10 topics with brief reviews: definitions, key facts, and class questions.
The urinary system is the primary filter of our body. In this section, we will examine the microscopic structure of the organs responsible for metabolite excretion and homeostasis maintenance. The central topic of the section is the kidney and its structural and functional unit, the nephron.
We will consistently cover the stages of urine formation, the detailed structure of the renal corpuscle and renal tubules. Special attention is paid to the unique renal blood supply (the "miraculous net"), as well as the renal endocrine system, including the structural components of the juxtaglomerular apparatus (JGA). In addition, you will find materials on the embryonic development of the urinary system and the structure of the urinary tract walls.
In practical classes and exams, the structure of the filtration barrier in the renal corpuscle and the mechanisms of the JGA are most frequently questioned, along with requests to identify parts of the nephron on a histological slide. Understanding these microscopic structures is the key to successfully passing histology and understanding the pathogenesis of renal diseases.
How does the fibrous capsule of the kidney differ from other connective tissue capsules?
Its histological composition contains smooth muscle myocytes. Their function is to maintain intrarenal pressure for plasma filtration and to assist in urine excretion.
How to quickly distinguish the renal cortex from the medulla under a microscope?
The main marker is the presence of renal corpuscles (capsules with glomeruli). They are located exclusively in the cortex. If there are no corpuscles in the field of view, you are looking at the medulla.
Why is high pressure maintained in the glomerular capillaries?
The efferent arteriole is anatomically noticeably narrower than the afferent arteriole. This creates resistance to outflow and increases the pressure in the glomerulus to 50–60 mm Hg, which is necessary for plasma filtration.
What is the difference between primary and final urine?
Primary urine (filtrate) is formed in the glomeruli with a volume of about 180 L/day and contains useful substances (glucose, amino acids). Final urine is formed after reabsorption and secretion, with a volume of 1.5 L/day; it lacks glucose and proteins, but has an increased concentration of metabolic waste products.
What does the renal filtration barrier consist of?
The barrier consists of three layers: fenestrated capillary endothelium, a three-layered glomerular basement membrane, and a layer of podocyte pedicels with filtration slits.
How to distinguish a proximal tubule from a distal tubule under a light microscope?
In the proximal tubule, the cytoplasm is turbid ("foamy") and the lumen is narrow and stellate due to the brush border. In the distal tubule, the cytoplasm is light and the lumen is wide, smooth, and clear because the brush border is absent.
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