# Urinary System of Man The human urinary system, also known as the excretory system, is responsible for filtering waste products from the blood and producing, storing, and eliminating urine. ## 1. Overview of the Urinary System **Components:** - **Kidneys (2):** Paired, bean-shaped organs that filter blood and produce urine. - **Ureters (2):** Tubes that carry urine from the kidneys to the urinary bladder. - **Urinary Bladder:** A muscular sac that stores urine. - **Urethra:** A tube that carries urine from the bladder to the outside of the body. ## 2. Structure of the Kidney The kidneys are the primary organs of the urinary system. ### Location and External Structure - **Location:** Lie on either side of the vertebral column, just below the liver in the peritoneal cavity. The *right kidney* is slightly lower than the left to accommodate the liver. - **Adrenal Glands:** Located on top of each kidney. - **Protective Layers (from outermost to innermost):** 1. **Renal Fascia:** A tough outer layer of connective tissue. 2. **Perirenal Fat Capsule:** A layer of fat that helps anchor the kidneys in place. 3. **Renal Capsule:** The innermost layer that directly covers the kidney. ### Internal Structure (Anatomy) The kidney is divided into three main internal regions. - **Renal Cortex:** The granular, outer region containing the nephrons (the kidney's functional units). - **Renal Medulla:** The middle region, containing multiple cone-shaped tissue masses called *renal pyramids*. - **Renal Columns:** Spaces between the pyramids through which blood vessels pass. - **Renal Papillae:** The tips of the pyramids, which point toward the renal pelvis. - **Renal Pelvis:** The central, funnel-shaped region located in the **hilum** (the concave side of the kidney). - It collects urine and channels it into the ureter. - Branches into *major calyces*, which further divide into *minor calyces* that receive urine from the renal papillae. ### Relationship between Structure and Function | Structure | Function | | :--- | :--- | | **Renal Capsule** | Protects the kidney from injury and connects it to surrounding tissues. | | **Renal Artery** | Supplies unfiltered blood to the kidneys (approx. 1.2 liters/minute at rest). | | **Renal Cortex** | Site of blood filtration (nephrons begin here) and production of the hormone erythropoietin (EPO). | | **Renal Medulla** | Contains most of the nephron tubules and collecting ducts, which concentrate and transport urine. | | **Renal Pelvis** | Collects urine from the calyces and funnels it into the ureter. | | **Renal Vein** | Carries filtered blood out of the kidneys and back to the heart via the inferior vena cava. | ## 3. Structure of a Nephron The nephron is the microscopic functional unit of the kidney responsible for forming urine. - **A. Renal Corpuscle:** The initial filtering component. - **Glomerulus:** A tangled network of capillaries where blood filtration begins. - **Bowman's Capsule:** A double-walled, cup-shaped structure that surrounds the glomerulus and collects the filtrate. - **B. Renal Tubule:** A long, coiled tube where the filtrate is processed into urine through reabsorption and secretion. 1. **Proximal Convoluted Tubule (PCT):** The first, highly coiled section after Bowman's capsule. 2. **Loop of Henle:** A U-shaped loop that dips into the medulla, consisting of a *descending limb* and an *ascending limb*. 3. **Distal Convoluted Tubule (DCT):** The final, highly coiled section. 4. **Collecting Duct:** Receives urine from multiple nephrons and carries it through the medulla to the renal pelvis. ## 4. Blood Circulation to the Nephron A specialized blood supply is crucial for the nephron's function. - **Path of Blood Flow:** 1. The **Renal Artery** branches into smaller arteries. 2. An **Afferent Arteriole** supplies blood *to* the glomerulus of each nephron. 3. Blood is filtered in the **Glomerulus**. 4. An **Efferent Arteriole** carries blood *away from* the glomerulus. 5. This arteriole branches into the **Peritubular Capillaries**, a network surrounding the PCT and DCT. 6. A specialized part of these capillaries, the **Vasa Recta**, runs alongside the Loop of Henle deep into the medulla. 7. The capillaries merge, and blood drains into the **Renal Vein**, which exits the kidney.