Question

Describe the order of blood flow through the nephron in order and explain each step of the pathway-

afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries, vasa recta

Answer 1

Renal Artery
Renal artery arises directly from abdominal aorta and enters the kidney through the hilus. While passing through renal sinus, the renal artery divides into many segmental arteries.
Segmental Artery
Segmental artery subdivides into interlobar arteries .
Interlobar Artery
Interlobar artery passes in between the medullary pyramids. At the base of the pyramid, it turns and runs parallel to the base of pyramid forming arcuate artery.
Arcuate artery.
Each arcuate artery gives rise to interlobular arteries.
Interlobular Artery
Interlobular arteries run through the renal cortex perpendicular to arcuate artery. From each interlobular artery, numerous afferent arterioles arise.
Afferent Arteriole
Afferent arteriole enters the Bowman capsule and forms glomerular capillary tuft. After entering the Bowman capsule, the afferent arteriole divides into 4 or 5 large capillaries.   
Glomerular Capillaries
Each large capillary divides into small glomerular capillaries, which form the loops. And, the capillary loops unite to form the efferent arteriole, which leaves the Bowman capsule.
Efferent Arteriole
Efferent arterioles form a second capillary network called peritubular capillaries, which surround the tubular portions of the nephrons. Thus, the renal circulation forms a portal system by the presence of two sets of capillaries namely glomerular capillaries and peritubular capillaries.
Peritubular Capillaries and Vasa Recta
Peritubular capillaries are found around the tubular portion of cortical nephrons only. The tubular portion of juxtamedullary nephrons is supplied by some specialized capillaries called vasa recta. These capillaries are straight blood vessels hence the name vasa recta. Vasa recta arise directly from the efferent arteriole of the juxtamedullary nephrons and run parallel to the renal tubule into the medulla and ascend up towards the cortex

When blood passes through glomerular capillaries, the plasma is filtered into the Bowman capsule. This process is called glomerular filtration.
Filtrate from Bowman capsule passes through the tubular portion of the nephron. While passing through the tubule, the filtrate undergoes various changes both in quality and in quantity. Many wanted substances like glucose, amino acids, water and electrolytes are reabsorbed from the tubules. This process is called tubular reabsorption.
And, some unwanted substances are secreted into the tubule from peritubular blood vessels. This process is called tubular secretion or excretion .   
A. Glomerular filtration
Glomerular filtration is the process by which the blood is filtered while passing through the glomerular capillaries by filtration membrane. It is the first process of urine formation. The structure of filtration membrane is well suited for filtration.
Filtration Membrane
Filtration membrane is formed by three layers: 1. Glomerular capillary membrane
2. Basement membrane
3. Visceral layer of Bowman capsule.
1. Glomerular capillary membrane
Glomerular capillary membrane is formed by single layer of endothelial cells, which are attached to the basement membrane. The capillary membrane has many pores called fenestrae or filtration pores with a diameter of 0.1 μ.
2. Basement membrane
Basement membrane of glomerular capillaries and the basement membrane of visceral layer of Bowman capsule fuse together. The fused basement membrane separates the endothelium of glomerular capillary and the epithelium of visceral layer of Bowman capsule.
3. Visceral layer of Bowman capsule
This layer is formed by a single layer of flattened epi­ thelial cells resting on a basement membrane. Each cell is connected with the basement membrane by cytoplasmic extensions called pedicles or feet. Epithelial cells with pedicles are called podocytes (Refer to Fig. 49.4). Pedicles interdigitate leaving small cleft­like spaces in between. The cleft­like space is called slit pore or filtration slit. Filtration takes place through these slit pores.
Process of Glomerular Filtration
When blood passes through glomerular capillaries, the plasma is filtered into the Bowman capsule. All the substances of plasma are filtered except the plasma proteins. The filtered fluid is called glomerular filtrate.
Ultrafiltration
Glomerular filtration is called ultrafiltration because even the minute particles are filtered. But, the plasma proteins are not filtered due to their large molecular size. The protein molecules are larger than the slit pores present in the endothelium of capillaries. Thus, the glomerular filtrate contains all the substances present in plasma except the plasma proteins.

B. Tubular reabsorption   
Tubular reabsorption is the process by which water and other substances are transported from renal tubules back to the blood. When the glomerular filtrate flows through the tubular portion of nephron, both quantitative and qualitative changes occur. Large quantity of water (more than 99%), electrolytes and other substances are reabsorbed by the tubular epithelial cells. The reabsorbed substances move into the interstitial fluid of renal medulla. And, from here, the substances move into the blood in peritubular capillaries.
Since the substances are taken back into the blood from the glomerular filtrate, the entire process is called tubular reabsorption.   
Reabsorption of the substances occurs in almost all the
segments of tubular portion of nephron.
1. Substances Reabsorbed from Proximal Convoluted Tubule
About 7/8 of the filtrate (about 88%) is reabsorbed in proximal convoluted tubule. The brush border of epithelial cells in proximal convoluted tubule increases the surface area and facilitates the reabsorption.
Substances reabsorbed from proximal convoluted tubule are glucose, amino acids, sodium, potassium, calcium, bicarbonates, chlorides, phosphates, urea, uric acid and water.
2. Substances Reabsorbed from Loop of Henle Substances reabsorbed from loop of Henle are sodium
and chloride.
3. Substances Reabsorbed from Distal Convoluted Tubule
Sodium, calcium, bicarbonate and water are reabsorbed from distal convoluted tubule.

C. Tubular secretion.
Tubular secretion is the process by which the substances are transported from blood into renal tubules. It is also called tubular excretion. In addition to reabsorption from renal tubules, some substances are also secreted into the lumen from the peritubular capillaries through the tubular epithelial cells.
Dye phenol red was the first substance found to be secreted in renal tubules in experimental conditions. Later many other substances were found to be secreted.   
Some substances are transported from blood into the
renal tubule.
With all these changes, the filtrate becomes urine.