33.1 Introduction to the Renal System

Kidneys

The kidneys have three main functions: filtration, reabsorption, and secretion. The kidneys filter 200 liters of fluid daily to remove waste products that exit the body in the urine. Collecting important nutrients, ions, and proteins protects the body against deficiencies. The individual processes for the three main functions are listed here and explained in greater detail in later sections of this chapter:

• Removal of the waste products of metabolism
• Regulation of electrolytes and fluid
• Acid–base balance regulation
• Maintenance of systemic blood pressure
• Erythropoietin secretion
• Vitamin D₃ metabolism
• Gluconeogenesis

Kidneys’ Anatomical Structure

The kidneys are two bean-shaped organs positioned in the retroperitoneum slightly above the waist, between the T12 and L3 vertebrae. The right kidney sits slightly lower than the left to accommodate the liver. Each kidney weighs approximately 135–150 g and is approximately 10–12 cm long. The adrenal glands are located on the upper pole of each kidney, and the spleen is connected anteriorly to the upper pole of the left kidney by the splenorenal ligaments.

The exterior of the kidney has three protective layers: the renal fascia, which is the outermost layer; the perirenal fat capsule; and the renal capsule, which consists of fibrous connective tissue. The interior of the kidney is divided into three distinct areas: the outermost cortex, the medulla, and the renal pelvis. The renal cortex contains nephrons, the functional units of the kidney, which merge into the collecting ducts and the convoluted tubules. The renal medulla contains 8 to 18 renal pyramids with the bases located adjacent to the cortex and the apices connecting to the minor calyces (Sorano et al., 2023). The minor calyces drain into the major calyces, which are large collecting spaces near the ureters’ superior edge. The renal papillae are openings at the bottom of the renal pyramids where urine enters the collecting ducts. The renal columns are cortical tissues that separate the pyramids and provide space for the interlobar arteries. The urine moves from the pyramids to the funnel-shaped renal pelvis in the center of the kidney to the ureters (see Figure 33.2).

Figure 33.2 The renal columns serve to divide the kidney into 6–8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Nephrons

The nephron is the functional unit of the kidney and contains these structures (see Figure 33.3):

• Glomerulus and Bowman’s capsule, which together comprise the renal corpuscle
• Proximal convoluted tubule, located in the renal cortex
• Descending loop of Henle
• Ascending limb in the renal medulla
• Thick ascending limb
• Distal convoluted tubule
• Collecting duct

Figure 33.3 Various portions of the nephron differ in their capacity to reabsorb water and specific solutes. While much of the reabsorption and secretion occur passively based on concentration gradients, the amount of water that is reabsorbed or lost is tightly regulated. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

There are approximately 1 million nephrons in each kidney. There are two types of nephrons, which are labeled according to their anatomical position. Eighty-five percent of the nephrons are cortical nephrons, located in the outer renal cortex and extending partially into the medulla. The nephron loops are short and are supplied by the peritubular capillaries. The juxtamedullary nephrons are positioned at the junction of the renal cortex and the renal medulla. The nephron loop extends into the medulla and is surrounded by the vasa recta, a network of capillaries originating from the efferent arteriole. The cortical portion of the juxtamedullary nephron is also surrounded by the peritubular capillaries, which aid in the concentration and volume control of urine.

Blood is filtered by the globe-shaped renal corpuscle, which contains the glomerulus and Bowman’s capsule. The glomerulus is a tufted structure of fenestrated capillaries essential to filtration. The glomerulus is enclosed in a double-layered structure, the Bowman’s capsule, which has two surfaces, the exterior parietal layer and the interior visceral layer. The parietal layer is composed of squamous epithelial cells. The visceral layer is composed of specialized epithelial cells called podocytes that encircle the glomerular capillaries. Extensions of the podocytes called pedicles also surround the capillaries, forming filtration slits or pores that also aid filtration. The space between the visceral and parietal layers, the capsular space or Bowman’s space, becomes the lumen of the renal tubule. The fluid that is filtered from the glomerular capillaries moves to the renal tubule as filtrate.

The mesangial cells, located around the glomerular capillaries, regulate the surface area that is available for glomerular filtration. The cells contract in response to the stretch of increased blood in the capillaries, decreasing the surface area. Conversely, the filtration surface area increases when the cells are relaxed. The cells contract in response to angiotensin II and endothelin and relax in response to atrial natriuretic peptides (ANPs) and nitric oxide.

The renal tubule consists of three different areas with specialized structures and functions. Initially, the filtrate flows through the proximal tubule, which has straight sections and “convoluted” sections. This is the longest segment of the renal tubule, and the surface of the lumen has microvilli that increase the filtration surface area.

The second area, the nephron loop (loop of Henle), has two sections: the descending limb and the ascending limb. The descending limb, which consists of simple squamous cells, is often identified as the thin descending limb, whereas the ascending limb is mostly composed of cuboidal epithelial cells and is called the thick ascending limb.

The terminal portion of the nephron, the distal tubule, also contains straight and convoluted sections; however, the structure is composed of simple cuboidal epithelial cells without the microvilli. The fluid leaves the distal tubule and enters the collection system.

Glomerular Filtration

Glomerular filtration is the initial step in removing waste and concentrating the urine. The filtration membrane consists of three layers: the fenestrated glomerular capillary endothelial cells, the basal lumina, and the podocytes in the visceral layer of the glomerular or Bowman’s capsule. The first layer, the openings in the fenestrated glomerular capillary endothelial cells, allows the movement of molecules up to 100 nanometers in size, which means that platelets and blood cells are not filtered from the capillaries. The basal lumina is a thin layer of tissue between the glomerular endothelial cells and the podocytes. The collagen fibers from this layer generate a mesh that creates a second barrier. This collagen mesh blocks most plasma proteins by size and also blocks negatively charged proteins regardless of size. The filtration slits or pores formed by the podocytes provide the third barrier to filtration. These openings block plasma proteins from entering the filtrate. The filtrate consists of the fluids and solutes that progress through the filtration membrane. Substances that are commonly included in the filtrate include water, glucose, electrolytes, amino acids, and smaller proteins.

The Glomerular Filtration Rate (GFR)

The kidneys filter approximately 200 liters of fluid every day (Ogobuiro & Tuma, 2022). The normal glomerular filtration rate is 120–125 mL per minute (Ogobuiro & Tuma, 2022). This value is influenced by the client’s age, weight, and muscle mass. Filtration is increased by the glomerular capillary hydrostatic pressure and Bowman’s capsule oncotic pressure, and filtration is opposed by the glomerular oncotic pressure and the Bowman’s capsule hydrostatic pressure. Note that the oncotic pressure in the Bowman’s capsule is normally near zero because proteins and cells do not enter the capsule.

Three elements determine the critical net filtration pressure required for homeostatic glomerular filtration:

• Glomerular capillary hydrostatic pressure: The pressure in the glomerular capillary bed is 55 mm Hg and is the major filtration force (Ogobuiro & Tuma, 2022).
• Glomerular capillary oncotic pressure: This pressure is determined by the oncotic pressure of the blood in the glomerulus. This pressure is higher than the average oncotic pressure because water is quickly filtered from the blood. This pressure opposes filtration because the increased osmotic pressure can pull the water from the filtrate into the arterioles.
• Bowman’s capsule hydrostatic pressure: The hydrostatic pressure is determined by the amount of fluid in the capsular space. The amount of fluid is determined by the rate at which the filtrate enters the capsular space, which is greater than the rate at which the filtrate empties into the lumen of the tubule. This pressure opposes filtration.

Tubular Reabsorption

The tubules are responsible for selective reabsorption of electrolytes, nutrients, and water from the filtrate and the return of these substances to the circulating blood volume. These mechanisms are responsible for moving water and substances from one area to another:

• Active transport: Active transport uses energy, ATP, to move a substance across a membrane from an area of low concentration to an area of higher concentration of that substance.
• Diffusion: Simple diffusion follows the concentration gradient and moves substances across a membrane from an area of higher concentration to an area of lower concentration. This process does not require energy expenditure.
• Facilitated diffusion: This process also moves substances along a concentration gradient; however, membrane receptors or channel proteins are required for the transfer (Ogobuiro & Tuma, 2022).
• Secondary transport systems: Secondary transport systems, including symport and antiport structures, each require energy. Symport structures move two or more substances in the same direction simultaneously. Antiport structures move two or more substances across the cell membrane in different directions.

Tubular Secretion

Tubular secretion is the movement of substances from the peritubular vessels into the tubular lumen to be excreted in the urine. The substances either are moved by passive diffusion from the peritubular capillaries into the interstitial space or are transported across the epithelial lumen of the nephron by active transport requiring ATPase. The proximal tubules secrete nitrogenous waste products such as urea, ammonia, and creatinine; excess hydrogen ions; and toxic substances including many protein-bound drugs that do not cross the glomerular basement membrane. The distal tubules secrete potassium and hydrogen ions in response to the hormonal control noted previously.

Common Conditions Affecting Kidney Function

Kidney function is affected by systemic disease, as discussed in the following section; however, several intrinsic/intrarenal conditions can affect renal homeostasis:

• Congenital abnormalities: These are alterations in anatomy or physiology that are present at birth. With renal agenesis and renal hypoplasia, the kidneys fail to develop in utero and the infant is born without kidneys (renal agenesis) or with only one kidney (renal hypoplasia). Renal agenesis is incompatible with extrauterine life, and the infant is often stillborn. The infant can survive with a single kidney; however, additional congenital abnormalities usually threaten the infant’s survival.
• Genetic disorders: Multiple forms of polycystic kidney disease can present in childhood or adulthood. These fluid-filled cysts interfere with urine formation and increase the risk of infection and hemorrhage. The disease course is complicated by hypertension, progressive loss of kidney function, and pain. Additional renal conditions, such as renal calculi, are also common in this client population.
• Neoplasms: General risk factors for renal neoplasms are similar to other types of cancer: smoking, obesity, hypertension, history of renal dialysis or transplant, and exposure to environmental toxins.
  • Wilms tumor/nephroblastoma: Presents in childhood with the onset of hypertension and kidney enlargement. It is the most common renal cancer in children and has a 90% five-year survival rate.
• Acute and chronic pyelonephritis (infection): The renal system has multiple protective mechanisms against infection; however, the most common infections ascend from the ureters, bladder, and urethra. The usual causative agent is Escherichia coli. Infections associated with indwelling urinary catheters are common in hospitalized clients, and these infections can progress to chronic renal failure in susceptible clients.
• Glomerulopathies: Glomerulopathies are disorders of the glomerular capillaries. These disorders can be due to systemic, autoimmune, or intrinsic renal alterations. Common manifestations include proteinuria, decreased GFR, hypertension, and edema. These disorders are progressive and commonly result in end-stage renal disease (ESRD).
  • Nephrotic syndrome: A disorder associated with massive proteinuria, edema, and hypertension. It commonly progresses to ESRD.
• Renal calculi: Also called nephrolithiasis, urolithiasis, or kidney stones; are commonly composed of calcium crystals. Risk factors for stone formation include hypertension, red meat ingestion, obesity, family history of nephrolithiasis, prolonged immobility, vesicoureteral reflux, and hyperparathyroidism. Stones may be asymptomatic and excreted without further intervention; however, large stones can move from the kidney pelvis to the ureteral junction or further in the ureter, causing obstruction and pain (renal colic). Lithotripsy or endoscopy may be necessary to remove the obstruction. Preventive measures include maintaining oral fluid intake at 64–96 fluid ounces per day (or approximately 2–3 liters), avoiding additional calcium intake, and limiting dietary sodium and protein (National Kidney Foundation, 2019). Clients with recurrent stone formation require chemical analysis of the stones to address dietary restrictions.