Download Structure and Histology of Urinary (Renal) System

Lecture Notes:
Structure and Function of Urinary (Renal) System
教学目标：
1. 肾脏大体解剖结构及血液供应
2．泌尿小管的组成，肾单位的组成及光电镜结构
3．肾小球旁器的组成和功能
4．肾脏的主要生理功能
5．肾脏血供的自身调节和调节机制
6．尿液的形成、重吸收及分泌
7．尿液形成的调节
8．肾脏、膀胱组织学切片观察（数字切片：10.71.148.59/dssweb）
Vocabulary
泌尿系统 urinary system 肾 kidney 肾门 renal hilum 肾盂 renal pelvis 输尿管 ureter
膀胱 urinary bladder 膀胱三角 trigone of bladder 尿道 urethra 肾单位 nephron
足细胞 potocyte 滤过屏障 filtration barrier 肾小体 renal corpuscle 肾小管 renal tubule
球旁复合体 juxtaglomerular complex 肾素 renin 间质细胞 interstitial cell
排泄 Excretion 髓袢 Loop of Henle 肾小球 Glomerulus
近球小体 Juxtaglomerular apparatus 直小血管 Vasa recta
致密斑 Macula densa 窗孔 Fenestration 裂孔膜 Filtration slit membrane
Anatomy of Urinary System:
Organs of the urinary system:
Paired kidneys filter blood and produce urine.
Paired ureters transport urine to the urinary bladder.
Urinary bladder stores urine.
Urethra transports urine to the exterior.
Function：
 Regulation of body fluid osmolality
 Production of hormones:
Erythropoietin, Kinins and Renin, Prostaglandins，1,25-dihydroxycholecalciferol
 Regulation of electrolyte and water
 Excretion of waste products
 Regulation of acid and base
Basic anatomy of the kidney
1）
• The kidneys are located deep to the posterior abdominal wall at the T12-L3 level. Each
kidney has a concave medial border.
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The kidney is invested by a tough fibrous capsule.
The hilum is the medial area where nerves, blood and lymph vessels, and the ureter enter
and/or exit the kidney.
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The expanded upper end of the ureter is the renal pelvis.
The renal pelvis divides into 2 or 3 major calices.
Each major calix branches into several minor calices.
2）
• The area surrounding a minor calix is a renal sinus.
• On the interior aspect, the kidney has a cortex and an inner medulla.
• The medulla consists of 15 to 20 portions, called renal pyramids.
• The apical part of a pyramid is a renal papilla.
• The areas in between the renal pyramids are called renal columns.
Each medullary pyramid, plus the cortical tissue at its base, constitutes a renal lobe.
– Kidneys – Blood Circulation
1- Blood enters the kidney through the renal artery, which branches into smaller
segmental arteries.
2- Each segmental artery branches into interlobar arteries; these then course as delineating renal
(medullary) pyramids.
3- The vessels further divide into arcuate arteries; these then travel in the cortical/medullary
boundary and run perpendicular to the interlobar arteries.
4- The arcuate arteries divide, as well, into small arteries, called interlobular arteries, in the
renal cortex.
（That is as much as we see with the naked eye. For the microscopic circulation that follows after
the interlobular artery）
The following structures are microscopic:
1- The interlobular arteries give rise to the afferent arterioles.
2- The afferent arterioles form a capillary (for now, we’ll call it the glomerular capillary).
3- This capillary collects in another arteriole, called the efferent arteriole (yes, this is unusual…
the vast majority of capillaries in the body collect into a vein; but this one is an exception,
instead it collects into another arteriole).
4- The efferent arteriole forms a second capillary with 2 parts – the peritubular capillary and the
vasa recta.
5- The blood from this second capillary system collects into the interlobular vein and the arcuate
vein.
Histology of the Urinary System – Kidneys
Nephrons. there are 1 to 1.4 million nephrons per kidney.
The functional unit of the kidney is the nephron. The nephron consists of several discrete
structures that, essentially, consist of a filtration apparatus connected with a series of tubules.
The tubule system of the nephron facilitates the recovery of nutrients, the excretion of waste, and
the maintenance of osmotic pressure.
The structure of the nephron：
The nephron consists of the following portions:
•
The renal corpuscle filtrates blood (aka filtration apparatus, as referred to in the
previous slide)
The renal corpuscule has 2 parts: the Bowman’s capsule and the glomerulus, which is a globular
network of capillaries inside the capsule.
• The proximal convoluted tubule
• The loop of Henle
• The distal convoluted tubule
The collecting tubule, which eventually joins with other collecting tubules to form a
collecting duct (the collecting duct is NOT considered part of the nephron)
Filtrate contains everything found in blood plasma except for proteins. As filtrate moves into
the collecting ducts, it has lost most of its water, ions and nutrients. The material that
remains at this point is known as urine.
There are 2 types of nephrons:
Cortical nephron
• Glomerulus in outer cortex
• Short loop of Henle
• 80% to 85% of nephrons are this type
Juxtamedullary nephron
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Glomerulus very close to medulla
Long loop of Henle
15% to 20% of nephrons are this type
Nephron and a portion of the kidney blood circulation.
Some relationships between the nephron and the vasculature, and the kidney regions are:
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The arcuate artery is at the border of the cortex and the medulla.
Most of the nephron is in the cortex.
Most of the loop of Henle is in the medulla.
Histology of the Urinary System – Kidneys – Nephrons – Cross-sections
A renal corpuscle：
(Note: remember that the renal corpuscle has 2 parts – the glomerular capsule, also known as
Bowman’s capsule, and the capillary, called glomerulus.)
The capillary (called glomerulus) is linked with the afferent and efferent arteries.
the capillary is covered by cells. These cells are called podocytes
Renal corpuscle includes the glomerulus and the Bowman’s capsule. Between these 2
structures is the urinary space.
Glomerulus :is described as a “tuft” or “ball” of fenestrated capillaries.
Bowman’s capsule lines the urinary space and consists of:
•
An outermost layer – simple squamous epithelium (properly referred to as the
parietal layer of Bowman’s capsule)
• An innermost layer – cells known as podocytes (more on next slide), which adhere
to the external surface of the glomerulus (properly referred to as the visceral layer of
Bowman’s capsule)
Podocytes: These are specialized cells with foot processes that give off small finger-like
projections, known as pedicels. These projections form interdigitations with other
adjacent podocyte pedicels, which give rise to slit valves (or filtration slits). Together
these slit valves function as a filter. They trap large molecules and particles here in order
to prevent them from entering the urinary space.
Function of renal corpuscles:

Blood flows into the glomerulus by way of an afferent arteriole; blood then circulates in
the glomerular capillaries, and finally exits through an efferent arteriole.

Water, solutes, and small molecules from the blood are able to pass through the walls of
the glomerulus, between the filtration slits of the podocytes, and into the urinary space,
where they form a fluid substance called filtrate (it’s not called urine yet).

It is important to note that cells and large proteins, such as serum albumin, are NOT able
to cross this barrier (the filtration barrier of the glomerulus) under normal conditions.
(The particulars of the filtration system are discussed in Physiology.)
Filtration system or barrier:
This is a 3-layered system – materials leaving the blood and entering the urinary space must go
from the glomerular capillary through:
1- The fenestrated endothelium of the glomerulus
2- The uniquely thick glomerular basement membrane (GBM)
3- The filtration slits of the podocyte (also known as the visceral layer of Bowman’ s
capsule)
until they get to the urinary space.
In addition to providing a physical barrier for cells and large proteins, the glomerular basement
membrane and the pedicels of the podocytes contain negatively charged glycosaminoglycans,
which act to repel negatively charged proteins, particularly serum albumin.
The structure between capillary loops in the glomerulus:
Mesangial cells are glomerular cells that are NEITHER podocytes NOR endothelial cells.
They are modified smooth muscle cells.
Mesangial cells
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Have phagocytic properties
Provide structural support for the glomerulus
Play a role in controlling the glomerular flow rate via their contractile ability
Proximal convoluted tubule：
The PCT can be recognized by several histological characteristics:
A simple cuboidal epithelium
A brush border (meaning the cell surface contains microvilli, which add to the
absorptive surface area of the cells)
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Intensely eosinophilic staining and“frothy”appearing cytoplasm
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Basal striations (parallel rows of mitochondria in infoldings of cell membrane) – these
can only be seen at extremely high magnification
In addition, the PCT often appears to be filled with “debris” – the lumen of this tubule tends to
not be clear. This is likely due to small plasma proteins adhering to the microvilli.
Function: The proximal convoluted tubule is the first segment of renal tubules where filtrate
enters as it exits the urinary space. PCT is the site at which a majority (~65% to 67%) of recovered
water, ions, and glucose are transported from the filtrate and added back to the blood.
The PCT cells actively transport glucose and ions from the lumen of the PCT through their basal
surface, and into the peritubular capillary plexus. Water is also drawn through these cells, as it
follows the osmotic gradient, created by the movement of Na+ and Cl-. These cells have
numerous mitochondria located in their basal cytoplasm.
Distal Convoluted Tubule:
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A simple cuboidal epithelium
NO brush border
Large clearly defined lumen
Paler cytoplasm (due to fewer organelles)
DCT cells have a tendency to show more nuclear profiles per tubule relative to PCT cells; owing
to the fact that PCT cells have more abundant cytoplasm.
Function: DCT cells are unique in that they are the site at which the kidney regulates Na+
concentration, via absorption and secretion of this ion, in a response to aldosterone secretion.
In addition, they play a role in regulating pH by absorbing and secreting bicarbonate and protons.
Collecting tubules: travel toward the medulla on their way to joining a collecting duct. In doing
so, they form medullary rays.
Medulla: Loops of Henle
In the renal medulla, a proximal convoluted tubule becomes straight – while sometimes they are
simply referred to as proximal tubules, they are more commonly referred to as thick descending
limbs of the loop of Henle.
The loop of Henle then becomes thin and, thus, it is referred to as the thin loop of Henle
(ascending and descending parts).
Finally, the loop of Henle becomes thick and, thus, it is called the thick ascending loop of Henle
(before becoming a DCT).
The loop of Henle is the portion of the nephron that is largely responsible for the kidney’s ability
to produce hypertonic (or concentrated) urine.
Juxtaglomerular apparatus (JGA):
Next to the macula densa, the smooth muscle cells of the afferent arteriole are also modified, and
are now called juxtaglomerular cells. Also at the vascular pole are lacis cells – extraglomerular
mesangial cells that seem to be similar to the mesangial cell – inside the renal corpuscle.
Function: maintains blood pressure
肾脏生理：
一、肾脏的血液供应和调节
健康成人两肾血流量 1200ml/min, 占心输出量的 20-25%
1. 肾脏血液供应的自身调节
肾血流量自身调节的机制：动脉血压在一定范围（80-160 mmHg）内发生变动时，肾血
流量相对稳定，肾小球滤过率相对稳定。
机制：1）肌源性机制
2）管-球反馈
2. 肾脏血液供应的神经体液调节
二. 尿液形成
1.肾小球的滤过作用
肾小球滤过率: 正常成人 125ml/min
1) 不同物质通过滤过膜的能力取决于被滤过物质分子的大小及所带的电荷。
滤过膜屏障:
A. 有效分子半径大于 4.2 nm
B. 带负电荷的物质
2) 滤过的动力: 有效滤过压
3) 影响滤过的因素:
 有效滤过压
 滤过膜的面积和通透性
 肾血浆流量
 滤过系数（Kf）
2. 肾小管和集合管的重吸收
1) 重吸收主要部位：近端小管
重吸收途径：跨细胞转运途径；细胞旁转运途径
2）Na+的重吸收：99%滤过的 Na+被肾小管和集合管重吸收
近端小管：Na+进入上皮细胞的过程与 H+的分泌以及葡萄糖、氨基酸重吸收耦连。
髓袢：Na+- K+- Cl- 同向转运体
远端小管、集合管：Na+- Cl- 同向转运，Na+- K+ 交换
3） 水的重吸收：重吸收的动力--渗透压， 通道—AQP
4）葡萄糖、氨基酸：在近端小管全部重吸收，继发性主动转运。
3. 肾小管和集合管的分泌作用
1）H+、K+的分泌：
维持机体内环境酸碱平衡。
近端小管是分泌 H+的主要部位，以 Na+-H+交换为主。
Na+ -H+交换与 Na+ -K+ 交换存在竞争机制，酸中毒要防止高血 K+ 。
2）NH3 的分泌：
蛋白质代谢产物的排泄途径。
肾小管,集合管分泌 H+,NH3 和生成 HCO3-对体内酸碱平衡的调节起重要作用.
三. 肾脏功能的体液调节
1）醛固酮：来自肾上腺皮质球状带，肾素-血管紧张素-醛固酮系统
2）ADH：来自下丘脑视上核和室旁核，作用于远端小管和集合管，促进水的重吸收。