BI0005 - Lecture 16 - Renal Function
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How much water is there in a human body?
By weight, the average human adult male is approximately 60% water.
However, there can be considerable variation in body water percentage based on a number of factors like age, health, weight, and gender.
In a large study of adults of all ages and both sexes, the adult human body averaged ~53% water.
What are the fluid compartments?
Fluid compartments in the mammalian body broadly comprise two compartments, each with several subdivisions:
Intracellular fluid (ICF), which makes up approximately 60-65% of body water, and extracellular fluid (ECF), which makes up the other 35-40% of body water (for all practical purposes, the only solvent in the body is water)
What is the intracellular fluid?
Intracellular fluid is found inside the two-layered plasma membrane of the body's cells, and is the matrix in which cellular organelles are suspended, and chemical reactions take place.
In humans, the intracellular compartment contains on average about 28 litres of fluid, and under ordinary circumstances remains in osmotic equilibrium with the ECF.
What can the Extracellular fluid compartment be further divided into?
The interstitial compartment
The intravascular compartment
The third space
What is the interstitial compartment?
The interstitial compartment (also called extravascular compartment or tissue space) is the space that surrounds the cells of a given tissue.
It is filled with interstitial fluid.
Interstitial refers to a "small opening or space between objects".
Together with the vascular space, the interstitial space comprises the extracellular space.
When excessive fluid accumulates in the interstitial space, edema develops.
Interstitial fluid provides the immediate microenvironment that allows for movement of ions, proteins and nutrients across the cell barrier.
This fluid is not static, but is continually being refreshed and recollected by lymphatic channels. In the average male (70 kg) human body, the interstitial space has approximately 10.5 litres of fluid.
In the lungs there is an interstitial space between capillaries (tiny blood vessels) and the alveoli (the microscopic air-filled sacs in the lungs responsible for absorbing oxygen from the atmosphere).
For gas exchange to occur, carbon dioxide must diffuse across the endothelium of the capillaries across the interstitial space, and across the alveolar epithelium; oxygen must diffuse the other direction, into the blood in the capillaries.
What is the intravascular compartment?
The main intravascular fluid in mammals is blood, a complex fluid with elements of a suspension (blood cells), colloid (globulins) and solutes (glucose and ions).
The average volume of plasma in the average (70 kg) male is approximately 3.5 liters.
The volume of the intravascular lumen is regulated in part by hydrostatic pressure gradients, and by reabsorption by the kidneys
What is the third space?
The third space is space in the body where fluid does not normally collect in larger amounts, or where any significant fluid collection is physiologically nonfunctional.
Major examples of third spaces include the peritoneal cavity and pleural cavity.
Still, small amount of fluid does exist normally in such spaces, and function for example as lubricant in the case of pleural fluid.
Also, the lumen of the gastrointestinal tract is often classified as belonging to the third space, although it has substantial fluid content physiologically.
In this classification system, the first and second space generally refer to the intravascular space (within vessels) and the extravascular space (the interstitial and intracellular spaces), respectively.
How do the size of body compartments change?
- Excessive sweating,
How do constituents in the compartments vary?
Extracellular fluid contains large quantities of Na+ and Cl-, and bicarbonate
Plasma contains large amounts of proteins
What role does sodium play in the volume of the extracellular compartment?
- The cell membrane and the capillary endothelium act as selective barriers
- - They maintain the state of chemical and electrical dis-equilibrium
- Water moves freely between cells and extracellular compartments down its concentration gradient
- -water move to dilute areas
Osmolarity of body compartments - Sum of partcles gives osmosal concentration = 285 mOsM
- Na determines the extracellular fluid volume
- -Na is the major cation: 145mM
If Na in the plasma increases and the concentration is kept constant, then the plasma volume will increase
Na is the major extracellular ion; an increase in total body Na will increase the volume of the plasma
What are the main functions of the kidneys?
To clear the blood of urea
To control the amount of water in the body
To regulate the composition of extracellular fluid
To produce erythropoietin and renin (hormones that regulate crythropoiesis and blood pressure)
Also involved in the balance of Ca ions
By forming urine, what key functions do the kidneys perform?
1. The kidney excretes the waste products of metabolism
2. The kidney regulates the body's content of water, sodium and potassium.
3. The kidney maintains the appropriate acid-base balance of plasma.
What is the structure of the human kidneys?
- The excretory system of mammals centers on a pair of kidneys.
- In humans, each kidney is about 10cm long and is supplied with blood by a renal artery and drained by a renal vein.
- Blood flow through the kidneys is voluminous.
- The kidneys acount for less than 1% of human body mas but receive roughly 25% of he blood exiting the heart.
Urine exits each kidney through a duct called the ureter
, and both ureters drain into a common urinary bladder
- During urination, urine is expelled from the bladder through a tube called the urethra, which empties to the outside near the vagina/penis.
- Urination is regulated by sphincter muscles close to the junction of the urethra and the bladder.
The mammalian kidney has an outer renal cortex
and an inner renal medulla
Microscopic extretory tubules and their associated blood vessels pack both regions.
- Weaving back and forth across the cortex and medulla is the nephron, the functional unit of the vertebrate kidney.
- A nephron consists of a single long tubule as well as a ball of capillaries called the glomerulus.
The blind end of the tubule forms a cup-shaped swelling, called bowman's capsule
, which surrounds the glomerulus.
Each human kidney contains about a million nephrons, with a total tubule length of 80km.
Briefly, how does filtration occur?
- Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman's capsule.
- The porous capillaries and specialised cells of the capsule are permeable to water and small solutes, but not to blood cells or large molecules such as plasma proteins.
Thus, the filtrate in bowman's capsule contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules.
Because filtration of small molecules is nonselective, the mixture mirrors the concentrations of these substances in blood plasma.
What is the pathway of the kidney filtrate?
From the bowman's capsule, the filtrate passes into the proximal tubule, the first of three major regions of the nephron.
Next is the loop of Henle, a hairpin turn with a descending limb and an ascending limb.
The distal tubule, the last region of the nephron, empties into a collecting duct, which receives processed filtrate from many nephrons.
This filtrate flows from all of the collecting ducts of the kidney into the renal pelvis, which is drained by the ureter.
What happens in the proximal tubule?
Reabsorption in the proximal tubule is critical for the recapture of ions, water, and valuable nutrients from the huge initial filtrate volume.
H2O, salts, HCO3-, H+, Urea, Glucose, amino acids, some drugs, are all actively or passively transported from the filtrate to the interstitial fluid, and then into the peritubular capillaries
Processing of filtrate in the proximal tubule helps maintain a relatively constant pH in body fluids.
As the filtrate passes through the proximal tubule, materials to be excreted become concentrated.
Many wastes leave the body fluids during the nonselective filtration process and remain in the filtrate while water and salts are reabsorbed. For example, Urea, is reabsorbed at a much lower rate than salt and water.
What happens at the descending loop of Henle?
Re-absorption of water continues as the filtrate moves into the descending limb of the loop of Henle. Here numerous water channels formed by aquaporin proteins make the transport epithelium freely permeable to water.
For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate.
This condition is met along the entire length of the descending limb, because the osmolarity of the interstitial fluid increases progressively from the outer cortex to the inner medulla.
As a result, the filtrate undergoes a loss of water and an accompanying increase in solute concentration at every point in its downward journey along the descending limb.
What happens at the ascending loop of henle?
Contains ion channels but not water channels, so is impermeable to water.
NaCl diffuses out of the permable tubule into the interstitial fluid. This movement helps maintain the osmolarity f the interstitial fluid in the medulla.
As a result of losing salt but not water, the filtrate becomes progressively more dilute as it moves up to the cortex in the ascending limb of the loop.
What happens at the distal tubule?
Aldosterone acts here to increase reabsorption of Na+
(but most Na+ is reabsorbed in the proximal tubule).
What happens at the collecting ducts?
The collecting duct carries the filtrate through the medulla to the renal pelvis.
As filtrate passes along the transport epithelium of the collecting duct, hormonal control of permeability and transport determines the extend to which the urine becomes concentrated.
What do hormones in the nephron do?
The nephron regulates the plasma levels of Na+ (aldosterone) and water (ADH)
Aldosterone 'fine tunes' the last 10% of Na+ reabsorption
ADH 'fine tunes' water reabsorption.
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