41.2 The Kidneys and Osmoregulatory Organs

Kidney Structure and Function

The kidneys are the body's primary osmoregulatory organs. They filter blood, remove metabolic waste, and precisely control the volume and solute concentration of body fluids. Beyond filtration, they also produce hormones and regulate blood pressure, making them central to whole-body homeostasis.

Role of Kidneys in Osmotic Balance

The kidneys keep blood osmolarity within a narrow range (~300 mOsm/L in humans) by adjusting how much water and solute they excrete or retain.

• Regulate blood osmolarity and volume by excreting excess water and solutes (sodium, potassium) or conserving them during dehydration or blood loss
• Control blood pressure by adjusting salt and water balance. When the kidneys retain more sodium, water follows osmotically, increasing blood volume and raising pressure.
• Maintain blood pH by excreting excess hydrogen ions or reabsorbing bicarbonate, keeping arterial pH near 7.4
• Produce key hormones:
  • Erythropoietin (EPO) stimulates red blood cell production in bone marrow
  • Calcitriol (active vitamin D) promotes calcium absorption in the gut
  • Renin initiates the renin-angiotensin-aldosterone system (RAAS), which raises blood pressure

Key Structures of the Kidney

Each kidney has distinct anatomical regions that correspond to different stages of urine processing.

• Renal cortex (outer region): Contains the glomeruli and convoluted tubules of nephrons. This is where ultrafiltration and most selective reabsorption occur (glucose, amino acids, ions).
• Renal medulla (inner region): Contains the loops of Henle and collecting ducts. The medulla's tissue is arranged in a steep osmotic gradient that allows the kidney to concentrate urine.
• Renal pelvis: A funnel-shaped cavity that collects urine draining from the collecting ducts and channels it into the ureter toward the bladder.
• Hilum: The indented region on the medial side where the renal artery, renal vein, nerves, and ureter enter or exit the kidney.

Nephron Structure and Urine Formation

Nephron Components

The nephron is the functional unit of the kidney. Each human kidney contains roughly 1 million nephrons. A nephron has two main parts:

1. Renal corpuscle (filtration site)

   • Glomerulus: a ball-shaped capillary network where high blood pressure forces small molecules (water, glucose, amino acids, ions) through fenestrated capillary walls into Bowman's capsule. Large molecules like plasma proteins and blood cells are too big to pass through and stay in the blood.
   • Bowman's capsule: a cup-shaped structure that surrounds the glomerulus and collects the filtrate.

2. Renal tubule (where filtrate is modified)

   • Proximal convoluted tubule (PCT): Reabsorbs most filtered nutrients, water, and ions back into the blood. Also secretes organic wastes like uric acid and creatinine into the tubule.
   • Loop of Henle: Dips into the medulla and establishes the osmotic gradient needed to concentrate urine (via countercurrent multiplication).
   • Distal convoluted tubule (DCT) and collecting duct: Fine-tune urine composition under hormonal control (aldosterone, ADH).

Three-Step Process of Urine Formation

1. Glomerular filtration

   • A passive, pressure-driven process. Blood pressure in the glomerulus pushes water and small solutes through the capillary walls into Bowman's capsule.
   • The net filtration pressure depends on the balance of hydrostatic pressure (pushing fluid out) and osmotic pressure of plasma proteins (pulling fluid back in). These are sometimes called Starling forces.
   • The resulting fluid is called filtrate. It resembles blood plasma but lacks proteins and cells.

2. Tubular reabsorption

   • The filtrate contains many substances the body needs to keep. Reabsorption reclaims ~99% of filtered water, all filtered glucose and amino acids, and most sodium and other ions.
   • Most reabsorption happens in the PCT. The loop of Henle reabsorbs additional water and salts.
   • Mechanisms include active transport (e.g., $Na^+/K^+$-ATPase pumps on the basolateral membrane), cotransport, and osmosis.

3. Tubular secretion

   • An active process that moves additional wastes and excess ions from the peritubular capillaries into the tubular fluid.
   • Key substances secreted: $H^+$ ions, $K^+$, ammonia, and certain drugs or toxins.
   • Occurs mainly in the DCT and collecting duct.
   • This step is critical for fine-tuning urine pH and eliminating substances that were not filtered at the glomerulus.

Kidney Functions in Homeostasis

Osmoregulation and Urine Concentration

The kidneys adjust urine concentration depending on the body's hydration state. The key mechanism is the countercurrent multiplier system in the loop of Henle, which builds an osmotic gradient in the renal medulla (from ~300 mOsm/L at the cortex to ~1200 mOsm/L deep in the medulla).

Antidiuretic hormone (ADH), released by the posterior pituitary, controls how much water the collecting duct reabsorbs:

• When you're dehydrated, ADH levels rise. ADH inserts aquaporin channels into collecting duct cells, making them permeable to water. Water moves out of the collecting duct by osmosis (following the medullary gradient), producing small volumes of concentrated urine.
• When you're well-hydrated, ADH levels drop. Fewer aquaporins are present, so less water is reabsorbed, and you produce large volumes of dilute urine.

Blood Pressure Regulation

The kidneys regulate blood pressure through the renin-angiotensin-aldosterone system (RAAS):

1. When blood pressure drops, specialized cells in the kidney (juxtaglomerular cells) release renin.
2. Renin converts angiotensinogen (from the liver) into angiotensin I, which is then converted to angiotensin II by ACE (angiotensin-converting enzyme) in the lungs.
3. Angiotensin II constricts blood vessels (raising pressure directly) and stimulates the adrenal cortex to release aldosterone.
4. Aldosterone increases $Na^+$ reabsorption in the DCT and collecting duct. Water follows the sodium, increasing blood volume and pressure.

Acid-Base Balance

Blood pH must stay near 7.35-7.45 for enzymes and proteins to function properly. The kidneys contribute to pH homeostasis in two ways:

• Excreting $H^+$ ions into the urine when blood is too acidic
• Reabsorbing or generating $HCO_3^-$ (bicarbonate) to buffer acids in the blood

These renal adjustments are slower than respiratory compensation (which takes seconds to minutes) but are more powerful for correcting sustained pH imbalances. Together, the lungs and kidneys prevent dangerous acidosis or alkalosis.