7.3 - The Kidneys

The Urinary System

The system is composed of the kidneys, ureters, bladder and urethra that is responsible for the excretion of wastes.

The Kidneys

- reddish brown, bean shaped structure that is formed by millions of nephrons and collecting ducts.

Structure of the Kidney

Renal capsule: Surrounds the kidney

Renal Cortex: Outer part of the kidney contains the top half of the nephron

Renal Medulla: The Inner part of the kidney, which consists of the bottom half of the nephron.

Renal Pyramid: Cone-shaped structures within the medulla

Renal columns: It separates the renal pyramids and is where blood vessels lie.

Renal pelvis: It funnels urine into the ureter.

Renal Hilum: Where vessels enter and leave the kidney.

Ureter: Carries urine to the bladder

Bladder: An organ in the pelvis that stores urine.

Urethra: A muscular tube that connects the bladder to the outside of the body, serving as the pathway for urine to exit the urinary system.

Nephron

- A microscopic functional unit of the kidney

- The role of the nephron is to: Form urine, remove waste from blood and regulate blood composition.

- Around 1.2 million in each kidney -> Increasing the SA for each process.

- Each has a complex network of blood capillaries -> Increasing efficiency of processes

Nephron Structures

- Renal Artery and Renal Vein

- Afferent Arteriole and Efferent Arteriole

- Peritubular Capillaries

- Renal Corpuscle (Glomerulus and Glomerular Capsule)

- Proximal convoluted tubule

- Descending Limb, Loop of Henle, and Ascending Limb

- Distal convoluted tubule -> Collecting Duct -> Renal Pelvis

The Structures of the Nephron - Renal Artery

- Carries blood to the kidneys

- Carries 1/4 of the blood from the heart, 1/8 to each kidney

- Efficient filtration -> 1.2L/ minute

The Structures of the Nephron - Afferent arteriole

- Carries blood from the renal artery to the glomerulus

- Has a wider diameter so more blood can be delivered.

!(B) IN THE DIAGRAM!

The Structures of the Nephron - Glomerulus

- Network of capillaries

- 1 cell-thick wall -> shorter distance for the transport of materials to the glomerular capsule.

- Permeable membrane -> increase efficiency for the transport of materials. Although large structures like erythrocytes, leukocytes and proteins cannot pass.

The Structures of the Nephron - Efferent arteriole

- Carries blood away from the glomerulus to the peritubular capillaries.

- Narrower diameter so less blood can be removed -> Increases the blood pressure in the glomerulus.

!(A) IN THE DIAGRAM!

The Structures of the Nephron - Peritubular Capillaries

- Secondary capillary network from efferent arteriole to renal venules.

- Surrounds the renal tubule and collecting duct -> increases surface area + less distance

!(C) IN THE DIAGRAM!

The Structures of the Nephron - Glomerular Capsule

- Expanded sac that surrounds the glomerulus.

- 1 cell-thick wall -> Short distance for transport of materials

- Fluid inside is called filtrate

The Structures of the Nephron - Proximal convoluted tubule

- A winding tube, carrying filtrate to the Loop of Henle.

- 1 cell-thick wall -> More efficient transport of materials

The Structures of the Nephron - Loop of Henle

- Tube carrying filtrate to the distal convoluted tubule

- Long bend -> Large surface area

- 1 cell-thick wall

The Structures of the Nephron - Loop of Henle: Descending Limb

- Large number of aquaporins -> More efficient transport of water out of the filtrate.

- Making the filtrate hypertonic

The Structures of the Nephron - Loop of Henle: Ascending Limb

- Few aquaporins -> More efficient transport of NaCl out of the filtrate.

- Making the filtrate hypotonic

The Structures of the Nephron - Distal Convoluted Tubule

- A winding tube carrying filtrate to the collecting duct. -> Winding = Large surface area

- 1 cell-thick wall

The Structures of the Nephron - Renal Vein

- Carries blood away from the kidney

The Structures of the Nephron - Collecting duct

- DCT from many nephrons connected, delivering filtrate, now called urine.

- Carries urine to renal pelvis

The Structures of the Nephron - Renal Pelvis

- A funnel-shaped chamber on the concave side of the kidney.

- Channels urine to the ureter

State how the renal artery, renal vein and ureter differ by the substances they carry.

- The renal artery carries unfiltered blood into the kidney.

- The renal vein carries filtered blood out of the kidney.

- The ureter carries urine away from the kidney.

The Formation of Urine

1. Glomerular Fitration

2. Selective Reabsorption

3. Tubular Secretion

Glomerular Filtration (Blood -> Nephron)

- Occurs in the renal corpuscle (glomerulus + Glomerular Capsule).

- High pressure in the glomerulus pushes water and small molecules out of the blood into the capsule.

- Large molecules like proteins and blood cells stay in the blood.

Tubular/ Selective Reabsorption (Nephron -> Blood)

- Happens along the renal tubule, specifically: The proximal convoluted tubule, the descending limb, the ascending limb, and the distal convoluted tubule.

- Useful substances are selectively reabsorbed from the filtrate back into the blood.

Facultative Reabsorption

- The amount of water reabsorbed is controlled by hormones depending on the body's needs.

Examples of the Substances involved in Tubular/ Selective Reabsorption at specific points in the Nephron: (2)

Descending Limb:

-> Water (mostly)

-> Not permeable to salt (Cannot leave the filtrate) -> filtrate becomes hypertonic -> increasing concentration gradient.

Ascending Limb:

-> Increase in solutes

-> Not permeable to water (Cannot leave the filtrate)

-> Mostly Na+ and Cl-

Examples of the Substances involved in Tubular/ Selective Reabsorption at specific points in the Nephron: (1)

Proximal Convoluted Tubule:

-> Glucose

-> Amino Acids (most)

-> Water

-> Ions: Na+, Cl-, K+, Ca2+ HCO3+

Distal Convoluted tubule:

-> Some Na+, Cl- and water.

-> Depending on the body, regulatory concentrations will be different.

Collecting Duct:

-> Water

-> Urea

Tubular Secretion (Blood -> Nephron)

- It occurs mainly in the proximal convoluted tubule and the distal convoluted tubule.

- Adds substances from the blood into the filtrate.

- Removes unwanted materials from the body and helps regulate blood pH. (Blood pH 7.35-7.45)

Examples of the Substances involved in Tubular Secretion at specific points in the Nephron:

Proximal convoluted tubules:

-> H+ -> regulates pH

-> NH4+ (Ammonium)

-> Creatinine

-> Drugs

Distal convoluted tubule:

-> H+

-> K+

-> NH4+

-> Drugs (Penicillin)

Collecting Duct:

-> H+ and K+

Ascending and Descending Limb:

-> No secretion

How is the glomerulus well suited to its function of filtration?

- It has high blood pressure, created by the afferent arteriole being wider than the efferent arteriole, which forces more fluid out of the blood.

- The blood is filtered through only two thin layers, one from the capillary wall and one from the capsule wall, allowing easy passage of small molecules.

- The walls are differentially permeable, which means small molecules like water and ions can pass through, but large molecules like red blood cells and proteins cannot.

- The large surface area of the glomerulus allows for more filtration at once.

- Over a million nephrons are present per kidney, this results in a large volume of filtrate being produced.

How is the nephron wall suited for it's function?

- The glomerular capsule and capillaries are only one cell thick, allowing for easy diffusion of substances.

- The tubule has many folds and a long loop, giving it a large surface area for reabsorption and secretion.

- Each kidney has over a million nephrons, which means there's a huge total surface area available to efficiently reabsorb useful substances and remove wastes.

- The constant flow of blood helps maintain a concentration gradient, which aids in the efficient movement of substances across the nephron wall.

Components in Urine

- Water (96%)

- Nitrogenous wastes (2%)

-> Urea

-> Uric Acid

-> Creatinine

- Various Ions (1.5%)

- Other/ Bile pigments (0.5%)

Components that should NOT be in Urine

- Leukocytes -> Too big to enter the glomerulus

- Erythrocytes -> Too big to exit the glomerulus

- Plasma Proteins -> Too big to exit the glomerulus

- Glucose -> Is fully absorbed