Excretion and the Structure and Function of the Human Kidney

Excretion in Humans

Urine Production

Urine is the main excretory product in humans, produced by the kidneys to remove waste products and maintain water and salt balance. The volume and composition of urine can vary depending on factors such as water intake and loss through sweat.

• Average Daily Production: An adult human produces about 1.5 dm3 (1500 cm3) of urine per day.

• Waste Content: Each litre of urine contains approximately 40 g of waste products and salts.

• Water Balance: Water is gained through drinking, food, and cell respiration, and lost via urine, sweat, exhaled air, and feces.

• Example: If a person drinks 1400 cm3 of water, total water intake (including food and metabolic water) may reach 2600 cm3.

Structure and Function of the Kidney

Gross Anatomy of the Kidney

The kidney is a vital organ responsible for filtering blood and forming urine. Its structure is divided into distinct regions, each with specific functions.

• Cortex: The darker outer region of the kidney, containing the filtering units called nephrons.

• Medulla: The middle layer, where tubules run down and eventually join up to form the tips of pyramids.

• Pelvis: A funnel-like structure that collects urine from the medulla and passes it to the ureter.

• Ureter: The tube that carries urine from the kidney to the bladder.

Nephron Structure

The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains about one million nephrons.

• Bowman's Capsule: A hollow cup of cells at the start of the nephron, surrounding a ball of capillaries called the glomerulus.

• Glomerulus: A network of capillaries supplied by small arteries (arterioles), where filtration occurs.

• Basement Membrane: A non-cellular layer between Bowman's capsule and glomerulus, acting as a filter.

• Renal Tubule: The tubule continues from Bowman's capsule, passing through the cortex and medulla, and includes the loop of Henle and collecting duct.

Ultrafiltration in the Bowman's Capsule

Ultrafiltration is the process by which blood is filtered under pressure in the Bowman's capsule, separating small molecules from larger ones.

• Process: Blood pressure forces water, ions, and small molecules (such as glucose and urea) through the filter formed by the capillary wall, basement membrane, and Bowman's capsule.

• Selective Filtration: Large molecules (e.g., proteins) and blood cells are retained in the blood, while smaller molecules pass into the capsule space as glomerular filtrate.

• Definition: Ultrafiltration is the separation of different-sized molecules under pressure through a semi-permeable membrane.

• Example: Glucose, urea, and ions are found in the glomerular filtrate, but proteins and blood cells are not.

Table: Composition of Urine (Inferred from Context)

The following table summarizes the main waste products and salts found in urine.

Component	Approximate Amount per Litre	Function/Origin
Urea	~20 g	Waste product from protein metabolism
Salts (e.g., NaCl)	~15 g	Excess ions from diet and metabolism
Other waste products	~5 g	Includes creatinine, uric acid, etc.
Water	960 g	Solvent for excretion

Additional info: Table entries inferred from standard urine composition data.

Key Terms and Definitions

• Excretion: The removal of metabolic waste products from the body.

• Nephron: The functional unit of the kidney, responsible for filtering blood and forming urine.

• Ultrafiltration: The process of filtering blood under pressure in the Bowman's capsule.

• Glomerular Filtrate: The fluid that enters the Bowman's capsule after filtration.

Relevant Equations

• Filtration Rate Equation:

$\text{Filtration Rate} = \text{Glomerular Pressure} \times \text{Filtration Coefficient}$

• Osmotic Balance Equation (Additional info):

$\text{Osmotic Pressure} = iMRT$

Additional info: Equations provided for academic completeness.