Kidney Function: RBF, GFR, and Acid-Base Balance

Each kidney contains a renal medulla, renal papilla, renal pyramids, and renal columns
The organs of the urinary system include 2 kidneys, 2 ureters, the urinary bladder, and the urethra
Kidneys are the main organs of homeostasis
Kidneys are positioned in the posterior abdominal wall, one on each side of the vertebral column
Medulla is formed of pyramids
Each kidney is divided into the renal cortex and the renal medulla
The cortex is the granular outer layer of the kidney
The renal medulla is the striated inner part of the kidney
It's cone shaped structures are renal pyramids with renal papillae apices, which drain into the renal pelvis
Normal renal blood flow (RBF) is 1200 ml/min
Normal glomerular filtration rate (GFR) is 125 ml/min
The kidneys receive blood supply from the renal artery, a branch of the abdominal aorta
25% of cardiac output (1200 ml blood/min) goes to the kidneys
98% of this blood goes to the cortex for filtration, which contains the glomeruli
The renal medulla receives the other 1-2% for the purpose of urine concentration
Homeostatic regulation, secretion, excretion, degradation, and metabolic functions include ECF volume, osmolarity, pH, blood pressure, electrolyte concentration and gluconeogenesis
Erythropoietin, renin, active from of vitamin D, and prostaglandins are responsible for secretion, and several hormones such as insulin, glucagon, & PTH are responsible for degradation
Urea, uric acid, and many drugs, are responsible for excretion
A nephron is the basic structural and functional unit of the kidney
Each kidney has approximately 1 million nephrons, each capable of forming urine
Nephrons are made of vascular and tubular components
Nephrons can either be cortical (85% of total nephrons) or juxtamedullary (15%)
75% of normal nephrons act as a reserve inside the body
Cortical nephrons are responsible for urine formation and Juxtamedullary nephrons are responsible for urine concentration
The cortex is in the outer 2/3 of the renal cortex, with the juxtamedullary nephrons being fewer and located in the inner 1/3 of the renal cortex, near the renal medulla
The nephron consists of a glomerulus and tubules
A glomerulus is formed by the invagination of a tuft of capillaries into the Bowman's capsule
It is supplied by an afferent arteriole and drained by a slightly narrower efferent arteriole
Glomeruli are present in the renal cortex, giving it a granular appearance and act as filters to the plasma
Renal tubules receive the filtrate from the glomerulus and change it into urine
Renal tubules are divided into proximal convoluted tubules, loop of Henle, distal convoluted tubules, and a collective duct
The cortical collecting duct (CCD) is located in the renal cortex
The medullary collecting duct (MCD) is located in the renal medulla
MCDs coalesce and drain into the renal pelvis
Cortical nephrons make up 85% of the total nephrons, and juxta-medullary nephrons make up 15% of the total
Cortical nephrons glomeruli are in the Outer cortex, whereas juxta-medullary nephrons glomeruli are located in the Inner cortex
The loop of Henle on the Cortical nephrons is Short, and only reaches the outer medulla
The loop of Henle on the Juxta-medullary nephrons is Long, and reaches the inner papillae
Cortical nephrons have juxtaglomerular apparatus and peri-tubular capillaries, whereas Juxta-medullary nephrons lack juxtaglomerular apparatus but have peri-tubular capillaries and vasa recta
Cortical nephrons are responsible for Urine formation, and Juxta-medullary nephrons are responsible for Concentration of urine
The juxta-glomerular apparatus is where the distal convoluted tubule (DCT) returns to it's parent glomerulus & contacts both afferent & efferent arteriole
The juxta-glomerular apparatus contains 2 types of cells:

Juxta-glomerular cells

Release renin into renal tissues

Macula Densa cells

Act as a sensor for sodium chloride load
The kidneys detect a fall in blood pressure by releasing renin

Renin turns Angiotensinogen into Angiotensin I

Angiotensin converting enzyme converts Angiotensin I into Angiotensin II
The renal angiotensin system effects stimulation in the the body
There is a Fall in blood pressure, causing the release of Renin
Angiotensinogen then turns into Angiotensin I, which becomes Angoiotensin II, resulting in Vasoconstriction, increased BP, sodium retention, and Aldosterone production
High pressure in the afferent arteriole (60 mmHg) and low pressure in the efferent arteriole (15 mmHg) favors filtration
The 3 Processes of urine formation that result in excretion within renal nephrons are filtration, reabsorption and secretion

Filtration

Some plasma gets filtered from renal glomeruli into Bowman's capsule to form filtrate, a passive and nonselective process

Reabsorption

Some filtrate and dissolved substances are reabsorbed to the peritubular capillaries from the renal tubules to preserve them in blood stream and limit loss in urine

Secretion

Unwanted or excess substances may be secreted from peritubular capillaries to renal tubules to be excreted in urine
Excretion of urine = filtration – reabsorption + secretion
Glomerular filtration involves passage of protein free plasma from the glomerular capillaries to the Bowman's capsule along the filtration membrane
It is the first, passive, and nonselective step of urine formation and allows the passage of both essential and non-essential substances
The fluid filtered creates glomerular filtrate, also known as the primary urine
The glomerular filtrate composition matches plasma composition, apart from traces of protein (protein free plasma)
Glomerular filtration rate (GFR) is the volume of fluid filtered from glomerular capillaries to Bowman's capsule per minute and is equal to 125 ml/min (180 L/day)
Glomerular filtration rate (GFR) is 180 L/day, while normal urine volume is 1.5 L/day
Tubular reabsorption reabsorbs the filtrate from the glomerulus, with 99% or more of it normally being reabsorbed
Essential substances are moved from tubular fluid to blood in the PTC plexus at 125ml/minute
During tubular reabsorption 124 ml is reabsorbed back to the blood in PTC
Tubular reabsorption and secretion mostly occur in the proximal convoluted tubules (PCT)
Substances move from tubular lumen to tubular cell via active or passive transport during this process
Sodium reabsorption plays a role body electrolyte and water homeostasis
99% of the filtered Na+ goes back into the blood through primary active transport that is coupled to the movement of other electrolytes
Sodium reabsorption in DCT and CD is controlled by aldosterone hormone
Different nephron segments have different shares of Na+ reabsorption
The proximal tubule accounts for 65%, the ascending limb of L.H accounts for 25%, and DCT & C.T accounts for 9%
Secreted by the adrenal cortex, aldosterone increases Na+ reabsorption and K+ secretion in DCT and CDs
Glucose and amino acids are absorbed through Sodium reabsorption in the PCT by co-transport, HCO-3 & chloride are absorbed by electrical gradient, water reabsorption passively occurs by osmosis, and H+ is secreted by 2ry-active counter transport
Glucose is reabsorbed along with Na+ by secondary active transport and filtered glucose is normally completely reabsorbed
If filtered glucose is high glucose appears in the urine, as is the case in patients with diabetes mellitus when kidneys can’t reabsorb all of the filtered glucose
Glucose is reabsorbed by secondary active transport in the proximal convoluted tubule (PCT), and passive facilitated diffusion in the baso-lateral border
The renal thereshold for glucose is the plasma concentration at which glucose begins to appear in urine, which its 180 mg%
During glucosuria, glucose is present in urine due to an Increase in blood glucose as in Diabetes Mellitus or a Decreased renal thereshold for glucose
Normal pH is 7.4. pH < 7.4 leads to Acidosis and pH >7.4 leads to Alkalosis due to abnormality
Metabolic acidosis occurs due to diarrhea and renal failure
Respiratory acidosis occurs due to lung disease

Alkalosis

Respiratory alkalosis occurs during hyperventilation
Metabolic alkalosis occurs due to vomiting
Renal failure is when the kidney fails to carry out its different functions, and it can be either acute or chronic