Chapter 19

Renal Physiology and the Urinary System

19.1 Functions of the Kidneys

The kidneys are vital organs responsible for maintaining homeostasis by regulating the composition and volume of blood and body fluids.

• Excretion of metabolic wastes: Removal of urea, creatinine, and uric acid.

• Regulation of blood volume and pressure: Adjusting water excretion and secreting renin.

• Regulation of osmolarity: Controlling solute and water balance.

• Regulation of ion concentrations: Balancing Na+, K+, Ca2+, and other ions.

• Regulation of pH: Excreting H+ and reabsorbing HCO3-.

• Production of hormones: Erythropoietin (stimulates RBC production), renin (regulates BP), and activation of vitamin D.

19.2 Anatomy of the Urinary System

The urinary system consists of the kidneys, ureters, bladder, and urethra, which work together to filter blood and excrete waste as urine.

• Renal pelvis: Collects urine from the kidney and channels it to the ureter.

• Retroperitoneal: The kidneys are located behind the peritoneum of the abdominal cavity.

• Renal cortex and medulla: The outer layer is the cortex; the inner layer is the medulla.

• Nephron: The functional unit of the kidney, consisting of a renal corpuscle and renal tubule.

• Cortical vs. juxtamedullary nephrons: Cortical nephrons are mostly in the cortex; juxtamedullary nephrons have long loops of Henle extending into the medulla and are important for concentrating urine.

Vascular Elements of the Kidney

• Blood flow: Renal artery → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries/vasa recta → renal vein.

• Renal portal system: Two capillary beds in series (glomerulus and peritubular capillaries/vasa recta).

Renal Corpuscle and Juxtaglomerular Apparatus

• Renal corpuscle: Composed of the glomerulus and Bowman's capsule; site of filtration.

• Juxtaglomerular apparatus: Specialized structure where the distal tubule contacts the afferent and efferent arterioles; regulates blood pressure and filtration rate.

• Urine formation: Fluid is considered urine once it enters the renal pelvis.

19.3 Overview of Kidney Function

The kidneys filter large volumes of plasma, reabsorb most of the filtered substances, and secrete additional wastes into the urine.

• Plasma filtered per day: ~180 L

• Urine excreted per day: ~1.5 L

• Fate of filtered fluid: Most is reabsorbed; only a small fraction is excreted as urine.

Three Processes of the Kidney

• Filtration: Movement of fluid from blood into the nephron at the glomerulus.

• Reabsorption: Movement of substances from the nephron back into the blood.

• Secretion: Active transport of substances from blood into the nephron.

• Bulk flow: Filtration occurs by bulk flow.

• Membrane proteins: Reabsorption and secretion use membrane proteins for transport.

The Nephron Modifies Fluid Volume and Osmolarity

• Bowman's capsule: Filtrate is isosmotic with plasma (300 mOsm).

• Loop of Henle: Volume decreases and osmolarity changes as fluid moves through.

• Collecting duct: Final urine volume is ~1.5 L/day.

Location in nephron	Volume of fluid	Osmolarity of fluid	Which process(es) take place here?
Bowman's capsule	180 L/day	300 mOsm	Filtration
Start of loop of Henle	54 L/day	Additional info: ~300 mOsm	Reabsorption
End of loop of Henle	18 L/day	Additional info: ~100 mOsm	Reabsorption
End of collecting duct (final urine)	1.5 L/day	Additional info: 50-1200 mOsm (varies with hydration)	Reabsorption, Secretion

• Final urine volume and osmolarity: Depend on reabsorption in the collecting duct, regulated by hormones (e.g., ADH).

• Excretion equation: $\text{Excretion} = \text{Filtration} - \text{Reabsorption} + \text{Secretion}$

19.4 Filtration

Filtration occurs at the renal corpuscle, where blood pressure forces water and solutes through the filtration barrier into Bowman's capsule.

The Renal Corpuscle Contains Filtration Barriers

• Glomerular capillaries: Fenestrated (have pores) to allow passage of water and small solutes.

• Mesangial cells: Support capillaries and regulate blood flow.

• Basal lamina: Gel-like layer that acts as a physical and charge barrier.

• Filtration barrier: Formed by capillary endothelium, basal lamina, and podocytes (with foot processes and filtration slits).

Capillary Pressure Causes Filtration

Pressure type	Average pressure measurement	Favors movement of fluid from where to where
Glomerular capillary hydrostatic pressure (PGC)	~55 mmHg	From glomerulus to Bowman's capsule (filtration)
Colloid osmotic pressure (π)	~30 mmHg	From Bowman's capsule to glomerulus (reabsorption)
Bowman's capsule hydrostatic pressure (PBC)	~15 mmHg	From Bowman's capsule to glomerulus (opposes filtration)
Net filtration pressure	~10 mmHg	Net movement from glomerulus to Bowman's capsule

• Glomerular filtration rate (GFR): The amount of filtrate produced per minute; average value is ~125 mL/min.

Regulation of GFR

• Controlled by: Resistance in afferent and efferent arterioles.

• Autoregulation: Maintains relatively constant GFR despite changes in blood pressure.

• Tubuloglomerular feedback: Macula densa cells sense NaCl and adjust afferent arteriole diameter.

• Hormonal and neural control: Sympathetic stimulation (norepinephrine) causes vasoconstriction; hormones like angiotensin II (vasoconstrictor) and prostaglandins (vasodilators) also regulate GFR.

• Podocytes and mesangial cells: Can contract to alter filtration surface area and GFR.

19.5 Reabsorption

Most filtered substances are reabsorbed from the nephron back into the blood, either actively or passively.

Active Transport of Sodium

• Na+ concentration: Filtrate entering the proximal tubule has [Na+] similar to plasma, but lower inside tubular cells.

• Mechanism: Na+ enters cells via apical membrane transporters and is pumped out basolaterally by Na+/K+-ATPase.

Secondary Active Transport: Symport with Sodium

• Examples: Glucose, amino acids, phosphate, and other molecules are reabsorbed via Na+-linked secondary active transporters.

Passive Reabsorption: Urea

• Urea: Moves by diffusion; a concentration gradient is created as water is reabsorbed, increasing urea concentration in the tubule.

Endocytosis: Plasma Proteins

• Mechanism: Filtered proteins are reabsorbed by endocytosis in the proximal tubule and degraded to amino acids.

Renal Transport Can Reach Saturation

• Transport maximum (Tm): The rate at which transporters are saturated.

• Renal threshold: Plasma concentration at which a substance first appears in urine.

• Filtration: Does not exhibit saturation; reabsorption and secretion can saturate.

• Glucose: Normally all filtered glucose is reabsorbed; appears in urine only if plasma concentration exceeds the renal threshold.

Peritubular Capillary Pressures Favor Reabsorption

• Low hydrostatic pressure and high colloid osmotic pressure in peritubular capillaries favor reabsorption of fluid from the nephron.

19.6 Secretion

Secretion is the active transport of substances from the blood into the nephron, allowing for the removal of additional wastes and regulation of ion balance.

• Examples: H+, K+, organic anions (e.g., penicillin, probenecid).

• Mechanism: Often involves tertiary active transport systems.

• Purpose: Enhances excretion of substances not filtered or filtered in small amounts.

19.7 Excretion

Excretion is the elimination of substances from the body in urine. The composition of urine reveals which substances have been filtered, reabsorbed, or secreted.

• Excretion equation: $\text{Excretion} = \text{Filtration} - \text{Reabsorption} + \text{Secretion}$

Clearance: Measuring Renal Function

• Clearance: The volume of plasma from which a substance is completely removed per unit time (mL/min).

• General equation: $\text{Clearance} = \frac{\text{Urine concentration} \times \text{Urine flow rate}}{\text{Plasma concentration}}$

• GFR estimation: Clearance of inulin or creatinine approximates GFR.

• Renal handling: Comparing clearance of a substance to GFR reveals whether it is reabsorbed or secreted.

19.8 Micturition

Micturition is the process of urination, involving storage and periodic release of urine from the bladder.

• Bladder capacity: About 500 mL.

• Prevention of leakage: Internal and external urethral sphincters.

• Micturition reflex: Stretch receptors in the bladder wall trigger contraction of the detrusor muscle and relaxation of the sphincters; voluntary control is exerted over the external sphincter.