chapter 10: the kidney

The kidney is the primary organ of the excretory system, aka the urinary system. The excretory system’s purpose is to conserve or eliminate water and remove bodily wastes by all mean possible. Urination (aka micturition)is how we get rid of most of our bodily waste and excess water. However, we excrete more than urine. We also get rid of waste and water by:

1. Respiration – Exhaling carbon dioxide and water vapor
2. Sweating – Excess water, salt, and waste products
3. Defecation  – Undigested food (poop, poo-poo, poopies, Windows Vista)

From Blood Plasma to Urine

At rest, your kidneys receive about 20% of your blood volume each minute. Twenty percent of the 20% of the blood plasma enters little filters in the kidney called nephrons. However, most of the filtred plasma returns to the blood after removing waste, excess water and solutes. (It takes about 45 minutes to filter the total blood plasma volume.) What remains in the nephrons is a solution that becomes urine. The urine will leave the kidneys via small, muscular tubes called ureters. Peristalsis pushes the urine down into the pelvic cavity, where it collects in the urinary bladder. As more and more urine enters the urinary bladder, the bladder begins to expand, and the expansion triggers the urge to urinate. During urination, a consciously controlled sphincter muscle opens, and urine flows out of the body through the urethra.  

Why Urine is Yellow 

Healthy urine’s color is a story involving red blood cells, bile, bacteria, and poop.  

Please don’t say we urinate poop. Please don’t say we urinate poop. Please don’t say we urinate poop.  

We don’t urinate poop, but the process that makes poopies brown is the same process that turns urine yellow.  

So we poop urine?  

No. If you did, you’d be a bird.   

Well, I am not a bird.  

Thanks for clarifying that.  

Though, I do like seeds.  

I will start to ignore you now and explain why urine is yellow.    

Your liver does many jobs, but one of its primary jobs is detoxifying the blood. Protein catabolism will produce ammonia, which is toxic. Our livers convert ammonia to urea, which is less toxic and quickly moves through cell membranes (high permeability). The kidneys and, to a lesser extent, the sudoriferous (sweat) glands excrete urea.  

So the yellow color comes from urea? 

No. Urea is the primary waste solute. However, the liver does create bilirubin, which is also a metabolite of protein catabolism.  

What is bilirubin? 

Good question. When a red blood cell dies, its hemoglobin enters the blood plasma. The spleen, liver, and red bone marrow break down hemoglobin into bilirubin, which travels to the liver and becomes bile. The liver and gallbladder secrete bile into the duodenum, where it emulsifies fats. The bile becomes a part of the slurry of undigested food and enters the large intestine, where bacteria convert it to stercobilin. Some of the stercobilin diffuses into the blood and oxidizes into urochrome (aka urobilinogen); the pigment gives urine its yellow color. The remaining stercobilin mixes with the undigested food (poop), turning it brown.

A Quick Review of Excretion and Secretion

Excretion refers to removing a substance that your body does not need. Urine is an example of excretion because it contains waste, excess water, and nutrients.  Secretion is a solution your body releases that has a function. Saliva is a secretion because it moistens food and contains digestive enzymes. Some secretions are also excretions, such as sweat. Sweat cools our body down (secretion), but it also has waste products (excretion).  

A Brief Review of Passive and Active Transport

Passive transport comprises diffusion, osmosis, and hydrostatic pressure. Diffusion and osmosis are the movements of solutes (diffusion) or water (osmosis) from a high concentration to a low concentration. Hydrostatic pressure is the movement of water down its pressure gradient. Osmosis and hydrostatic pressure both involve the passive transport of water. But osmosis is water diffusion through a semipermeable membrane, and hydrostatic pressure is water pushed (pressure) and does not have to occur through a membrane. 

Active transport requires a cell to use energy to move substances across a cell membrane. Protein pumps (Na⁺/K⁺ pump), phagocytosis (think macrophage), and exocytosis (“puking” cells) are examples of active transport.

Kidney Gross Anatomy and Physiology

You have two kidneys; one on the right side of the abdominal cavity and one on the abdominal cavity’s left side. The kidneys are deep to the digestive organs and are surrounded by a layer of protective adipose tissue.  The kidney receives blood via the renal arteries, and the renal veins transport blood away from them. 

A cross-section of a kidney gives a detailed look at its gross anatomy. The outer cortex and inner medulla house all of the nephrons (tiny filters). Each kidney has about a million nephrons, and like most neurons, the number is finite – in other words, once you lose them, they are gone for good. Deep to the medulla is the minor and major calyxes, which funnel urine from the nephrons into the renal pelvis. At the renal pelvis, urine leaves the kidney and moves towards the urinary bladder via a ureter. 

The kidney’s primary job is to maintain blood plasma osmolarity via:

1. Filtration: The removal of water and small solutes from the blood plasma
2. Reabsorption – Putting almost all of the water and solutes that were filtered out back into the blood plasma
3. Secretion – Regulates blood pH and plasma K+ levels
4. Excretion – Removal of the final product (urine)

During filtration, reabsorption, and secretion, the kidneys: 

1. Maintain plasma osmolarity (water to solute contraction)
2. Remove metabolic waste, such as urea 
3. Regulate blood plasma pH

Nephron Basics

Nephrons are microscopic tubes that filter blood plasma. Structurally they look as if a curly straw and a paperclip had a baby. The nephron’s many twists and turns ensure that most filtered plasma returns to the blood.  

Nephrons have two primary segments and a segment they share with other nephrons. The nephron parts are listed below from their proximal to their distal end.

1. The glomerular capsule (aka Bowan’s capsule) is a Pac-Man-shaped structure where the blood plasma that leaked out of the capillaries first filters through.   
2. The renal tube comprises, in order, the proximal convoluted tubule, the loop of Henle (no relation to Don Henley of The Eagles), and the distal convoluted tubule. 
3. Collection ducts are not a part of the nephron. They connect to multiple distal convoluted tubules, where they collect each nephron’s filtrate. The combined filtrate becomes urine (pee, pee-pee, tinkle, nitrogenous waste product 4000, Windows Vista).  

The nephron has three primary functions:

1. Filtration – The passive transport of some of the blood plasma into the nephron, becoming the glomerular filtration fraction, aka the filtrate (solution in the nephron).    
2. Reabsorption – Most of the water and solutes in the filtrate are reabsorbed back into the blood,
3. Secretion – The capillaries secrete H⁺ into the nephron if the blood pH is too acidic and secrete K⁺ if blood K⁺ is too high.  

Urine is the filtrate left in the nephron after filtration, reabsorption, and secretion is complete. There is a formula for this:

Filtration – reabsorption + secretion = Excretion 

Let’s do a sample problem:

An average human filters 50 grams of urea daily, reabsorbs 35 grams, and secretes 10 grams. How much urea does a person excrete in a day?

50 g (filtration) – 35 g (reabsorption) + 10 g (secretion) = 25 g of urea excreted per day 

So, what percentage of the daily filtered urea is excreted?

25 grams?

No. That is the amount excreted. To find the percentage, you need to use the following formula:

\bf{(}\bf{Amount\:Excreted}\:-\:Amount\:Secreted)\:\times\:100\%=Percent\: Excreted

 Example

\bf{(}\bf{25\:g}\:-\:50\:g)\:\times\:100\%=50\%\:of\:filtered\:urea\:is\:excreted