Urinary System and Kidneys: Structure and Function

Urinary System and Kidneys

Overview

The urinary system is essential for the removal of metabolic wastes and the regulation of blood composition. It consists of several organs that work together to filter blood, form urine, and excrete waste products from the body.

• Kidneys

• Ureters

• Urinary bladder

• Urethra

Functions of the Urinary System

Excretion

• Urea: Produced from the breakdown of amino acids.

• Uric acid: Produced from the breakdown of nucleic acids.

• Creatinine: Produced from the breakdown of creatine phosphate, a muscle source of ATP.

Regulation of Blood

• Volume: Controls the amount of water excreted.

• Salts: Regulates electrolyte balance.

• Acids & Bases: Maintains acid-base homeostasis.

Kidney Gross Anatomy

Location and Protection

• Located on the posterior abdominal wall.

• Protected by a layer of fat tissue.

Shape and Structure

• Kidney bean-shaped organs.

• Surrounded by a tough capsule and fat for protection.

• Topped by the adrenal glands (which produce hormones such as adrenaline and corticosteroids).

Frontal Section: Regions of the Kidney

• Cortex: The outer region.

• Medulla: The middle region, containing renal pyramids.

• Renal sinus: The inner cavity where urine drains.

Medulla Details

• Renal pyramids: Cone-shaped tissues in the medulla.

• Renal papilla: The tip of each pyramid, where urine is funneled into the minor calyx.

• Renal columns: Extensions of cortical tissue between pyramids.

Microscopic View: The Nephron

Nephron Structure and Function

The nephron is the structural and functional unit of the kidney, responsible for urine formation. Each kidney contains about 1 million nephrons.

• Performs filtration, reabsorption, secretion, and excretion.

Nephron Components

• Glomerulus: A small capillary bed with a 'leaky' endothelium, allowing filtration of blood plasma. Receives blood from the afferent arteriole and drains into the efferent arteriole.

• Bowman's capsule: Surrounds the glomerulus and collects the filtrate.

• Proximal convoluted tubule (PCT): Located in the cortex; lined with simple cuboidal epithelium. Responsible for reabsorption of plasma and secretion/excretion of substances.

• Loop of Henle: Dips into the medulla and returns to the cortex. Has a descending limb (permeable to water) and an ascending limb (permeable to ions). Responsible for reabsorption of water and ions.

• Distal convoluted tubule (DCT): Located in the cortex; also lined with simple cuboidal epithelium. Functions in reabsorption and secretion/excretion.

• Collecting duct: Receives urine from multiple nephrons, passes through the medulla, and empties into the minor calyx at the renal papilla.

Filtration Membrane

• The glomerular filtration membrane is 'leaky,' allowing water, small proteins, ions, glucose, amino acids, and urea to pass into the nephron, but preventing cells and large proteins from passing.

Nephron Functions

• Filtration: Occurs at the glomerulus; blood plasma is filtered into Bowman's capsule.

• Reabsorption: Most water, ions, and nutrients are reabsorbed from the PCT and DCT back into the blood.

• Concentration of Urine: The loop of Henle and DCT concentrate urine by reabsorbing water and ions.

• Secretion/Excretion: Additional wastes are secreted into the tubules for excretion.

Filtrate Flow and Urea Recycling

• Filtrate flows from Bowman's capsule → PCT → loop of Henle → DCT → collecting duct.

• Urea is lost in the urine, but also contributes to the concentration of the interstitial fluid and can re-enter the descending limb of the loop of Henle.

Water Reabsorption and Concentration Gradient

• The concentration of interstitial fluid increases from the cortex (300 mOsm) to the inner medulla (1200 mOsm).

• Water reabsorption occurs along the nephron, with about 65% reabsorbed in the proximal tubule, 15% in the loop of Henle, and 19% in the distal tubule. Only about 1% of the original filtrate remains as urine.

Collecting Tubules

• Located in both cortex and medulla.

• End at the renal papilla and empty into the minor calyx.

Hormonal Regulation: Antidiuretic Hormone (ADH)

• ADH (antidiuretic hormone) increases the number of water channels in the DCT and collecting duct, enhancing water reabsorption.

• Without ADH, water is not reabsorbed, resulting in large amounts of dilute urine.

• Higher levels of ADH increase water permeability and the number of water channels, allowing the body to conserve water.

Differences between Solute Concentrations in Urine and Plasma

The composition of urine differs significantly from plasma, reflecting the kidney's role in excretion and homeostasis.

Solute	Urine	Plasma
Ions (mEq/L)
Sodium (Na+)	147.5	138.4
Potassium (K+)	47.5	4.4
Chloride (Cl-)	153.3	106
Bicarbonate (HCO3-)	1.9	27
Metabolites and Nutrients (mg/dl)
Glucose	0.009	90
Lipids	0.000	600
Amino acids	0.018	5.0
Proteins	0.000	7.5 g/dl
Nitrogenous Wastes (mg/dl)
Urea	1800	10-20
Creatinine	150	1.0-1.5
Ammonia	60	0.02-0.05
Uric acid	40	3-7

Example: Clinical Application

Abnormal concentrations of solutes in urine can indicate kidney dysfunction or metabolic disorders. For example, the presence of glucose in urine (glycosuria) is a sign of diabetes mellitus.

Additional info: The adrenal glands, located atop each kidney, secrete hormones such as aldosterone (which regulates sodium and potassium balance) and cortisol (involved in stress response).