Detailed Notes on Urine Formation and Excretion

D3 - Excretion: Lesson 2 - Urine Formation

Excretory System Review

Key components of the excretory system:
- Inferior vena cava
- Adrenal gland
- Aorta
- Renal artery
- Renal hilum
- Renal vein
- Kidney
- Ureter
- Urinary bladder
- Urethra

Kidney Review

Key structures within the kidney:
- Nephron: The functional unit of the kidney.
- Proximal convoluted tubule
- Peritubular capillaries
- Distal convoluted tubule
- Efferent arteriole
- Glomerulus: A network of capillaries where filtration occurs.
- Bowman's capsule: Surrounds the glomerulus and collects filtrate.
- Cortex: The outer region of the kidney.
- Afferent arteriole
- Renal pyramid
- Medulla: The inner region of the kidney.
- Loop of Henle: A U-shaped tube that concentrates urine.
- Vasa recta: Capillaries surrounding the Loop of Henle.
- Collecting duct: A tube that collects urine from multiple nephrons.

Urine Formation Overview

I Can…: Explain the function of the nephron in maintaining blood composition (water, pH, and ions) and describe the function of the kidney in excreting metabolic waste.

Intake vs. Output

Typical daily fluid intake and output:
- Intake: 2,500 mL/day
  - Metabolic Water: 200 mL
  - Food: 700 mL
  - Drink: 1,600 mL
- Output: 2,500 mL/day
  - Feces: 200 mL
  - Expired air: 300 mL
  - Cutaneous transpiration: 300 mL
  - Sweat: 100 mL
  - Urine: 1,500 mL

Urea

Urea formation:
- The amount of urea formed is proportional to the amount of proteins consumed.
- Excess proteins are converted to carbohydrates in the liver.
- This process produces ammonia as a byproduct, which is toxic.
- Ammonia combines with carbon dioxide to form urea: $Ammonia + Carbon \, Dioxide = Urea$

Nitrogenous Wastes

Nitrogenous wastes:
- Proteins are broken down into amino acids.
- Amino acids produce ammonia ( $NH_2$ ) as a byproduct.
- Ammonia is converted to urea ( $H2N - C - NH2$ ) in the liver.
- Urea and uric acid are excreted in urine.
- Energy is needed to produce these compounds, and water is needed to excrete them.
- Chemical structures of urea ( $H2N-C-NH2$ ) and uric acid (O=C\NC\N') are shown.

Urine Formation Processes

Four crucial processes in urine formation:
- Glomerular filtration: Moves water and small solutes from blood plasma to the nephron.
- Tubular reabsorption: Reabsorbs useful solutes (e.g., sodium) from the nephron back into the bloodstream.
- Tubular secretion: Additional wastes move from the capillary into the nephron.
- Water reabsorption: Removes water from the filtrate and returns it to the blood.

Steps of Urine Formation

Urine formation involves four key steps:
1. Glomerular filtration: Creates a plasma-like filtrate of the blood in Bowman's capsule.
2. Tubular reabsorption: Removes useful substances from the filtrate and returns them to the blood for reuse in the tubule.
3. Tubular secretion: Adds wastes from the blood to the filtrate in the capillary.
4. Water reabsorption: Removes water from the filtrate and returns it to the blood for reuse.

Glomerular Filtration Factors

Two important factors for kidney filtration:
1. Capillaries in the glomerulus are extra permeable due to tiny pores in tissue walls, allowing small molecules to pass through while keeping large proteins out.
2. Blood pressure in the glomerulus is four times higher than in other capillary beds, allowing up to 2000L to pass through the kidneys each day!

Glomerular Filtration Pathway

Pathway of blood during glomerular filtration:
- Blood flows from the glomerulus to the afferent arteriole to Bowman’s capsule and then to the proximal tubule, where filtrate is formed.
Glomerulus: Functions as a high-pressure, semi-permeable filter.
- Water ( $H_2O$ ), salts, glucose, amino acids, hydrogen ions ( $H^+$ ), and urea can pass through.
- Blood plasma proteins, erythrocytes (red blood cells), and platelets cannot pass through.

Urea Production

Urea production and excretion:
- Urea passes through the glomerulus.
- Excess protein consumption leads to the breakdown of amino acids in the liver.
- This results in the production of ammonia ( $NH_4$ ), a nitrogenous compound that can become toxic if it accumulates.
In the liver:
- $2NH4 + CO2 \rightarrow urea$
- Urea is released into the bloodstream and excreted through the kidneys into the urine.

Tubular Reabsorption

Tubular reabsorption in the proximal tubule:
- 65% of the filtrate that passes through the proximal tubule is reabsorbed and returned to the body.
- Reabsorption is achieved by active and passive transport mechanisms.
- Anions and water return to the blood through diffusion and osmosis, respectively.

Proximal Tubule Characteristics

Proximal tubule structure and function:
- The cells of the proximal tubule have many mitochondria, which provide energy for active transport of glucose and sodium.
- Anions and water return to the blood through diffusion and osmosis, respectively.

Loop of Henle

Loop of Henle transport mechanisms:
- Water moves by osmosis into the capillary.
- The descending limb is permeable to water, allowing water to diffuse from the filtrate into surrounding capillaries.
- The ascending limb is impermeable to water.

Distal Tubule

Distal tubule characteristics:
- Reabsorption of sodium ( $Na^+$ ) from the filtrate into capillaries depends on the body’s needs.
- Passive absorption of negative ions occurs by electrical attraction; decreased filtrate concentration triggers osmosis (movement of water out of the tubule).
- Potassium ( $K^+$ ) and hydrogen ( $H^+$ ) ions are actively transported into the tubule to maintain blood pH.
- Drugs and metabolites are secreted into the distal tubule.

Distal Tubule and Aldosterone

Aldosterone's role:
- Reabsorption of $Na^+$ is aided by aldosterone, a hormone that makes the distal tubules more permeable to NaCl.
- As a result, water is also reabsorbed to maintain the osmotic gradient.
- Consider the consequences of hyper- vs. hyposecretion of aldosterone.

Collecting Duct

Collecting duct function:
- Filtrate entering the collecting duct still contains a lot of water.
- The collecting duct passes through the salty medulla, allowing passive reabsorption via osmosis.
- Reabsorption of water concentrates the filtrate, forming urine.

Collecting Duct and ADH

ADH's role:
- Direct reabsorption of water is aided by antidiuretic hormone (ADH).
- ADH increases the permeability of the collecting ducts to water ( $H_2O$ )
- Consider the consequences of hyper- vs. hyposecretion of ADH.

Tubular Secretion

Tubular secretion process:
- Secretion is the movement of wastes from the blood into the proximal tubule, distal tubule, or collecting duct of a nephron.
- Examples of secreted substances include ammonia, excess hydrogen ions ( $H^+$ ), and minerals.
pH control:
- pH is controlled by the secretion of excess hydrogen ions ( $H^+$ ) and the restoration of bicarbonate ions ( $HCO_3^-$ ) in the blood.

Summary Diagram

The diagram summarizes transport mechanisms in different parts of the nephron:
- Proximal tubule: Reabsorbs nutrients, NaCl, $HCO3^-$ , and $H2O$ , and secretes $H^+$ .
- Descending limb of the Loop of Henle: Reabsorbs $H_2O$ .
- Thick segment of the ascending limb: Reabsorbs NaCl.
- Distal tubule: Reabsorbs NaCl and $H_2O$ , and secretes $K^+$ and $H^+$ .
- Collecting duct: Reabsorbs $H_2O$ and urea.

Overall Summary

Reabsorption of water ( $H2O$ ), NaCl, glucose ( $C6H{12}O6$ ), potassium ( $K^+$ ), amino acids, and urea begins in the proximal tubule.
Majority of water reabsorption occurs in the descending limb of the Loop of Henle.
The Loop of Henle extends down into the extremely salty medulla, allowing water to be absorbed via osmosis.
The ascending limb of the Loop of Henle is not permeable to water.
- Salt (NaCl) is reabsorbed through diffusion (in the lower portion) and active transport (in the upper portion).
In the distal convoluted tubule, additional water and salt can be reabsorbed if necessary, requiring the action of hormones.

Urine Formation Summary Table

1. Glomerulus and Bowman's Capsule
- Filtration of water and dissolved solutes occurs as blood is forced through walls of glomerulus into Bowman's capsule by fluid pressure in capillaries.
- Substances transported: water ( $H_2O$ ), salt (NaCl), urea, uric acid, and minerals.
2. Proximal Tubule
- Selective reabsorption of nutrients from the filtrate back into the blood occurs by active and passive transport.
- Within the proximal tubule, pH is controlled by the secretion of hydrogen ions ( $H^+$ ) and the reabsorption of bicarbonate ions ( $HCO_3^-$ ).
3. Descending limb of Loop of Henle
- Water ( $H_2O$ ) out of the tubule and into the blood.
4. Ascending limb of Loop of Henle
5. Distal Tubule
6. Collecting Duct