第九章 尿液的生成与排泄
2011-06-08 18:26:57   来源：   作者：  评论：0 点击：

（二）肾血浆流量
如果血浆中某物质经过肾脏循环一周能被完全清除，即肾静脉中该物质的浓度为零，则该物质的清除率即为肾的血浆流量。换句话说，该物质每分钟尿中的排出量（Us×V），应等于每分钟通过肾脏血浆中所含的量。如果每分钟通过肾脏的血浆量为X，血浆中该物质的浓度为Ps，则Ux×V =X×Ps。

因为肾小球滤过率仅仅是血浆流量的20％，从血浆清除该物质必须通过肾小球滤过和肾小管分泌两种方式才能完成（图9-22）。现在还不知道通过肾脏能够完全清除的物质。但是有一种化合物对氨基马尿酸（paraamino-hippuric acid，PAH）经过肾脏大约有90％可从血浆中清除，所以PAH的清除率可用以作为近似的肾血浆流量。举例来说，假定PAH的血浆浓度为1mg/100ml，尿内的浓度为585mg/100ml，尿量为1ml/min
则：
如果PAH的排泄率为90％，实际的血浆流量为585ml/min被0.9除得出值为650ml/min。因为血浆占全血量的55％，则总血流量为：
650ml/min×100/55 = 1182ml/min
根据求得的肾血浆量还可以计算肾小球的滤过分数（Filtration Fraction）。滤过分数为肾小球滤过率和肾血浆流量之比，如果肾血浆流量为650ml/min，肾小球滤过率为125ml/min，

（三）评估肾小管的功能
如果测出肾小球滤过率（GFR）和肾脏排出尿液该物质的排泄率（UsxV），便可以推测肾小管的净重吸收或净分泌。举例来说，尿中该物质的排泄率（UsxV）小于该物质的滤过负荷（GFRxPs），说明该物质必定存在着肾小管的重吸收。相反，如果该物质的排泄率大于滤过负荷，这表明尿中显示的该物质为肾小球滤过率加上肾小管分泌的总和。
测定肾小管的重吸收可根据下列例子来计算。假定测得某人的尿量为1ml/min，尿中Na＋浓度（UNa）为70mEq/L = 70μEq/ml，血浆Na+浓度（PNa）为140mEq/L = 140μEq/ml，肾小球滤过率（inulin清除率）= 100ml/min，由此得出
肾小球滤过Na+负荷为GFR×PNa =100ml/min×140μEq/ml=14000μEq/min
尿中Na+排泄率为uNa×V =70μEq/ml×1ml/min =70μEq/min
所以肾小管的Na＋重吸收为Na+的滤过负荷与尿Na＋排泄量之间的差，即
14000μEq/min－70μEq/min =13930μEq/min
同理可以测定肾小管的分泌，即该物质的排泄量减去该物质的滤过负荷。

第七节 尿的排放
(Micturition)
尿液是由肾脏连续不断地生成，但是膀胱将尿液排出体外的过程是间歇性进行的。因为尿液在肾脏形成后，经过输尿管蠕动直接流入膀胱贮存。只有达到相当分量时才引起排放。膀胱的贮尿过程进行得很慢，并无意识的感觉。而排尿是随意的。通常是在短时间内完成的。当尿液贮存时可以不发生排尿，这是处于一种克制状态。相反，当排尿时则尿的贮存受到抑制。膀胱的排尿是通过逼尿肌和平滑肌及横纹肌相混合的内外括约肌来完成的。
一、膀胱与尿道的神经支配
(The innervation of the bladder and ureter)
（一）膀胱与尿道的结构特征
肾脏形成的尿液从肾盏和肾盂离开，流经输尿管进入膀胱。输尿管是肌性管道，长约30cm，在膀胱后部靠近基底、膀胱颈之上进入膀胱。膀胱由两部分组成：膀胱底（fundus）或体，主要是贮存尿液；漏斗形的膀胱颈，与尿道相连。膀胱颈长约2～3cm，也叫做后尿道。在女性后尿道是尿道的终点，即尿液的排出点。在男性尿液流经后尿道进入前尿道，经过阴茎延伸，尿液经过尿道外口排出体外。
膀胱是由平滑肌组成的，形成复杂的交织网络。根据肌纤维走行的方向，膀胱壁可以分为三层：外层为纵形肌，中层为环形肌，内层为网状肌。当排尿时它们一起收缩，故也叫做逼尿肌(detrusor muscle)。膀胱颈部的平滑肌纤维形成了内括约肌(internal sphincter)，它不受意志的控制，具有紧张性，防止膀胱内尿液外流。尿道穿越泌尿生殖膈(urogenital diaphragm)。泌尿生殖膈具有骨骼肌层，形成外括约肌(external sphincter)，它受意识的控制。输尿管、膀胱和尿道具有显著的皱折，所以很容易膨胀。在膀胱和尿道中这些皱折叫做嵴皱(rugae)。当膀胱充满尿液时，嵴皱展平，膀胱容积增加，但膀胱内压只有很小的变化。膀胱的容积可以从很小的容积10ml，随着尿意可增加达400～500ml，而伴随着压力的变化只有5cmH2O柱左右。表明膀胱有很大的顺应性(compliant nature)。
（二）膀胱和尿道的神经支配
膀胱和尿道的神经支配在控制排尿中非常重要。膀胱逼尿肌和颈部内括约肌接受交感和副交感神经支配，交感神经由腰髓发出，经腹下神经α-肾上腺素能纤维到达膀胱。兴奋时使逼尿肌松驰，内括约肌收缩，阻止尿液的排放。副交感神经起自骶髓2～4节段，由盆神经毒蕈碱性纤维到达膀胱。兴奋时使逼尿肌收缩，内括约肌舒张，促进排尿。
膀胱外括约肌的骨骼肌纤维受阴部神经支配，此神经是由骶髓发出的躯体神经。兴奋时使外括约肌收缩。兴奋停止时外括约肌松驰。
上述的三种神经都含有传入纤维。腹下神经中含有传导膀胱痛觉的传入纤维，盆神经的感觉纤维分布在膀胱底部，传导膀胱充满牵张感受器发出的传入冲动。阴部神经则传导尿道感觉的传入冲动（图9-23）。
二、排尿反射
(The micturition reflex)
排尿是膀胱排空的过程，它涉及两个方面：（1）膀胱渐进性地充盈尿液，直到压力升高到临界值；（2）进行排尿反射，使膀胱中尿液排空。在正常情况下，膀胱没有尿液时，膀胱内压力大约为零。当尿液贮积30～50ml以前膀胱内压力仅增高5～10cmH2O。尿量增加到200～300ml膀胱内压也只有很小的增加。这种压力近似恒定的水平是由膀胱壁本身内在紧张性的结果。膀胱内超过300～400ml或更多的尿液时则压力才迅速增高。逼尿肌出现节律性收缩，呈上冲的尖峰波（图9-24）。它们是膀胱中牵张感受器发动牵张反射的结果，并产生排尿的欲望。
排尿活动是一种反射活动，即排尿反射(micturition reflex)。当膀胱充盈达到一定程度400～500ml时，膀胱壁牵张感受器受到刺激。从膀胱基底部发出的感觉信号通过盆神经传入到达脊髓排尿反射的初级中枢。冲动并上传到达脑干和大脑皮层的排尿高级中枢，引起充胀感觉和尿意。由排尿中枢发出的传出冲动再通过盆神经副交感纤维返回到膀胱，引起逼尿肌强烈收缩。膀胱的平滑肌是合胞体。当逼尿肌兴奋时也引起膀胱颈的肌肉收缩。由于膀胱出口的肌纤维呈丛行和辐射状，所以收缩时使内括约肌开放。尿液在膀胱内压力推动下使其流经后尿道，并刺激尿道感受器产生传入冲动，使膀胱进一步加强收缩。同时通过皮层抑制阴部神经活动，外括约肌发生随意性松驰，尿液排出体外。尿液通过尿道时还可反射性加强排尿中枢的活动。通过这种正反馈作用使排尿反射一再加强，直至尿液排净。在排尿末期，尿道海绵体肌的收缩可将剩余尿道中的尿液排出。通常排尿时腹肌和膈肌也配合收缩，使腹内压升高，有利于尿液的排出。
正常的排尿活动受高级中枢大脑皮层的控制，它可以易化或抑制脊髓初级排尿中枢的活动，但以抑制为主，所以人的意识可以控制排尿。婴幼儿高级中枢对脊髓排尿中枢的控制能力尚未发育完善，排尿不由意识控制，而出现排尿的次数多，且易发生夜间遗尿现象。当脊髓损伤时，脊髓排尿中枢与大脑皮层高级中构失去联系，排尿不受意识控制，膀胱充盈到一定程度后，通过低级中枢引起反射性排尿，而出现尿失禁的现象。
Summary
The urinary system is composed of the kidneys, bladder, and accessory structures. The kidneys produce urine, a fluid waste product whose composition and volume vary.
The six functions of the kidneys are regulation of extracellular fluid volume, regulation of osmolarity, maintenance of ion balance, homeostatic regulation of pH, excretion of wastes and foreign substances, and production of hormone. The most important function of the kidneys is the homeostatic regulation of the water and ion content of the blood.
I. Structure of the kidneys:
Each kidney has about 1 million nephrons. Each nephron in the kidneys consists of a renal corpuscle and a tubule.
1. Each renal corpuscle comprises a capillary tuft, termed a glomerulus, and a Bowman's capsule, into which the tuft protrudes.
2. Each glomerulus is supplied by an afferent arteriole, and an efferent arteriole leaves the glomerulus to branch into peritubular capillary, which supply the tubule.
3. The tubule extends out from Bowman's capsule and is subdivided into many segments, which can be combined for reference purposes into the proximal tubule, loop of Henle, distal convoluted tubule, and collecting duct. Beginning at the level of the collecting ducts, multiple tubules join and empty into the renal pelvis, from which urine flows through ureters to the bladder.
II. Basic Renal processes
1.The three basic renal processes are glomerular filtration, tubular reabsorption, and tubular secretion. In addition, the kidneys synthesize and /or catabolize certain substances. The excretion of a substance is equal to the amount filtered plus the amount secteted minus the amount reabsorbed.
2. Urme formation begins with glomerular filtration - approximately 180L/d - of essentially protein-free plasma into Bowman's space.
(1) Glomerular filtrate contains all plasma substances other than proteins and substances bound to protein.
(2) Glomerular filtration is driven by the hydrostatic pressure in the glomerular capillaries and is opposed by both the hydrostatic pressure in Bowman's space and the osmotic force due to the proteins in the glomerular capillary plasma.
3. As the filtrate moves through the tubules, certain substances are reabsorbed into the peritubular capillaries.
(1) Sbstances to which the tubular epithelium is permeable are absorbed by diffusion because water reabsorption creates tubule-interstitium concentration gradients for them.
(2) Tubular reabsorption rates are generally very high for nutrients, ions, and water, but are lower for waste products. Reabsorption may occur by diffusion or by mediated transport.
(3) Many of the mediated-transport systems manifest transport maximums, so that when the filtered load of a substance exceeds the transport maximum, large amounts may appear in the urine.
4. Tubular secretion (movement from the peritubular capillary into the tubules), like glomerular filtration, is a pathway for entrance of a substance into the tubule.
Ⅲ. Renal regulation
Renal function is regulated by neural and hormonal influences.The most important of these are:
1. renal sympathic nerves
2. renin-angiotensin system
3. aldosterone
4. atrial natriuretic peptide
5. antidiuretic hormone
6. prostaglandins
7. parathyroid hormone
Ⅳ. Clearance
Clearance is an abstract concept that describes how many milliliters of plasma passing through the kidneys have been totally cleared of a substance in a given period of time. The clearance of inulin can be used to calculate the glomerular filtration rate. In clinical settings, creatinine is used to measure GFR.
If a person's GFR is known, it is possible to calculate the filtration rate of a substance.
If less of a substance appears in the urine than was filtered, then some was reabsorbed. If more of the substance appears in the urine than was filtered, there is net secretion of the substance. If the same amount of the substance is filtered and excreted, then the substance is neither reabsorbed nor secreted.
Clearance values are also used to determine how the nephron handles a substance filtered into it. If the clearance of the substance is less than the inulin clearance, the substance has been reabsorbed. If the clearance rate of the substance has been secreted into the nephron.
V. Micturition
Urine is stored in the bladder until released by urination, also known as micturition.
1. In the basic micturition reflex, bladder distention stimulates stretch receptors that trigger spinal reflexes; these reflexes lead to contraction of the detrusor muscle, mediated by parasympathetic neurons, and relaxation of the external urethral sphincter, mediated by inhibition of the motor neurons to this muscle.
2. Voluntary control is exerted via descending pathways to the parasympathetic nerves supplying the detrusor muscle and the motor nerves supplying the external urethral sphincter.