Question

In a list format, provide the pathway of a water molecule from absorption into the systemic capillaries of the small intestines to its exit out the urethra. Be detailed in your list (i.e. do not just say it passes through the nephron, but rather list the segments of the nephron).

Answer 1

Even though water absorption occurs in the small intestine, the major part of systemic absorption occurs in the large intestine. There are numerous microvilli projections present in the lining wall of the small intestine.The water is absorbed into the network of capillaries through the walls of the villi by diffusion.As the absorption of digested food into the blood increases the water potential of the content in the ileum water is drawn into the blood by osmosis. As water is the the main part of blood, it will travel along with the blood vessels to reach the kidney through the renal arteries.

Arterial supply of the kidney is from the renal artery.THE RENAL ARTERY divides into anterior and posterior divisions at the hilum of the kidney.The anterior division mainly supplies apical,upper,middle as well as the lower segments and the posterior division supplies the posterior segments via the segmental branches.Each segmental artery divides into lobar branches after supplying the renal sinuses.The lobar arteries are further divided into 2-3 interlobar arteries which passes between the pyramids.Here each interlobar artery divides into the arcuate arteries as reaches the base of the associated pyramids and run parallel to the surface of the kidneys between the pyramids and the overlying cortex.Here, each arcuate arteries give rise to many interlobular arteries which passes radially to the surface of the kidney.The interlobular arteries gives afferent arterioles which pass to the capillary network of glomeruli and unite to form efferent arterioles.The efferent arterioles form the peritubular capillaries around the nephron loop.

urine formation occurs in 3 steps.

1. glomerular filtration
2. reabsorption
3. Secretion

the filtration is carried out by the glomerulus. which is known as to glomerular filtration.The glomerular capillary blood pressure causes the filtration of the blood. through 3 layers

1. endothelium of glomerular blood vessels
2. .epithelium of bowman's capsule
3. the basement membrane between these 2 layers

the epithelium of bowman's capsule called podocytes are arranged in s single layer manner so as to leave a space in between known as slit pores. when the blood flows through the capillary network of bowman's capsule, it gets filtered through the walls of the capillaries, through which the water soluble small molecular wt substances such as glucose and amino acids and nitrogenous waste are filtered from the blood. Blood cells and proteins are not filtered out from the blood. This filtration is called ultra filtration. On an average 1100- 2100 millilitre of blood is filtered through the kidneys per minute..

Blood enters the afferent arteriole through the bowman's capsule and passes through the tuft of capillaries called glomerulus. The diameter of efferent arteriole which coming out of the glomerulus is less than the afferent arteriole; thus creating a high pressure inside the glomerulus to ultra filtration to occur. Hydrostatic and osmotic pressure in the capillary makes the plasma and solutes to be filtered from the blood into the capsular space of nephron through the filtration membrane.The filtration membrane allows only small particles to pass through it, this include water glucose amino acids inorganic contents and various metabolic waste products like urea,Creatinin etc. This fluid is called glomerular filtrate.
The amount of filtrate produced per minute is called glomerular filtration rate.
The efferent arteriole after exiting from the glomerulus forms a network of capillaries around the renal tubule called peritubular capillaries.
In the first part of renal tubule i.e.proximal convoluted tubule(PCT), the two third of the filtrate are reabsorbed to the peritubular capillaries . In this process water and solutes are driven out of the epithelium to the extracellular space, which are absorbed into the blood capillaries;Mainly sodium-ion. The sodium reabsorption is important because it creates an osmotic force to drive the water out of the the PCT. It also creates and electrical force for the negatively charged particles to come out.The sodium potassium pump in the epithelial cell play the major role in driving out of the sodium ion into the extracellular space. This creates a concentration gradient that favors sodium diffusion from the tubular fluid into the cell. Sodium is taken up by symport proteins which also takes glucose and amino acids along with it. Nearly all glucose and amino acids are reabsorbed at this stage. About half of the nitrogenous waste are also reabsorbed into the blood.
Tubular secretion of of drugs, metabolic waste products , and H+ Ion take place from the peritubular capillaries into the PCT.The process of reabsorption and secretion continuous through the nephron loop (called loop of Henle) and distal convoluted tubule(DCT). The main function of loop of Henle is to create and maintain an osmolarity gradient in the medulla that enables the collecting ducts to concentrate the urine at the later stage. Ascending limb of loop of Henle actively pump sodium out of the tubule to make the medulla salty. The descending limb of loop of Henle is permeable much for water but not for sodium. The water gets out from the filtrate and the the filtrate become more concentrated as it reaches the bottom.
On the other hand the ascending limb is permeable to ions but not for water. As a result, the filtrate loses sodium as it comes up and become more diluted at the top of the loop. The medulla is in equilibrium with the loop and have more salinity gradient from top to the bottom. Reabsorption and secretion at the distal convoluted tubule are under control of various hormones. This is how kidney adjust the composition of the urine according to the body's need. The collecting duct receives the tubular fluid from several nephrons. The main function of the collecting duct is to concentrate urine by controlling the water. This is made possible by the osmolarity gradient created by the loop of Henle. As the filtrate goes down the collecting duct more and more water are removed out .The collecting duct is also under the hormonal control of antidiuretic hormone(ADH) to control the the water level in the blood according to the body state of hydration.For example, in case of a dehydration more water are reabsorbed from the collecting duct into the blood.

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Once millions of nephrons have each made urine it flows into the minor calyces the major calyces and finally into the renal pelvis.From the it goes down the ureter which has a muscular lining which helps to push urine along the ureter.The urrters insert into the bladder at the ureterovesical junction at a sideways angle so that when the bladder becomes full,
it compresses the opening to the ureters preventing the backflow of urine. It is basically a one way valve that prevents urine from refluxing backwards from the bladder into the ureters. The bladder itself is like a balloon its muscular wall has many folds called rugae that can contract when the bladder is emptied of urine and can expand when it is filled with urine. In the layers of the bladder wall are a mucosal layer that has a transitional epithelium,which is structure allowing for bladder distention while maintaining a barrier between urine and the body.In addition there is a thick muscular layer called detrusor muscle that helps with bladder contraction during urination and it has fibrous adventitia outer layer. On average the bladder can hold around 750 millimetres of urine.The floor of the bladder has has more the triangular region called the trigone region with two corners at the ureterovesical junction and the third corner being the internal urethral orifice where the bladder meets the urethra.The trigone region is very sensitive to expansion and once it stretches for a certain extension the bladder sends a signal to the brain that it's time to pee. The urethra is a thin muscular tube that drains urine from the bladder starting from the internal urethral orifice to the external opening.By contracting the skeletal muscles around the external sphincter urination can be stopped voluntarily.The act of urination involves close coordination between the nervous system and muscles of the bladder. Once the volume of bladder is greater than about 300 to 400 ml pressure on the bladder wall increases and send signals to the urination or micturation centre in the spinal cord.This is called the micturition reflex which causes contraction of the bladder and relaxation of the internal sphincter and external sphincter. This allows the urine to flow through the urethra and out of the body.