Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Internal Organs
Urinary System
Kidneys

The Kidneys (renes) — right and left — are bean-shaped (Atl. Fig. 203). They are reddish-brown and relatively firm to the Touch. The vertical dimension of the Kidney is 10—12 cm, its width is 5—6 cm, its thickness is 4 cm, and its weight is 120 g. The anterior surface is convex, and the posterior surface is flatter. The lateral margin is convex, and the medial margin is concave, facing downward and slightly forward. In its middle, There is a deep depression — the renal sinus, which contains the renal hilum. The renal artery, renal vein, Ureter, and nerves pass through the hilum. Lymph Nodes are also located here. The upper pole of the kidney is flatter, and the Adrenal gland lies adjacent to it. The lower poles of the kidneys are further from the spine than the upper poles.

The kidneys are covered by a thin but dense fibrous capsule. The inner layer of this capsule contains smooth Muscle Cells, whose slight contraction helps maintain the pressure required for filtration processes within the kidney. Thin Connective Tissue interlobular septa extend from the fibrous capsule into the renal cortex. Externally, the kidneys are surrounded by a layer of adipose tissue (perirenal fat capsule), which is particularly thick on the posterior side. It plays an important role in maintaining the kidney's position in the lumbar region. Weight loss and a reduction in adipose tissue can lead to kidney mobility or Nephroptosis.

Externally, the kidneys are enclosed by the renal fascia, whose two layers, anterior and posterior, envelop the kidney along with its fat capsule and the Adrenal Glands. The fascia holds the kidney in a specific position. Other factors contributing to kidney fixation include their Blood Vessels, intra-abdominal pressure, support from adjacent Organs, and the fat capsule, which secures the kidney within the fascial layers. Both layers of the fascia descend along the posterior abdominal wall to the Urinary Bladder, enveloping the ureter. The posterior layer of the fascia crosses from one kidney to the other over The surface of the vertebral bodies. The anterior layer also crosses to the opposite side over the surface of the major vessels (the Abdominal Aorta and INFERIOR VENA CAVA). Connective tissue fibers extend from the fascia to the fibrous capsule through the adipose tissue.

The kidneys are located retroperitoneally in the lumbar region, where they lie against the posterior abdominal wall on either side of the last two thoracic and first two lumbar vertebrae. The XII rib passes approximately across the middle of the kidney. The right kidney lies 2—3 cm lower than the left. The right kidney is in contact with the Liver, the transverse colon, and the duodenum, while the left kidney is in contact with The Stomach, Pancreas, jejunum, and Spleen. The adrenal gland lies adjacent to the upper pole of each kidney.

A Cytology/practical/54.html">Longitudinal section of the kidney (Atl. Fig. 204) reveals that its parenchyma consists of two layers: an outer reddish-brown renal cortex, 5—7 mm thick, and an inner, denser, and lighter renal medulla. The cortex is located at the periphery of the kidney and extends deep into the medulla as renal columns (columns of Bertin). The medulla is divided by these renal columns into 15—20 renal pyramids, whose apices point toward the renal sinus and bases face outward. A medullary pyramid, along with its overlying cortex, forms a renal lobe. A human kidney consists of 6—18 such lobes.

The apices of every 2—3 pyramids merge to form a renal papilla. On average, there are 7—8 such papillae in each kidney. Each papilla is enclosed by a minor calyx, which marks the beginning of the Urinary Tract. The calyces are funnel-shaped and merge to form 2—3 major calyces, which then unite to form the renal pelvis. The pelvis is a funnel-shaped cavity, flattened from front to back, hidden within the renal sinus, and transitioning into the ureter at the renal hilum.

The wall of the calyces and pelvis consists of three layers: an inner mucosa, a middle muscularis, and an outer adventitia (connective tissue layer).

The Structural and functional unit of the kidney is the nephron (Fig. 4.37). There are more than a million of them in each kidney. The nephron begins with a blind, cup-like expansion with a double-walled Structure — the glomerular capsule (Bowman's capsule), lined with simple cuboidal epithelium. Between the two layers of the capsule lies a space that communicates with the lumen of the tubule extending from the capsule. Within the capsule is a glomerulus of blood capillaries, which together with the capsule forms the renal corpuscle. The proximal convoluted tubule (1st order) originates from the capsule and transitions into the descending limb of the nephron loop (Loop of Henle). The ascending limb of the loop continues into the Distal convoluted tubule (2nd order). This tubule empties into straight collecting ducts, through which urine flows into the renal pelvis.

The tubules of many nephrons empty into each collecting duct. Together, they form a lobule of renal tissue (Fig. 4.38). These lobules are not separated from each other by connective tissue septa. The core of a lobule is formed by a branching collecting duct. Surrounded by the loops of Henle, they form medullary rays in the cortex above the pyramids. Medullary rays are clearly visible in the renal cortex, whereas they are indistinguishable in the medulla. Interlobular Arteries ascend into the cortex roughly along the borders of adjacent lobules. Near the renal capsule, the medullary rays are significantly thinner than near the medulla. This is because fewer nephron tubules connect to the collecting duct in the peripheral part than in the deeper PARTS OF THE lobule.

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Fig. 4.37. Diagram of nephron structure:

1 — glomerular capsule; 2 — proximal convoluted tubule; 3 — distal convoluted tubule; 4 — collecting duct; 5 — descending and 6 — ascending limbs of the loop of Henle

Fig. 4.38. Structure of a renal lobule (after Ham and Cormack):

1 — lobule; 2 — medullary ray; 3 — interlobular artery; 4 — proximal convoluted tubule; 5 — renal corpuscle; 6 — juxtamedullary nephron; 7 — distal convoluted tubule; 8 — collecting duct; 9 — loop of Henle; 10 — renal papilla; 11 — medulla and 12 — cortex; 13 — renal capsule; 14 — cortical nephron

Depending on the Location of their glomeruli, nephrons are classified as cortical nephrons, which are located in the outer layers of the renal cortex, and juxtamedullary nephrons, located deep within the kidney near the renal columns. Most nephrons, whose glomeruli lie in the outer cortex, have short loops of Henle that extend only shallowly into the medulla. In juxtamedullary nephrons, the glomeruli lie close to the medulla, and their loops are long, reaching the apices of the pyramids (Fig. 4.38).

The renal (Malpighian) corpuscle, as previously noted, consists of a capillary glomerulus surrounded by the glomerular capsule. Afferent arterioles enter these glomeruli within the capsules (Figs. 4.39, 4.40; Atl. Figs. 205Б, 206А). The blood capillaries from each glomerulus coalesce into an efferent arteriole, which has a smaller diameter than the afferent one. This difference in arteriole diameters helps maintain high blood pressure within the glomerular capillaries. This is where filtration of metabolic wastes occurs and primary urine is formed. The pressure in the glomerular capillaries is highly stable, remaining constant even when systemic blood pressure rises. Consequently, the filtration rate also remains virtually unchanged.

Fig. 4.39. Glomerular capsule:

1 — proximal convoluted tubule; 2 — basement membrane of the capsule; 3 — lumen of blood capillary; 4 — glomerular basement membrane; 5 — capsular epithelium (parietal layer); 6 — juxtaglomerular cells; 7 — afferent arteriole; 8 — distal convoluted tubule; 9 — efferent arteriole; 10 — glomerular epithelial cells (podocytes); 11 — capillary endothelium

The inner layer of the capsule (visceral layer) is formed by an epithelium known as the glomerular epithelium (Fig. 4.39). This epithelium almost completely envelops individual capillaries and forms a basement membrane around them. The epithelium of the outer wall of the capsule — the parietal layer — is called the capsular epithelium. Both layers transition into each other at the vascular pole of the capsule. The space enclosed between them is known as Bowman's or the capsular space.

The diameter of the renal corpuscle ranges from 150 to 250 µm. It has an oval shape.

The muscular layer is more developed in the wall of the afferent arteriole than in the efferent one. Near the glomerular capsule, these two vessels diverge; this area is called the vascular pole of the glomerulus. The ascending limb of the loop of Henle bends in this region and makes close contact with the wall of the afferent arteriole. The loop then transitions into the distal convoluted tubule.

At the point of contact with the convoluted tubule, juxtaglomerular cells are located within the Middle layer of the afferent arteriole wall.

Juxtaglomerular cells, characterized by a rounded Nucleus and cytoplasmic granules containing the biologically active substance renin, are in close contact with the arteriole endothelium and the Cells of the distal convoluted tubule wall. Juxtaglomerular cells participate in The regulation of arterial pressure, functioning as vascular baroreceptors. When blood pressure drops in the afferent arteriole, these cells respond to the lack of stretch by releasing renin into the bloodstream. As a result of its interaction with Plasma Proteins, smooth muscle cells in the arteriole walls contract, narrowing their lumen and raising blood pressure. Furthermore, this stimulates the secretion of aldosterone, a hormone of the adrenal cortex, which acts on the nephron tubules to reduce The excretion of sodium and Water from the body. When blood pressure rises, renin secretion ceases. In addition, juxtaglomerular cells may serve as a site for The production of Erythropoietin, which stimulates red blood Cell production in the Bone Marrow.

Fig. 4.40. Capillary glomeruli of renal corpuscles. Corrosion cast, scanning Electron Microscopy

Filtration of substances present in Blood Plasma occurs within the glomerular capsule. These substances pass into the capsular lumen through the capillary endothelium, the basement membrane, and the epithelium of the visceral layer of the capsule. The Cytoplasm of endothelial cells is extremely thin, and most of it contains numerous pores (fenestrae) with a diameter of 100 nm. The basement membrane is thicker here than in other parts of the body. It is synthesized by epithelial cells and degraded on the opposite side by macrophages located near the blood capillaries. The epithelial cells have FOOT-like processes, which is why they are called podocytes (from Greek podos — foot). The Cell body is separated from the capillary by a space filled with glomerular filtrate (Fig. 4.41). Long cell processes extend along the capillary, wrapping around it like a sleeve and interdigitating with the processes of neighboring cells. Between these processes, there are filtration slits 20–30 nm wide, closed by a slit Diaphragm.

All Components of the described barrier exhibit selective permeability. Macromolecular substances with a molecular diameter exceeding 100 nm are retained by the endothelium. The passage of smaller molecules is restricted by the filtration action of the basement membrane, and finally, the slit diaphragms spanning the gaps between podocyte processes prevent molecules larger than 6–9 nm from entering the capsular space. It has also been established that the barrier's efficiency depends on how close the glomerular capillary blood flow is to normal, as well as on the protein COMPOSITION OF THE blood plasma. For example, albumin filtration through the barrier does not occur in the presence of high-molecular-weight proteins, which likely block the pores. The ability to penetrate the barrier also depends on the electrostatic charge of the molecule. Negatively charged molecules, such as albumins, are repelled by the negatively charged glycocalyx covering the podocyte membranes. Despite the presence of this complex barrier, some protein molecules still pass into the lumen of the nephron capsule.

There are more than 1 million nephrons in the human kidneys. Their total excretory surface area reaches 5–8 m², which is 3 to 5 times greater than the body surface area. Typically, only 1/3 of the nephrons function at any given time, while the rest serve as a physiological reserve. This explains why a person can tolerate the surgical removal of one kidney. In such cases, the remaining kidney only slightly increases in size.

Fig. 4.41. Relationship between podocytes and endothelial cells in the nephron capsule: A — diagram; B — electron microscopy (diagram); 1 — basement membrane; 2 — filtration slits; 3 — pedicels and 4 — cytoplasm of podocytes; 5 — fenestrated endothelium; 6 — nuclei of podocytes; 7 — erythrocyte; 8 — fenestrae; 9 — microfilaments

The length of the proximal convoluted tubule is about 14 mm. It originates from the nephron capsule. Externally, the tubule is covered by a basement membrane. The tubule wall is formed by a single layer of epithelial cells. Their luminal surface is covered by a brush border of microvilli (Fig. 4.42). The basal cell membrane forms numerous folds. When a large volume of filtrate (primary urine) accumulates, the tubule lumen becomes rounded, and the cells of its wall become low columnar. With a small volume of filtrate, the tubule lumen narrows, and the cells take on the appearance of a tall columnar epithelium.

In the proximal convoluted tubules, reabsorption of water and sodium (up to 85%) occurs, as well as calcium, phosphate, sulfates, and several other ions contained in the filtrate. These substances enter the capillaries into which the efferent arteriole of the glomerulus branches. Intracellular ion transport processes require significant Energy Expenditure, which is supplied by numerous Mitochondria in the basal portions of the cells. These tubules also reabsorb proteins, glucose, Amino Acids, creatinine, and Vitamins that entered the filtrate from the blood plasma. The tubule cells are also capable of secretion, releasing Metabolic waste products, drugs, etc., into the lumen.

Fig. 4.42. Types of cells forming the walls of the nephron tubules (after Hees H.):

1 — proximal convoluted tubule; 2 — distal convoluted tubule; 3 — loop of Henle; 4 — collecting duct

The proximal tubule enters the medullary ray and continues as the descending limb of the nephron loop (loop of Henle). The initial segment of the loop is structurally similar to the convoluted tubule, but its diameter subsequently decreases. Only about 15% of the glomerular filtrate enters the lumen of the loop of Henle. The cells of the wall of this part of the loop allow water to pass from the lumen into the interstitial fluid. Sodium is transported across the walls of the ascending limb into the surrounding interstitial fluid, making it hypertonic. The epithelium of this segment of the loop is impermeable to water. This is of great functional significance, especially in juxtamedullary nephrons, whose loops extend to the apices of the renal pyramids. Due to the increased ion concentration in the interstitial fluid bathing the structures of the medullary rays, particularly the collecting ducts, water is reabsorbed within them. As a result, the final urine entering the renal pelvis via the collecting ducts is hypertonic.

As it moves further from the proximal tubule, the lumen of the loop narrows, and its lining epithelium becomes simple squamous (Fig. 4.42).

1700 liters of blood. Blood flow varies in different parts of the kidney: its maximum rate occurs in the renal cortex, which contains the glomeruli and convoluted tubules.

The Blood supply to the kidney is provided by the renal artery, which branches off the abdominal aorta. Near the renal hilum, the artery divides into two branches, which give rise to five segmental arteries, each supplying its respective part of the kidney (Atl. Fig. 205). These arteries give rise to interlobar arteries, which in turn give rise to arcuate and interlobular arteries that alternate with the medullary rays. Most of the arteries (over 90%) go to the renal cortex, and their terminal branches extend into the renal capsule, forming the capsular plexus. The remaining arteries pass into the renal medulla toward the nephron loops. Intralobular branches arise from the interlobular arteries to supply individual nephrons, and afferent arterioles run from them to the glomeruli. The efferent arterioles of the glomeruli branch again into capillaries, forming dense networks around the convoluted tubules. Blood from the capillaries then drains into venules and small Veins, which merge into interlobular veins. In general, the veins correspond to the arteries. Upon merging, they form the renal vein, which empties into the inferior vena cava.

Plexuses of Lymphatic vessels accompany the arterial and venous vessels. Valves are found in the large lymphatic vessels located in the renal hilum.