5.1 The Urinary System & Fluid Balance
Key Takeaways
- The nephron is the microscopic functional unit of the kidney, comprising the renal corpuscle (glomerulus and Bowman's capsule) and a tubular system responsible for urine formation.
- Urine formation occurs through three sequential processes: non-selective glomerular filtration, selective tubular reabsorption (primarily in the proximal convoluted tubule), and tubular secretion.
- The Renin-Angiotensin-Aldosterone System (RAAS) regulates systemic blood pressure and sodium balance by releasing renin when renal perfusion drops, leading to vasoconstriction and increased sodium/water reabsorption.
- Antidiuretic Hormone (ADH) from the posterior pituitary inserts aquaporin-2 water channels into the collecting ducts to increase water reabsorption, producing concentrated urine during dehydration.
- The kidneys maintain acid-base homeostasis by selectively reabsorbing bicarbonate ions (HCO3-) and secreting hydrogen ions (H+) to keep blood pH strictly between 7.35 and 7.45.
Overview of the Urinary System
The urinary system plays a vital homeostatic role in the human body by filtering metabolic waste products from the bloodstream, regulating blood volume and arterial pressure, maintaining extracellular fluid (ECF) electrolyte concentrations, and balancing blood pH within the narrow physiological range of 7.35 to 7.45. The primary metabolic wastes eliminated by the kidneys include urea (derived from amino acid deamination in the liver), uric acid (derived from nucleic acid catabolism), and creatinine (derived from skeletal muscle creatine phosphate breakdown).
Gross Anatomy of the Kidneys & Urinary Tract
The kidneys are paired, bean-shaped, retroperitoneal organs positioned against the posterior abdominal wall at levels T12 to L3. Each kidney is encased in a protective fibrous renal capsule and surrounded by adipose tissue. The medial concave border contains the renal hilum, the entry and exit point for the renal artery, renal vein, nerves, and ureter.
Internally, a sagittal section of the kidney reveals three distinct structural regions:
- Renal Cortex: The outer, light-colored region containing the renal corpuscles (glomeruli and Bowman's capsules) and proximal and distal convoluted tubules.
- Renal Medulla: The inner, darker region containing 8 to 18 cone-shaped renal pyramids. The bases of the pyramids face the cortex, while their apices (renal papillae) point inward. The pyramids consist mainly of parallel segments of loops of Henle and collecting ducts. Renal pyramids are separated by cortical extensions called renal columns (columns of Bertin).
- Renal Pelvis: A large, funnel-shaped cavity that collects urine. Minor calyces enclose the papilla of each pyramid and converge to form major calyces, which empty into the renal pelvis. Urine flows from the renal pelvis into the ureter, travels to the urinary bladder via peristalsis for temporary storage, and is excreted through the urethra during micturition.
Renal Blood Supply Pathway
The kidneys receive approximately 20% to 25% of resting cardiac output (~1.2 liters of blood per minute) through the renal arteries. The vascular path through the kidney follows a strict sequence:
Microscopic Structure: The Nephron
Each kidney contains approximately 1 million nephrons, which serve as the structural and functional units of urine formation. A nephron consists of two main parts: a renal corpuscle where fluid is filtered and a renal tubule where the filtered fluid is processed.
1. Renal Corpuscle
Located in the cortex, the renal corpuscle consists of:
- Glomerulus: A high-pressure, fenestrated capillary bed supplied by the afferent arteriole and drained by the efferent arteriole.
- Bowman's Capsule (Glomerular Capsule): A double-walled cup surrounding the glomerulus. The outer parietal layer consists of simple squamous epithelium. The inner visceral layer consists of specialized branching epithelial cells called podocytes. Podocyte foot processes (pedicels) interdigitate to create filtration slits surrounding the fenestrated capillary endothelium.
2. Tubular System
- Proximal Convoluted Tubule (PCT): Located in the cortex, lined with simple cuboidal epithelial cells possessing a prominent brush border of microvilli and abundant mitochondria. The PCT is the primary site of reabsorption, reclaiming ~65% of filtered water and $Na^+$, 100% of filtered glucose and amino acids, and major fractions of $HCO_3^-$, $K^+$, and $Cl^-$.
- Loop of Henle (Nephron Loop): Extends into the medulla. Consists of a descending limb (thin, highly permeable to water via aquaporin-1 channels, poorly permeable to solutes) and an ascending limb (thick segment, impermeable to water, actively transports $Na^+$, $K^+$, and $Cl^-$ out of the lumen into the medullary interstitium via $Na {-}K {-}2Cl$ cotransporters). This differential permeability establishes the osmotic gradient in the medulla necessary for concentrating urine.
- Distal Convoluted Tubule (DCT): Located in the cortex, lined with simple cuboidal cells lacking a microvilli brush border. The DCT performs refined tubular reabsorption and secretion under hormonal regulation ($Na^+$ reabsorption via aldosterone, $Ca^{2+}$ reabsorption via parathyroid hormone).
- Collecting Duct: Receives fluid from multiple DCTs. Passes through the renal medulla to the renal papilla. Under the control of Antidiuretic Hormone (ADH), principal cells in the collecting duct adjust final water reabsorption, determining whether urine is dilute or concentrated.
Three Steps of Urine Formation
Urine formation occurs continuously through three distinct physiological mechanisms:
| Process | Location | Mechanism & Primary Solutes Involved |
|---|---|---|
| 1. Glomerular Filtration | Glomerulus to Bowman's space | Passive, non-selective pressure filtration. Blood hydrostatic pressure forces water, glucose, amino acids, urea, and ions across the filtration membrane. Formed elements (RBCs, WBCs, platelets) and large proteins (albumin) are retained in blood. |
| 2. Tubular Reabsorption | PCT, Loop of Henle, DCT, Collecting Duct | Selective movement of water and essential solutes from tubule lumen back into peritubular capillaries. Active transport ($Na^+$, glucose via secondary active transport) and passive osmosis ($H_2O$). |
| 3. Tubular Secretion | PCT, DCT, Collecting Duct | Active transfer of unwanted substances from peritubular blood into tubule lumen. Eliminates $H^+$, $K^+$, $NH_4^+$, creatinine, and drug metabolites. Key mechanism for blood pH regulation. |
Renal Threshold & Transport Maximum
Reabsorption of solutes like glucose relies on specific membrane transport proteins. The maximum rate at which a solute can be reabsorbed is called the Transport Maximum ($T_m$). When plasma glucose exceeds the renal threshold (~180 mg/dL), glucose filtering through the glomerulus exceeds $T_m$. Unreabsorbed glucose remains in the filtrate and spills into urine (glucosuria), pulling water with it via osmosis (osmotic diuresis), leading to polyuria and polydipsia characteristic of untreated diabetes mellitus.
Hormonal Regulation of Fluid & Blood Pressure Balance
1. Renin-Angiotensin-Aldosterone System (RAAS)
When systemic blood pressure or blood volume drops, or when sodium delivery to the macula densa cells of the juxtaglomerular apparatus (JGA) decreases:
- Juxtaglomerular (JG) cells in the wall of the afferent arteriole release the enzyme renin into the blood.
- Renin cleaves plasma angiotensinogen (produced by the liver) into Angiotensin I.
- Angiotensin-Converting Enzyme (ACE), primarily located in pulmonary capillary endothelium, converts Angiotensin I into Angiotensin II.
- Angiotensin II causes:
- Direct systemic vasoconstriction of arterioles (raising total peripheral resistance and blood pressure).
- Stimulation of the adrenal cortex to secrete aldosterone.
- Stimulation of the hypothalamus to trigger thirst and ADH release.
- Aldosterone acts on principal cells of the DCT and collecting duct, increasing the synthesis and insertion of $Na^+/K^+$ ATPase pumps and $Na^+$ channels. This promotes active $Na^+$ reabsorption into blood and $K^+$ secretion into urine. Water follows $Na^+$ passively via osmosis, expanding ECF volume and raising blood pressure.
2. Antidiuretic Hormone (ADH / Vasopressin)
Synthesized by the hypothalamus and stored/released by the posterior pituitary in response to elevated plasma osmolality (dehydration) or low blood volume. ADH binds V2 receptors on collecting duct cells, inserting aquaporin-2 water channels into apical membranes. Water moves rapidly out of the tubule lumen into hypertonic medullary capillaries, producing a low volume of concentrated urine.
3. Atrial Natriuretic Peptide (ANP)
Released by cardiac atrial cells in response to atrial stretching caused by high blood volume. ANP directly opposes RAAS by inhibiting renin and aldosterone release, increasing Glomerular Filtration Rate (GFR), and reducing $Na^+$ reabsorption in collecting ducts. This promotes natriuresis (sodium excretion) and diuresis (water loss), reducing blood pressure.
Which segment of the nephron is responsible for reabsorbing 100% of filtered glucose and amino acids under normal physiological conditions?
During dehydration, how does Antidiuretic Hormone (ADH) act to conserve body water and produce concentrated urine?
What sequence of events occurs in the Renin-Angiotensin-Aldosterone System (RAAS) when renal blood flow drops?