal convoluted tubule (PCT): The first section of the nephron where reabsorption of essential substances (like glucose, amino acids, water, and electrolytes) occurs.
* Loop of Henle: A hairpin-shaped loop that descends into the medulla of the kidney and then ascends back up. It plays a crucial role in concentrating the urine by reabsorbing water.
* Distal convoluted tubule (DCT): The final segment of the nephron where further reabsorption and secretion of ions and other substances occur.
* Collecting duct: A larger tube that collects urine from multiple nephrons and transports it to the renal pelvis and ureter.
2. Epithelial cells: These cells line the nephron tubules and other structures of the excretory system. They have specialized functions based on their location:
* Podocytes: Cells in Bowman's capsule that form filtration slits, allowing the passage of small molecules but preventing larger proteins from entering the nephron.
* Proximal convoluted tubule epithelial cells: Specialized for reabsorption, with microvilli to increase surface area.
* Loop of Henle epithelial cells: Have varying permeabilities to water and solutes, allowing for the concentration gradient necessary for water reabsorption.
* Distal convoluted tubule epithelial cells: Involved in both reabsorption and secretion of ions, depending on the body's needs.
3. Smooth muscle: Found in the walls of the ureters, bladder, and urethra. It helps propel urine through the system.
4. Blood vessels: The excretory system is highly vascularized, with arteries, veins, and capillaries supplying the nephrons with blood for filtration and removing waste products.
5. Connective tissue: Provides structural support for the kidneys and other organs within the system.
Functions of the Excretory System:
* Waste removal: Filters metabolic waste products (urea, creatinine, uric acid) from the blood and eliminates them in urine.
* Fluid balance: Regulates the amount of water in the body by reabsorbing water and excreting excess water.
* Electrolyte balance: Regulates the levels of electrolytes (sodium, potassium, calcium, etc.) in the blood.
* Blood pressure regulation: Produces hormones (renin, erythropoietin) that influence blood pressure and red blood cell production.
The excretory system relies on a complex interplay of specialized cells, tissues, and organs to maintain homeostasis within the body.
