The Urinary System: Filtering Blood and Balancing Fluids

1Overview

Every minute of cellular metabolism (Lesson 2) generates waste, and every meal and drink changes the amount of water, salt and acid in the body. The urinary system removes metabolic wastes and adjusts the composition of blood within narrow limits, so it is central to homeostasis. This lesson covers the structure of the kidneys and nephrons, the three steps that form urine, the hormonal control of water and salt balance, the role of the kidneys in acid-base balance, and the storage and release of urine.

2Organization of the urinary system

The urinary system consists of two kidneys, two ureters, the urinary bladder and the urethra. The kidneys are bean-shaped, about 11 to 14 cm long, and sit against the back wall of the abdomen on each side of the spine, protected by the lower ribs and padded by fat. Although they make up less than 1% of body mass, they receive about 25% of the heart's output (Lesson 8), a sign of how much blood-processing they do.

Functions

• Excretion of wastes: urea (from amino acid breakdown), creatinine (from muscle), uric acid and drug and toxin metabolites.
• Regulation of blood volume and pressure: by changing the amount of water and sodium excreted and by releasing renin.
• Regulation of ion concentrations: sodium, potassium, calcium and others.
• Regulation of pH: by excreting hydrogen ions and conserving bicarbonate.
• Hormone production: erythropoietin (stimulates red blood cell production, Lesson 8) and calcitriol (active vitamin D, which supports calcium absorption in the digestive system).

Internal structure

A kidney has an outer cortex, an inner medulla arranged in renal pyramids, and a central renal pelvis that collects urine and leads into the ureter. Each kidney contains about 1.3 million microscopic functional units called nephrons. A nephron has a renal corpuscle (the glomerulus, a ball of capillaries, enclosed in Bowman's capsule) and a long renal tubule that is divided into the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and a connection to a collecting duct that is shared among nephrons. Blood enters the glomerulus by an afferent arteriole and leaves by an efferent arteriole, and then flows around the tubule in peritubular capillaries.

3Forming urine: filtration, reabsorption and secretion

Glomerular filtration

The wall of a glomerular capillary has pores, a basement membrane and slits between specialized cells called podocytes. Together these let water and small solutes (ions, glucose, amino acids, urea) pass into Bowman's capsule but hold back blood cells and most proteins. The fluid formed is the filtrate, essentially plasma without proteins. The rate is the glomerular filtration rate (GFR), about 125 mL per minute in males and 105 mL per minute in females, which equals roughly 180 L a day. The filtrate volume is about 60 times the volume of plasma, so the plasma is filtered many times each day.

Filtration is driven by pressure. Blood pressure inside the glomerulus (about 55 mm Hg) pushes fluid out, and two forces push back: capsular hydrostatic pressure (about 15 mm Hg) and blood colloid osmotic pressure (about 30 mm Hg, produced by plasma proteins). The net filtration pressure is 55 - (15 + 30) = 10 mm Hg.

Tubular reabsorption

Of the 180 L filtered per day, about 99% returns to the blood, and only about 1.5 L leaves as urine. About two thirds of the filtered water and sodium and nearly all the glucose and amino acids are reabsorbed in the proximal convoluted tubule, which has a brush border of microvilli much like the intestine (Lesson 11). Transport uses the processes of Lesson 3: sodium is pumped out of the tubule cells into the blood by the sodium-potassium pump, and glucose and amino acids follow sodium by co-transport, while water follows by osmosis. Reabsorption of glucose has a limit, the transport maximum. In uncontrolled diabetes mellitus (Lesson 7) the blood glucose exceeds this limit, so glucose appears in the urine and carries water with it, which causes frequent urination and thirst.

Tubular secretion

In secretion, substances move from the blood into the tubule. Hydrogen ions, potassium ions, ammonium and many drugs such as penicillin are removed this way, which clears substances that filtration alone would leave behind and helps adjust blood pH.

Secretion therefore acts as a second route into the tubule, in addition to filtration, and it is especially important for substances bound to plasma proteins that filtration cannot remove. The result of the three steps is summarized by a simple relation: amount excreted = amount filtered - amount reabsorbed + amount secreted.

Two kinds of nephron and clearance

About 85% of nephrons are cortical nephrons, with short loops that stay mostly in the cortex. The remaining 15% are juxtamedullary nephrons, whose long loops reach deep into the medulla and are responsible for the strongest concentrating power. The longer the loop, the saltier the medulla can become and the more concentrated the urine can be.

Clinicians estimate kidney function from the clearance of creatinine, a muscle waste that is filtered and hardly reabsorbed. If a blood test shows a falling clearance, GFR is falling and nephrons are being lost. A urine test (urinalysis) can detect protein or blood, which suggests a damaged filtration barrier, or glucose, which suggests that the transport maximum has been exceeded.

4Concentrating urine and water balance

The loop of Henle and collecting duct allow the kidneys to produce urine that is much more concentrated than blood. The descending limb of the loop is permeable to water but not to salt, and the ascending limb pumps out salt but is impermeable to water. This arrangement, called the countercurrent multiplier, builds an increasingly salty fluid in the medulla. Where the collecting duct passes through this region, water can leave by osmosis, but only if the duct is permeable to water.

Antidiuretic hormone (ADH), released from the posterior pituitary (Lesson 7) when osmoreceptors in the hypothalamus detect high blood osmolarity, inserts water channels (aquaporins) into collecting duct cells. Water then flows out of the duct, so a small volume of concentrated urine forms. When the body has too much water, ADH release falls, the duct stays impermeable, and a large volume of dilute urine forms. Alcohol suppresses ADH release, which explains the increased urination after drinking.

5Hormonal regulation of blood pressure and sodium

The kidneys sense and regulate blood pressure through the renin-angiotensin-aldosterone system. When blood pressure or sodium falls, cells next to the glomerulus release the enzyme renin, which starts a chain that produces angiotensin II. Angiotensin II constricts blood vessels and stimulates the adrenal cortex to release aldosterone (Lesson 7). Aldosterone makes the distal tubule and collecting duct reabsorb sodium, and water follows, increasing blood volume and pressure.

A counterbalancing hormone is atrial natriuretic peptide (ANP), released by the heart's atria when they are stretched by high blood volume. ANP promotes sodium and water loss and lowers blood pressure. The two systems act in opposition, a typical homeostatic arrangement. Drugs that block the renin-angiotensin system (ACE inhibitors) and diuretics that increase urine output are common treatments for high blood pressure (Lesson 14).

6Acid-base balance and the bladder

Acid-base balance

Normal blood pH is 7.35 to 7.45. As Lesson 1 showed, the bicarbonate buffer system keeps pH stable, with a bicarbonate to carbonic acid ratio of about 20 to 1. The lungs adjust the carbon dioxide (acid) component within minutes by changing the breathing rate (Lesson 10), whereas the kidneys adjust the bicarbonate component more slowly, over hours to days, by secreting hydrogen ions into the filtrate and by making new bicarbonate. In metabolic acidosis, for example in uncontrolled diabetes, the lungs compensate by breathing deeply. In respiratory acidosis, caused by poor ventilation, the kidneys compensate by retaining bicarbonate.

Storage and release of urine

Urine is carried to the bladder by peristalsis of the ureters (about 30 cm long). The bladder wall is made of smooth muscle called the detrusor and can hold 500 to 600 mL, although the urge to urinate begins at about 200 mL. The urethra is about 4 cm in females and about 20 cm in males; the shorter female urethra is one reason urinary tract infections are more common in females. Two sphincters control release: an involuntary internal sphincter and a voluntary external sphincter made of skeletal muscle.

In the micturition reflex, stretch receptors send signals to the spinal cord, which stimulates parasympathetic nerves to contract the detrusor and relax the internal sphincter. Higher brain centres, including the cerebral cortex, allow the reflex to be delayed by keeping the external sphincter closed until a suitable time. Infants lack this control until about age two or three, because the nerve pathways that carry voluntary control from the brain to the external sphincter mature gradually.

7Interdependence and health

The urinary system illustrates all three big ideas of the course. Homeostasis is maintained by negative feedback loops that sense volume, pressure and pH. Gene expression and the environment interact because salt intake, hydration and disease alter how the kidney works over a lifetime. The organ systems are interrelated because the kidney cannot work without a steady blood supply from the heart, hormones from the endocrine system and bicarbonate and carbon dioxide balance with the lungs.

Disorders show the interdependence. Kidney stones are crystals of calcium salts or uric acid that obstruct a ureter. Urinary tract infections occur when bacteria ascend the urethra. Chronic high blood pressure and diabetes are the two leading causes of chronic kidney disease, in which nephrons are lost gradually. When kidneys fail, dialysis uses a machine to filter the blood artificially, or a kidney transplant replaces the organ. Because the body has more kidney tissue than it needs, a person can live well with one kidney, although the remaining kidney carries a higher workload over many years. Maintaining a healthy blood pressure, controlling blood glucose and limiting salt intake all protect the nephrons.

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