LESSON 23 cortas

The urinary system has an endocrine function and produces hormones such as renin-angiotensin, erythropoietin, and prostaglandins.

The kidneys of domestic animals have a great variety of forms, such as smooth and bean-shaped (cat, dog, goat, and sheep); smooth, elongated, and flattened (pigs); smooth and heart-shaped (horse); and lobed and oval (bovines).

The urinary system regulates the volume and composition of body fluids, retains fluid components necessary to maintain homeostasis, and removes metabolic waste products, water excess, and electrolytes from the body.

The urinary system carries out the process of filtration of the blood, reabsorption, and secretion to form and remove urine. The urinary bladder acts as a store and intermittent evacuator for urine, while the ureters and urethra conduct urine.

The basement membrane in the glomerulus filtration barrier is composed of three layers: an outer and inner low electron density layer (lamina rara externa and interna), and a thick, electron-dense middle layer (lamina densa). It contains type IV collagen fibers, glycoproteins laminin in the lamina densa, and heparan sulfate and fibronectin in the laminae rara. The basement membrane functions as part of the glomerular filtration barrier, selectively filtering molecules based on size, charge, and blood pressure.

The proximal convoluted tubule is the longest segment of the nephron and is lined by simple cuboidal to low columnar epithelium with numerous microvilli (brush border), invaginations, and evaginations. Its cells contain large numbers of mitochondria, an endocytic apparatus, and a perinuclear Golgi complex and developed smooth and rough endoplasmic reticulum. The proximal convoluted tubule collects ultrafiltrate and is involved in absorption and excretion phenomena.

The proximal tubule is the longest segment of the nephron, consisting of the proximal convoluted tubule and proximal straight tubule. It is lined by simple cuboidal to low columnar epithelium with numerous microvilli, invaginations, and evaginations. The cells contain large numbers of mitochondria, an endocytic apparatus, and a perinuclear Golgi complex and developed smooth and rough endoplasmic reticulum. On the other hand, the distal tubule consists of two segments: a straight tubule (ascending) and a convoluted tubule, which form part of Henle's loop and are lined by a simple cuboidal epithelium. Both tubules are involved in absorption and excretion phenomena, with the proximal tubule being more complex in structure and function.

The macula densa is part of the juxtaglomerular apparatus and is located at the vascular pole of the renal corpuscle in the straight distal tubule. It plays a role in the regulation of renal blood flow and glomerular filtration rate by sensing the sodium chloride concentration in the distal tubule and signaling for changes in afferent arteriole resistance.

The thin tubule is a simple squamous epithelium and is the continuation of the straight proximal tubule in the renal medulla. It is involved in passive water reabsorption. The thick tubule, on the other hand, is part of the loop of Henle and is responsible for active reabsorption of sodium, chloride, and potassium.

The glomerular filtration barrier selectively filters molecules based on size, charge, and blood pressure. It consists of the filtration slits, basal lamina, and fenestrated endothelium, which collectively regulate the passage of substances into the urinary space. This selective filtration process is crucial for maintaining the composition of the blood and the formation of urine.

Cats have lipid droplets in the epithelial cells of the proximal convoluted tubule, while dogs have lipid droplets in the epithelial cells of the straight tubule. These lipid droplets serve as an energy reserve and may play a role in osmoregulation and metabolic adaptation in these animals.

Podocytes, with their underlying basement membrane, are separated from fenestrated capillary endothelium and play a crucial role in the filtration process by forming filtration slits and contributing to the maintenance of the glomerular filtration barrier. They help in the selective passage of substances from the capillaries to the urinary space.

The juxtaglomerular apparatus, located at the vascular pole of the renal corpuscle, plays a key role in the regulation of renal blood flow and glomerular filtration rate. It consists of the macula densa, granular cells, and extraglomerular mesangial cells, which collectively regulate blood pressure and the secretion of renin, thereby influencing systemic blood pressure and electrolyte balance.

The glomerular filtration barrier plays a crucial role in maintaining homeostasis by selectively filtering molecules based on size, charge, and blood pressure. This process allows for the retention of essential substances in the blood while facilitating the excretion of waste products and excess ions in the urine, thereby contributing to the regulation of body fluid volume, electrolyte balance, and acid-base equilibrium.

The structure and function of the basement membrane in the glomerulus filtration barrier are significant for the regulation of selective filtration. The three-layered composition and the presence of specific proteins and glycoproteins allow for the controlled passage of substances based on their size, charge, and blood pressure, thereby contributing to the maintenance of blood composition and the formation of urine.

Both the proximal tubule and distal tubule are involved in the processes of absorption and excretion in the nephron. The proximal tubule, with its complex structure and abundance of organelles, is responsible for the reabsorption of water, ions, and nutrients from the ultrafiltrate, as well as the secretion of certain substances into the tubular fluid. The distal tubule, with its role in the regulation of electrolyte balance and acid-base equilibrium, further modifies the tubular fluid before it enters the collecting duct.