General organization of the urinary system
The urinary system is functionally and anatomically divided into two main groups of organs. The first group comprises the urine-forming organs, which are represented directly by the kidneys (renes). The second group includes the urine-conveying pathways: the ureters, the urinary bladder, and the urethra.
The structure of the urethra exhibits pronounced sex-related differences. In men, this pathway is long, anatomically closely related to the reproductive system, and passes through the prostate gland, the urogenital diaphragm, and the corpus cavernosum of the penis. In women, the canal is arranged much more simply: it is significantly shorter and passes exclusively through the urogenital diaphragm.
Renal capsules
Externally, the kidney is reliably protected by three layers, which are located from the periphery to the center in the following order:
- Connective tissue fascia — the outermost layer.
- Adipose capsule — a layer of adipose tissue underlying the fascia.
- Fibrous capsule (capsula fibrosa) — a thin, smooth plate directly adjacent to the organ parenchyma.
An interesting histological feature of the fibrous capsule is that, in addition to connective tissue, it contains smooth muscle elements (myocytes). This makes it resemble the capsule of the spleen. The contraction of these myocytes is thought to help maintain intrarenal pressure for effective plasma filtration, as well as facilitate the excretion of the formed urine from the kidney.
Structure of the parenchyma
The renal tissue located beneath the fibrous capsule comprises urine-forming structures that are visually and topographically divided into two zones:
- Cortex (cortex). It occupies a peripheral, subcapsular position and stands out with a dark red color on a fresh section. Portions of the cortex extend inward between the medullary structures, forming renal columns.
- Medulla (medulla). It is located deeper than the cortex and has a lighter color. It is organized in the form of 8–12 renal pyramids. Thin processes called medullary rays extend from the medulla into the thickness of the cortex.
Intrarenal pathways and microcirculation
Inside the organ, a system of cavities begins that collects urine (the renal calyx-pelvis system). The apices of the renal pyramids—the renal papillae—numbering from one to three, open into minor calyces. There are a total of 8–9 minor calyces in the kidney. By fusing, they form 2–3 major calyces, which then transition into the renal pelvis (pelvis). Topographically, only the upper part of the pelvis is located inside the kidney, while its lower part protrudes outward from the hilum of the organ and continues into the ureter.
At the microscopic level, the basis of renal function is built on the interaction of the vascular system and epithelial tubules. The key hemodynamic principle here is the blood flow through two capillary networks. The first network forms capillary glomeruli hidden within the capsule, and the second network densely intertwines the system of epithelial tubules.
Briefly: answers to topical questions
What parts does the nephron consist of?
The tubular part of the nephron consists of five consecutively transitioning segments.
- Proximal convoluted tubule — begins at the Bowman's capsule and forms loops in the cortex near the renal corpuscle.
- Proximal straight tubule—a short segment following the convoluted one.
- Thin tubule — forms part of the loop of Henle; the descending limb of the loop is represented by the thin tubule, and the ascending limb begins as its continuation.
- Distal straight tubule — forms part of the loop of Henle after the initial segment of the ascending limb, which is represented by the thin tubule.
- Distal convoluted tubule — is a continuation of the ascending limb of the loop of Henle, is located in the cortex, and empties into the collecting duct.
What type of epithelium lines the minor and major renal calyces?
The minor and major renal calyces, acting as intrarenal urinary pathways, are lined with transitional epithelium (urothelium).
This specific type of stratified epithelium is adapted to stretching and consists of three layers:
- Basal layer — small cells with oval nuclei lying on the basement membrane.
- Intermediate layer — polygonal cells.
- Superficial layer — very large (often binucleated) cells whose shape changes from dome-shaped to flattened when the organ is stretched.
What types of cells are part of the juxtaglomerular apparatus of the kidney?
The juxtaglomerular apparatus (JGA) of the kidney includes three elements.
- Macula densa (macula densa)—a specialized region of the distal convoluted tubule wall.
- Juxtaglomerular cells — located in the middle layer of arterioles, predominantly the afferent arteriole.
- Juxtavascular cells (Goormaghtigh cells) — located in the space between the two arterioles and the macula densa.
The main function of the JGA is the synthesis of the enzyme renin, participating in the regulation of blood pressure.
To remember the order of the renal capsules from outside to inside, use the phrase: "Fatty Fascicles Fibrous" (Fascia, Adipose capsule, Fibrous capsule).
Class and exam questions
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.
What is the specific feature of the blood supply system of the kidney?
Blood in the kidney passes successively through two capillary networks. The first network is located in the glomeruli, and the second intertwines the epithelial tubular system.
How is the renal pelvis formed?
The renal pelvis is formed by the fusion of 2–3 major calyces, which in turn are formed from 8–9 minor calyces that collect urine from the renal papillae.
What else to study on this topic
- Histological structure of the epithelial tubules intertwined by the secondary capillary network.
- Organization of the first capillary network: structure of the renal corpuscle and its capsule.
- Microscopic structure of the renal papillae and their contact with minor calyces.
- Topography of the renal hilum and the exit of structures from the organ.
- Cellular composition of the fibrous capsule and the distribution of myocytes.
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