Section 11 - Renal System

This chapter can be confusing. I recommend reading the book as the lecture notes are kind of jumbled.

What are the two main functions of the urinary system?

1. Excretion of metabolic waste products

2. Regulation of extracellular fluid, both volume and concentration

   1. Primarily extracellular water

Functionally, it is the ___ that controls the activities of the urinary system

Kidneys

Function of the Kidneys

Filtration of metabolite waste from the blood and water retention (which will then return to circulation later on)

What 3 shapes can the kidney take on? In what species is each shape found?

Bean-shaped

• Most species, such as dogs, cats, goats, etc

Heart-shaped

• The right kidney in horses

Lobulated

• Cattle

Where does blood supply from the kindey come from?

Directly off of the descending aorta of the heart

How does filtered blood from the kidneys return to circulation?

Directly back to the vena cava via renal veins

The right kidney sits ___ on the aorta and has a slightly ____ blood pressure

Higher, higher

Which kidney is better at filtration? Left or right?

Right, due to its position on the aorta and its higher blood pressure

Kidney: Hilus

An indentation in the outer structure of the kidneys where blood vessels, nerves, and ureter enter or exit the kidney

Kidney: Cortex

Outer region of the kidney which houses the structures that filter metabolites out of the blood by allowing significant water to cross out of the blood

Kidney: Medulla

Inner region of the kidney (just below the cortex) where water is regulated.

• Water is reabsored, concentrating metabolites prior to voiding.

• Region of collection

Kidney: Renal Pelvis

An extension of the ureter into the kidney where urine is directly deposited by the medula for transport to the bladder

What is the filtered fluid that enters the renal pelvis called? Does it change any further?

Urine. Once the filtered fluid enters the renal pelvis, it remains unchanged

• Exception is in horses, where it changes further in the bladder

Ureters

Highly muscular tubes that peristaltically pump urine unidirectionally away from the renal pelvis to the bladder.

How to the ureters enter the bladder? What does it form?

The ureters enter the bladder at an angle, forming a functional one-way valve that prevents backflow into the kidney. This is termed the ureterovesicular junction.

Bladder

A highly elastic, muscular storage vessel that can expand and fill with urine.

• Epithelium can change configuration and allow for expansion

• When the bladder is full, muscle contractions will lead to its emptying

Neck of the Bladder

Highly elastic structure that forms an internal sphincter which is normally closed.

• Opens during contraction to allow bladder to empty

External sphincter

A skeletal sphincter muscle that makes up the junction between the bladder and the urethra. Prevents urine escape when its contracted.

• Sacral spinal cord allows urine to escape through the neck and external sphincters

Urethra

A single muscular tube that peristaltically pumps urine outside of the body

What is urine expulsion a combination of?

Sphincter relaxation and muscles of the bladder contracting

Nephron

The functional unite of the kidney. Consists of a filtration zone and a water regulatory region.

Glomerulus

Two specialized porous capillary beds located in the cortex of the kindey involved in blood filtration. High in pressure.

• Akin to arterial end of capillary

• Porous state allows larger molecules to cross the endothelium

Peritubular capillary

Low pressure section of the glomeruli where blood perfuses (permeates) from the glomeruli

• akin to venous end of capillary

Loop of Henle

The water regulating portion of the nephron

The ability of the nephron to reabsorb water is determined by what?

How far it extends into the medulla

Cortical Nephron

Type of nephron where the glomeruli are located high within the cortex and the loop of henle reaches only to the top of the medulla (left)

Juxtamedullary Nephron

Type of nephron where the glomeruli sit just above the cortical-medullary junction and the loop of henle plunges deep into the medulla (right)

What do both the cortical and juztamedullary nephrons do?

Creates and maintains a significant osmotic pressure that moves water filtered by the glomerulus back into the body

Do the overall number and type of nephrons vary between species?

Yes.

• Those that depend more on metabolic water would have more juxtamedullary nephrons

Bowmans capsule

A funnel-like structure that encapsulates the glomerulus and collects filtered materials from it. Connects directly to the proximal tubule.

How does pressure differentiate between the glomerulus and the bowmans capsule?

When pressure in the glomerulus is high, the pressure in the bowmals capsule is low.

How does the bowman’s capsule maintain its lower pressure?

via the loss of filtrate, both hydrostatic and osmotic

How is blood supplied to the glomerulus? How does it leave?

Supplied via the afferent arteriole and leaves via a veniole to the renal vein

How does the pressure difference between the glomerulus and the bowmans capsule allow the nephron to function?

Because liquid moves from areas of high to low pressure, the difference allows for the fluid within the glomerulus to pass through the clefts in the capillary bed to then be filtered into the bowmans capsule.

• This process is primarily passive

What is the fluid thats filtered into the bowmans capsule called?

Glomerular filtrate (GF)

Proximal tubule

Transports materials from the bowmans capsule to the loop of henle

When GF (Glomerular filtrate) enters the proximal tubule, what is it then called?

Tubular fluid

What parts of the nephron are responsible for water regulation?

The proximal tube, loop of henle, and distal tubes.

• The tubules

What percentage of of filtered water continues on as a component of urine?

1%

Loop of Henle

Contains descending tube, ascending tube, area of reabsorption, and connects to the distal tubule

Distal tubule

Transports materials from the loop of henle to the collecting ducts

Vasa Recta

Surrounds the loop of henle and regulates water and ion reabsorption

Juxtaglomerular apparatus

Specialized region where blood supply and the distal tubules are connected. This regulates kidney blood flow, rate of filtration, and the release of renin (an enzyme.

How is urine formed?

1. Glomerular filtration — filters materials passing through the glomerulus and into the bowmans capsule (GF) which then becomes tubular fluid upon entering the tubules

2. Tubular reabsorption — reabsorption of water and ions back into the blood supply as the fluid travels through the tubules

3. Finally ends when the fluid enters the renal pelvis, where it remains unchanged until expelled

Where does reabsorption take place in the nephron?

Entire length of the tubules

Where is true urine found?

Renal pelvis

Renal Blood Flow (RBF) and its importance

Rate of bloow flow to the kidneys, effectively equals the renal plasma flow

• Important for the formation of glomerular filtrate

Glomerular filtration rate (GFR)

The portion of plasma filtered across the glomeruli

Filtration Fraction (FF)

Ratio of Glomerular filtration rate (GFR) to Renal blood flow (RBF)

• GFR:RBF

Protein crossing the glomeruli and entering urine represents a what state?

Diseased

Dilation of the afferent arteriole leads to what?

Increased RBF, hydrostatic pressure, and potentially higher filtration

Constriction of the afferent arteriole leads to what?

Decreased RBF but higher hydrostatic pressure.

Constriction and Dilation of the afferent arteriole causes different things, but ultimately maintains what?

The filtration fraction (FF)

Tubular Reabsorption

Movement of substance in the FF back across the tubular epithelium into the interstitual fluid or capillary

Tubular Secretion

Movement of a substance not filtered at the glomerulus across the tubular epithelium into the tubular fluid

Why is Glucose unique in the urinary system?

Due to its size, it is easily filtered into the tubular fluid and reabsorped at 100% back into the body.

ADH

Antidiuretic hormone — controls the permeability of the epithelial cells to water

• Without it, excessive water would leave as urine

What factors affect ADH release?

1. Decreased blood volume

   1. Monitored by volume receptors (pressure) in the walls of major vessels. Stimulates ADH release to increase water retention

2. Cold environments

   1. Inhibits ADH, causing increased urine production and thirst

3. Alcohol

   1. Increases urination; water loss exceeds alcohol consumption

Angiotensin II

Controls GFR as well as maintains FF when RBF is decreased

Aldosterone

Regulates extracellular potassium (K) and requires reabsorption of sodium (Na)

• Secretion is under the control of ADH

Parathyroid Hormone

Regulates calcium (Ca) reabsorption and the formation of the active form of vitamin D

• Promotes Ca absorption in the gut via cacitriol

Micturition

Emptying of the bladder

• Transfer of urine to the urinary bladder due to differences in hydrostatic pressure

The stretching of the bladder’s epithelium triggers what?

Pressure sensors that send signals to the control centers

Brain stem’s function in urination

Prevents contraction of the bladder and maintains contraction of the sphincter

When the bladder is full, what occurs?

Sensors in the bladder stimulate the cerebral cortex, allowing voluntary control of micturition.

Once mictruition starts, can it stop?

No. A second reflex is triggered that insures complete emptying of the bladder.

Bladder contractions continue until what?

No further flow is sensed through the urethra.

Why is urine formed?

Ultimately, to maintain balance of extracellular fluid by removing waste

• General materials found in urine are present in extracellular fluid

If materials are in high concentration in the body, will they be excreted in urine? Low concentrations?

If the materials are in high concentration, they will be excreted. If in low concentration, they will be conserved.

What gives urine its yellowish color?

Bilirubin, which has been oxidized

In most animals, urine is watery. In what animal (that we learned of) is this not the case?

Horses, as their urine is syrupy

Urine has a nitrogenous component known as?

Urea (in most species)

Where is urea formed? What does it require? Toxicity?

Urea is formed from ammonia in the liver, of which is highly toxic. It requires a large amount of energy to be made. However, once transformed into urea, it is relatively non-toxic in comparison to ammonia.

Polyuria

Increase urine output

Oligouria

Decreased urine output

Anuria

No urine output

Dysuria

Painful urine output

Continence

Maintenance of urine in the bladder between urinations

Incontinence

Leaking of urine from bladder with sphincter closed

What factors lead to incontinence?

Age, injury (especially to spinal cord), and disease

Renal Clearance

Measurement of the ability of a kidney to remove compounds from plasma

Are most substances fully cleared upon each pass through the kidney?

No, only a few substances are. They must be freely filtered at the glomerulus and not reabsorbed

What is the normal pH of the body?

7.4

How does the kindey balance pH?

By filtering and secreting excess ions into tubular fluid and by creating buffers.

What are the 3 buffers that the kidney makes? What do these work together to form?

All 3 form the isohydric principle

• Bicarbonate buffer — binds to hydrogen ions in the extracellular fluid, reacts with strong acids/bases

• Biphosphate buffer — maintains intracellular pH (similar to bicarbonate)

• Protein system buffers — Contain significant free anions which readily absorb and donate hydrogen

What do avians form instead of urea? Why?

Uric acid. Formed instead to maintain water, as creation of urea not only necessitates loss of water, but requires a large amount of energy

• Important for animals that lay eggs.

In animals, of the 3, which would always indicate a diseased state?
Calcium, bilirubin, RBC

RBC will always indicate a diseased state

What is urine (utilize the term “blood plasma”) basically?

Urine is what is left of blood plasma after it has gone through filtration, reabsorption, and secretion.

What two hormones regulate water movement in the kidney?

ADH and Aldosterone

Does reabsorption of materials from the lumen of a nephron always require energy?

No. It can require energy (such as for glucose, ions, etc), but it can also be passive due to diffusion and osmosis (water, urea, some ions)