8.1 Structure and function of the kidneys

The kidneys are vital organs that keep our bodies in balance. They filter blood, remove waste, and control fluid levels. These bean-shaped powerhouses also make hormones and help regulate blood pressure. Without them, we'd be in big trouble.

Let's dive into the kidney's structure and function. From the outer cortex to the inner medulla, each part plays a crucial role. We'll explore how blood flows through the kidneys and how they maintain homeostasis in our bodies.

Kidney Anatomy and Location

Gross Anatomy

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• The kidneys are paired, bean-shaped organs located in the retroperitoneal space of the posterior abdominal wall
• The right kidney is slightly lower than the left due to the size of the liver above it
• The hilum is a concave opening on the medial side of each kidney where the renal artery, renal vein, and ureter enter and exit the kidney
• The fibrous renal capsule is a thin, transparent layer of dense irregular connective tissue that surrounds each kidney and provides protection

Internal Structure

• The renal cortex is the outer region of the kidney, containing the glomeruli, proximal and distal convoluted tubules, and cortical collecting ducts
• The renal medulla is the inner region of the kidney, consisting of cone-shaped renal pyramids containing the loops of Henle, medullary collecting ducts, and renal papillae
• The renal pelvis is a funnel-shaped structure that receives urine from the major calyces and is continuous with the ureter
• The major and minor calyces are cup-shaped structures that collect urine from the renal papillae and drain into the renal pelvis (e.g., the major calyx receives urine from several minor calyces, while each minor calyx collects urine from the papilla of a renal pyramid)

Kidney Blood Supply and Innervation

Renal Vasculature

• The renal arteries arise from the abdominal aorta and branch into the segmental arteries, interlobar arteries, arcuate arteries, and interlobular arteries before forming the afferent arterioles that supply the glomeruli
• The efferent arterioles drain blood from the glomeruli and form the peritubular capillaries, which surround the renal tubules and provide oxygen and nutrients
• The renal veins follow a similar pattern to the arteries, with the interlobular veins, arcuate veins, interlobar veins, and segmental veins draining into the renal vein, which empties into the inferior vena cava (e.g., the interlobular veins drain the peritubular capillaries, while the arcuate veins are formed by the convergence of several interlobular veins)

Renal Innervation

• The kidneys receive sympathetic innervation from the renal plexus, which arises from the celiac plexus and contains postganglionic fibers from the T10-L2 spinal segments
• Sympathetic stimulation causes vasoconstriction of the renal arteries, reducing renal blood flow and glomerular filtration rate, while also increasing renin secretion and sodium reabsorption (e.g., during fight-or-flight response or decreased blood volume)
• Parasympathetic innervation to the kidneys is minimal and does not significantly influence renal function

Kidney Functions in Homeostasis

Fluid and Electrolyte Balance

• The kidneys regulate blood volume and pressure by adjusting the excretion of water and sodium, with increased excretion leading to a decrease in blood volume and pressure, and vice versa
• The kidneys maintain electrolyte balance by selectively reabsorbing or secreting ions such as sodium, potassium, calcium, magnesium, and chloride to keep their concentrations within normal ranges (e.g., the reabsorption of sodium in the proximal tubule and loop of Henle, or the secretion of potassium in the distal tubule and collecting duct)

Acid-Base Balance and Waste Excretion

• The kidneys regulate acid-base balance by excreting excess hydrogen ions and reabsorbing bicarbonate ions to maintain a normal blood pH of 7.35-7.45 (e.g., the secretion of hydrogen ions in the proximal tubule and collecting duct, or the reabsorption of bicarbonate in the proximal tubule)
• The kidneys excrete metabolic waste products such as urea, creatinine, and uric acid, which are filtered from the blood and eliminated in the urine to prevent their accumulation to toxic levels
• The kidneys also excrete foreign substances, such as drugs and toxins, to maintain a clean internal environment

Endocrine Functions

• The kidneys produce and secrete hormones such as erythropoietin (stimulates red blood cell production in the bone marrow), renin (regulates blood pressure through the renin-angiotensin-aldosterone system), and calcitriol (active form of vitamin D, regulates calcium homeostasis)
• The kidneys also convert vitamin D to its active form, calcitriol, which promotes calcium absorption in the intestine and reabsorption in the kidney, maintaining normal calcium levels in the blood

Kidney Cortex vs Medulla

Anatomical Differences

• The renal cortex is the outer region of the kidney, while the renal medulla is the inner region
• The cortex contains the glomeruli, proximal and distal convoluted tubules, and cortical collecting ducts, while the medulla contains the loops of Henle, medullary collecting ducts, and renal papillae
• The cortex has a granular appearance due to the presence of glomeruli and convoluted tubules, while the medulla has a striated appearance due to the parallel arrangement of the loops of Henle and collecting ducts

Functional Differences

• The cortex receives a larger proportion of renal blood flow compared to the medulla, as the peritubular capillaries surrounding the cortical nephron segments require more oxygen and nutrients
• The medulla has a higher osmolarity than the cortex due to the countercurrent multiplier system in the loops of Henle, which creates a concentration gradient that aids in water reabsorption (e.g., the ascending limb of the loop of Henle actively transports sodium, while the descending limb is permeable to water, resulting in a hypertonic medullary interstitium)
• The cortex is the site of the majority of the filtration, reabsorption, and secretion processes in the nephron, while the medulla is primarily involved in the concentration of urine
• The cortex contains the juxtaglomerular apparatus, which regulates renal blood flow and renin secretion, while the medulla contains the vasa recta, which are specialized capillaries that maintain the medullary concentration gradient